-t! v - • · -t! v - • nasa cr-145335 * ... alouette iii sud aviation sa341g! aerospatiale...

By

John J. Dougherty, HILawrence D. Barrett

-T! V - •

NASA CR-145335 *

Prepared under Contract No. NAS1-13624By

Boeing Vertol CompanyPhiladelphia, Pennsylvania

for

NASANational Aeronautics and

Space Administration

April 1978

https://ntrs.nasa.gov/search.jsp?R=19780013150 2018-08-26T05:05:52+00:00Z

NASACR-145335

RESEARCH REQUIREMENTS TO IMPROVE"" ^RELfABlLITY OF CIVIL HELICOPTERS

By John J. Dougherty, IIIand Lawrence D. Barrett

-Prepared-under Contract No. NAS1-13624 byBoeing Vertol Company

Philadelphia, Pennsylvania

for

_ NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

ABSTRACT'

This report documents the basic research and development required, as well as otherchanges possible within the state of the art, to achieve a substantial improvement in thereliability of civil helicopters.

Federal Aviation Agency civil helicopter Malfunction or Defect (M or D) reports werereviewed 'to identify and prioritize the problems, supplemented with military helicopter dataonly where deemed necessary.

The costs of unreliability in terms of unscheduled on-aircraft maintenance manhoursand the costs of repair components and materials are included.

A listing of specific R&D recommendations is provided with an estimation of the pay-offs, timing, and development costs.

This report was prepared by the Boeing Vertol Company for the National Aeronauticsand Space Administration, Langley Research Center, under NASA Contract NAS1-13624.William Snyder was NASA Program Manager for these studies. The Boeing Project Managerwas Kenneth T. Waters.

. iv

This study has shown that 78 percent of the reliability problems of civil helicopters canbe categorized into 30 problems. These problems were analyzed to determine causal factorsand to recommend corrective action. Of the 30 problems that were analyzed, the followingtable lists their relative impact by subsystem:

_ _ _ _ _ _ \ * _ . . . . _ _ . : . .<•• • •>_ Unscheduled _

Subsystem

Propulsion (Turbine power) t

Drive

RotorAirffameLanding Gear (Floats)*

Fuel

Hydraulics

RelativeFailure Rate (%)

35.3

13.9

12.219.99.4

5

4.1

(Maintenance Repair Partsr "" -Man-lours (%) ' Cost (%)

A 25.1 66

35 [ 21.3

19.7 11.410.1 - 15.6

1.1

2.8

1.2

tOnly turbine-powered helicopters were included in this study.

*Ari aggressive reliability improvement program has virtually eliminated floatsfrom the problem list subsequent to the data received for this study.

In terms of unscheduled maintenance manhours and cost of repair parts, the propulsion,drive, and rotor subsystems represent over 80 percent of the reliability problem. Since thesesubsystems also have a major impact on mission reliability (aborts) and safety, it is clear thatmajor emphasis should continue to be placed on improving these subsystems. Because of thesignificant number of problems and the unscheduled maintenance manhours involved, air-frame reliability also needs improvement.

This study has defined research and development that should be conducted to sub-stantially improve reliability of civil helicopters and thereby reduce operation and maintenancecosts. The highest priority programs are listed below with estimated nonrecurring costs andtiming in months:

Nonrecurring Time($x 1,000) (mo)

Propulsion (turbine engines)• Fuel-control and-governor:

Study failure modes and developdiagnostic fault-isolation techniques

• Compressor/turbine failures:Study failure modes andapplication of inlet separators

400

400

36

36

__ _ _ _ _ _ Nonrecurring Time($ x 1,000) (mo)

Drive•; Transmission gear and bearing spalling:

Materials development per reference 1 500 36 r• Gear scuffing.and spalling:

Lubricant development 150 24• Main and tail rotor gearbox housing

cracks: Vibration reduction perreference 2 ' . 6,300 60

Rotor• Tension-torsion blade retention:

'Develop bearingless main rotor, reference 15 :/ — —• Main and tafl rotor control bearings: /

Continue development of elastomericand dry=lube bearings 800 36

Airframe• Structural cracking and unbending:

Implement composite airframestructures program, reference 3, and — —implement-vibration reductionprogram, reference 2

Many areas of unreliability can be improved by application of existing technology. Amongthe more prominent are:

1. Changeover to fiberglass composite rotor blades

2. Use of more effective turbine-engine inlet separators and screens to reduce FOD andcompressor erosion

3. Introduction of an aggressive engine failure analysis and fix program

4. Improve durability of airframe fasteners and small hardware

5. Redesign and testing to reduce spalling of transmission bearings

6. Redesign to improve lubrication of drive shaft hanger bearings

7. Redesign main transmission housings on one model helicopter to reduce mounting lugcracking

8. Use. the latest gear materials and improved process and quality controls to reduce toothspalling and scuffing problems

9. Improve corrosion resistance of rotor components through better coatings that aredamage-resistant

10. Apply the latest technology in hydraulic actuator seals to reduce seal leaking

11. ^ Product improvement of fuel pressure switches and fuel pumps

! vi

5 OF OCMTEHTS ^_'_ "_*s- -,,.. ', - , - - • -. - '~~*

Page

ABSTRACT . ... ..; . . . . . , j iiiI|

FOREWORD "7." 7TiTTT™TTT~. . ~~ . . .'""." ~ ""."T" ".".'7 "V. j iv

SUMMARYv--,:-.--v--.-i~--'--T--^- v v-,-'.. . . . . , - - - . - , . . . . | v: . . - ' . ' " ; ' ' ! •

LIST OF ILLUSTRATIONS : viu

LIST OF TABLES :

1.0 INTRODUCTION . . .

2.0 BACKGROUNDrTHE IMPACT OF RELIABILITY . . . . ... ; 5

3.0 THE RELIABILITY PROBLEM \ 7Z.I Problem Identification i 73.2 Subsystem Problem Discussion 19

~3:3~ ^Mission-Reliability"; ;.-; ; . . . . . . . . . ; . . . ' 38

4.0 RELIABILITY IMPROVEMENT PROGRAM . . . . . . . . . i 42i

5.0 CONCLUDING REMARKS ! 55

' APPENDIX:, CIVIL HELICOPTER RELIABILITY PROBLEMS . . . . . . [ 61

REFERENCES 103

. vii . /

Figure Page

1 Current and predicted flight hours of U.S. general aviationhelicopters 3

2 Percentages of civil helicopter flying hours by kind of flying . . . . 4

3 Distribution of reported reliability problems by helicoptersubsystem 8

4 Top 20 most frequently reported reliability problems ofcivil helicopters 9

5 Top 20 maintenance manhour problems of civil helicopters . . . . 10

6 Top 15 relative repair parts cost problems of civil helicopters . ... 11

7 Thirty analyzed reliability problems represent more thanthree-quarters of the malfunctions or defects reported tothe FAA 13

8 Major reliability problems of civil helicopters by relativefailure rate of subsystems 16

9 Major reliability problems of civil helicopters by relativeunscheduled maintenance manhours on subsystems 17

10 Major reliability problems of civil helicopters by relativecosts of repair parts for subsystems 18

11 Distribution of propulsion subsystem reliability problems 21

12 Distribution of airframe subsystem reliability problems 23

13 Distribution of drive subsystem reliability problems 26

14 Distribution of rotor subsystem reliability problems 31

15 Relative reliability of landing gear subsystem (float leaks) 33

16 Relative reliability of hydraulic subsystem (actuator leaks) . . . . 35

vui

Figure • _ Page

17 Distribution of fuel subsystem reliability problems 37

18 Distribution of engine subsystem reliability problemson U.S. Navy aircraft 41

IX

Table : Page

1 ' 1975 U.S. General-Aviation Flight Hour Distribution forTurbine^Powered Helicopters .

2 , "Combined "Reliability Problemi Summary . . . . . . . . . . . . "j 14 ;i . . ' . . . . : ' ' . " " .

3 ! Characteristics of the. Military Drive System '28 |

Flight Success of the OH-58A Helicopter, August 1970 !. Through June 1972 . . j 39

Army OH-58A Data on Items Most Frequently Causing Flight !Aborts, August 1970 Through June 1972 ! 40

6 Type of Action Required for Reliability Improvement 43 !

A A7 Reliability Improvement Action . . j 44 ,

8 ~ "Recommended" Reliability improvement R&D Programs v . . . . . ! 56 ;

x • .

-- • - •- ' - - - —-»s«j~,*" '',-"",./.'";''. ' "''"..' . ' ""* . , _v' \""_ _. j . . . . . .

The major reliability problems of the civil helicopter fleet as reported by helicopteroperational and maintenance personnel are documented in this report. An assessment of eachproblem is made to determine if the reliability can be improved by application of presenttechnology or whether additional research and development are required. ,

The reliability impact is measured in three ways:

• The relative frequency of each problem in the fleet.

• The relative on-aircraft manhours to repair, associated with each fleet problem.

• The relative cost of repair materials or replacement parts associated with each fleetproblem.

The primary source of problem identification and prioritization is the FAA Malfunctionor Defect report (M or D), FAA Form 8330. This report may be filed by pilots, operators,and maintenance or repair personnel and is mandatory for the certified domestic repairfacility. The data reporting is not mandatory for all personnel and all types of malfunctions,particularly for those malfunctions not immediately critical to flight safety. This FAA M orD report is, however, the most complete source of reliability data with inputs from all segmentsof the civil helicopter fleet that is known to the authors. Thus it is best suited as a data sourcefor any"quantitative" evaluation of the helicopter reliability problem.

This data was supplemented in a few cases by military data, when the same helicopter orsubsystem was used in both civil and military applications. All data from sources other thanthe FAA M or D report is referenced in the text.

The data reviewed covered the period of 1971 through 1976 and covered only turbine-engine aircraft. This was done because virtually all reported data for the reciprocating-engine-powered helicopters was for the Bell Model 47, a model last manufactured in 1973 and con-sidered atypical of the present-day product.

The flight hour base of the turbine-engine-powered civil helicopter fleet during this 6-yearperiod was approximately three million hours. The annual total civil helicopter fleet flighthours in Figure 1 are shown for the past nine years with yearly usage projected to 1985. Theturbine-powered helicopter annual usage has been steadily increasing, exceeding that of thepiston-engine-powered aircraft several years ago. The proportion of the total fleet hourstaken by the turbine-engine-powered machines is expected to continue to rise as-the civilhelicopter fleet is modernized.

A typical distribution of flight hours among the various helicopter models is shown inTable 1, which gives the 1975 turbine-powered helicopter fleet records. The Bell Model 206

dominates this.civil fleet and the civil helicopter reliability records reflect the high usage ofthis helicopter.

Typical missions for the civil helicopter fleet are given in Figure 2, which gives the per-centage of flying hours expended in 1974-75 for each type of flying.

TABLE 1. 1975 U.S. GENERAL AVIATION FLIGHT HOUR DISTRI-BUTION FOR TURBINE-POWERED HELICOPTERS

1

Manufacturer Model

1 Single Engine

Bell 206

Bell 205

FH1100i Hughes 369

Alouette III

Sud Aviation SA341G

! Aerospatiale SA315B

SNIAS SA318C

Twin Turbine

Bell 212

i Sikorsky S-61, S-64, S-58

/ Boeing Vertol 107, BO- 1C

f Total Turbine-Powered FligF

Percentage ofTurbine-Powered iFlight Hours

r " -<,

'56.9

\ \5.6 : .

4.4 1

n.i j

6.5

r • 15.515

it Hours 820,549

Data from reference 4

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TYPE OF FLYING i

AIR TAXI (CHARTER, OFFSHORE DRILLING- PERSONNEL AND EQUIPMENT)

INDUSTRIAL (CONSTRUCTION- PROFESSIONAL PILOTS)

CORP/EXEC (PROFESSIONAL PILOTS)

AGRICULTURAL (SEEDING, SPRAYING)

OTHER (POLICE, FIRE, SAR, MISC)

INSTRUCTION, TRAINING

BUSINESS (NONPROFESSIONAL PILOTS)

PERSONAL (PLEASURE)

PERSONAL 1.35

DATA BASE: 1974 AND 19752,960,070 FLYING HOURS

CORPORATE/EXECUTIVE

12.3

REFERENCE 4.-. A

Figure 2. Percentages of civil helicopter flying hours by kind of flying

•"5(J

r OF'RELIABILITY ].What is reliability? Webster's dictionary gives the definition, "suitable or fit to be relied

upon, worthy of dependence or reliance giving the same results on successive trials .. . ." / ,(ref. 5). / ,

Reliability to the military is "the probability that an item will perform its intended func-tion for a specified interval under stated conditions" (ref. 6). Demonstration of reliabilityis usually expressed in terms of mean time between failure (MTBF), or failure rate, 1 .

MTBF

Each civil helicopter operator probably has his own priorities as to what factors are mostimportant in demonstrating improved reliability. There may be agreement among operatorsthat the highest priority of corrective action should be directed at preventing malfunctionsthat result in mission aborts. These are the reliability problems that most affect the helicop-ter task completion and also have flight safety implications. Fortunately these are a smallpart of the helicopter reliability problem; one summary of reliability data for the OH-58 air-craft (the military version of the Bell Model 206) showed that less than 0.4 percent of mal-functions requiring repair or replace maintenance actions resulted in mission aborts (ref. 7).

The mean time between failure (MTBF) of a helicopter is certainly an important reliabilityparameter. "The adverse-effect of a low MTBF is increased if the malfunction has the follow-ing characteristics:

•. has safety of flight implications

• prevents mission completion

• causes extensive downtime of the helicopter

• requires many maintenance manhours to correct

• requires expensive repair parts

• requires not-normally-inventoried replacement parts.

A civil helicopter operator may be very tolerant of a helicopter with a high malfunctionrate (and a resulting low MTBF) if he has no mission-affecting failures, the helicopter avail-ability-is high, and the frequent maintenance actions are very inexpensive to correct in termsof manhours and materials. Some light bulb and rivet/screw failures are examples of this typeof malfunction. Conversely, a high-reliability helicopter in terms of a high MTBF may beconsidered unacceptable if the relatively few malfunctions are of a nature that adversely affectavailability or flight safety and result in a high repair cost per flight hour. Engine failures fallin this category.

In this report the three parameters chosen as indexes for determining the impact ofreliability are the failure frequency, on-aircraft manhours to repair, and the cost of repairparts.""The rationale for the selection of these indexes is as follows:

• Submission of the FAAMorD report is not obHgatory for many Kelicbpter personnel;the defects that are reported tend to be those considered most relevant by the operationalor maintenance personnel, primarily serious defects and those endangering the safeoperation of the aircraft. Thus the reported failure frequency obtained from this datasource is weighted toward the more significant failures.

• The FAA M or D reports contain no information identifying malfunctions resulting inmission aborts, nor do they give information from which the impact of the malfunctionon helicopter availability can be determined. Therefore this parameter could not beevaluated from the reliability data. The subject is discussed separately, however.

• The on-aircraft manhours to repair and the cost of repair materials and components canbe determined from the malfunction descriptions and relative failure frequencies ob-tained from the FAA M or D data, either by maintenance engineering analysis proceduresor, on some models of helicopter, from the extensive data records of equivalent militarymodels. These are important parameters in the determination of an operator's manpowerrequirements and operating expense.

• Helicopter downtime and the normal inventory of spare parts are not parameters de-terminable from the FAA M or D reports and are somewhat dependent on the size ofeach individual operator's fleet and maintenance shop, as well as his operating practices,and so were not possible to evaluate in this study.

3.0~'~THF3. 1 Problem Identification

Over 1,500 Federal Aviation Agency Malfunction or Defect. reports (FAA M or D) cover-ing the turbine-engine-powered civil helicopter fleet for the years 1971 through 1976 werereviewed to determine the subsystems and components responsible for the civil helicopterreliability problem. The distribution of reported malfunctions by subsystem is given inFigureS.

The frequency of malfunction of each component was taken as a standard for problemidentification. Major problems were determined in three ways:

• Problems with the highest reported frequency of occurrence.

• Problems requiring the highest on-aircraft unscheduled manhours to repair or replaceassociated components. The reliability impact was determined by the magnitude of theproduct of the manhours per problem occurrence and the frequency of problem occurrence.

• Problems with the highest costs of repair materials and replacement components. Thereliability impact was determined by the product of cost per problem occurrence andthe frequency of problem occurrence.

The 20 most frequently reported problems were identified for further analysis; Figure4 shows the relative failure rate of each of these problems.

These 20 problems were reported in over 70 percent of the FAA M or D reports andthus any alleviation of these problems would have a large impact on improving reliability.

A second review was made to find the problems that required the largest expenditure ofunscheduled on-aircraft maintenance manhours. Figure 5 shows the relative impact of thetop 20 reliability problems where the product of unscheduled maintenance manhours perproblem occurrence and the frequency of problem occurrence is used as a standard forevaluation. Seven of these problems were not previously identified on the 20 most frequent-ly reported list.

A third review of problems was made to determine those problems having the greatestimpact on reliability when the cost of repair materials and replacement components was con-sidered. Figure-6 shows the relative impact of the top reliability problems when the productof the cost of repair materials and replacement components per problem occurrence and thefrequency of problem occurrence is used as a standard for evaluation. As might be expected,several'pfoblems with a relatively low failure rate had a large" impact' on reliability repair costand vice versa. Again, several new problems were identified for a major reliability problemlist.

HYDRAULICS

INSTRUMENTATION/AVIONICSELECTRICALFLIGHT CONTROLS

Figure 3. Distribution of reported reliability problems by helicopter subsystem

8

RELATIVE FAILURE RATE

FUEL CONTROL

AIR FRAME CRACKS

FLOAT LEAKS

FUEL GOVERNOR

TRANSMISSION BEARING SPALLS

ACTUATOR LEAKS

COMPRESSOR FAILURES

BLADE CRACKS AND CORROSION

TENSION-TORSION PACK FAILURES

TAIL ROTOR TRANSMISSION MOUNTS

TRANSMISSION HOUSING CRACKS

HANGER BEARINGS

FUEL PRESSURE SWITCH

SWASHPLATE SUPPORT CRACKS

FUEL PUMP

SMALL HARDWARE

SKIN UNBOND

ENGINE LUBE

FUEL PUMP SPLINE WEAR

FUEL CELL LEAKS

NOTE: AREA OF EACH PROBLEM BAR ISPROPORTIONAL TO PROBLEMMAGNITUDE.

DATA SOURCE: FAA M OR D RECORDS

. Figure 4. Top 20 most frequently reported reliability problems of civil helicopters

RELATIVE MANHOUR REQUIREMENT

Q

TRANSMISSIONBEARING SPALL

AIRFRAME CRACKS

FUEL CONTROL

COMPRESSOR FAILURES



FLOAT LEAKS-

FUEL GOVERNORS

GEAR SCUFFING/SPALLING

ACTUATOR/SERVO LEAKS

HUB BEARING FAILURE,

TAIL ROTOR TRANSMISSION MOUNT CRACKS

HUB/SWASHPLATE SUPPORT CRACKS

MAIN ROTOR MAST BEARING RETAINER NUT

TURBINE FAILURE ,

BLADE CORROSION AND CRACKS

TAIL ROTOR GRIP BRG,

MAIN ROTOR MAST CORROSION

HANGER BEARING FAILS

SWASHPLATE BEARING FAILS

NOTE: AREA OF EACH PROBLEM BAR IS PROPORTIONALTO PROBLEM MAGNITUDE.

Figure 5. Top 20 maintenance manhour problems of civil helicopters

10

f

COMPRESSORFAILURES ,

FUEL CONTROL

TRANSMISSION BEARING SPALL

TURBINE FAILURES



BLADE CRACKS AND CORROSION

FUEL GOVERNOR

GEAR SCUFFING AND SPALLING ,

HUB BEARING FAILURE,

MAIN ROTOR MAST RETAINER NUT,

HUB/SWASHPLATE CRACK;

TAIL ROTOR GRIP BEARING !

TRAIL ROTOR TRANSMISSION MOUNT

SWASHPLATE BEARING FAILS

NOTE: AREA OF EACH PROBLEM BAR IS PROPORTIONAL ,TO PROBLEM MAGNITUDE. \

Figure 6. Top 15 relative repair parts cost problems of civil helicopters

11

Thirty reliability problems were selected for further analysis by combining the lists ofhigh frequency of occurrence, high maintenance manhours, and high materials/componentscost problems. This combined list accounted for more than three-quarters of the reportedmalfunctions, as shown in Figure 7.

A breakdown of these 30 problems by subsystem is given in Table 2, which shows theproblems arranged within each subsystem in order of decreasing frequency of occurrence,decreasing unscheduled manhour requirements, and decreasing repair parts cost.

No one problem in the instrumentation, avionics, electrical, or flight controls had suf-ficient reported failures to be represented on this major problem list.

Figure 8 shows the reliability impact by subsystem of these 30 major problems based onfrequency-of reported-problem occurrence. Figure 9 shows the subsystem reliability impactbased on the unscheduled maintenance manhours associated with these problems. Figure 10shows the subsystem reliability impact based on the relative cost of repair materials and re-placement components associated with these problems.

In reviewing Figures 8, 9, and 10, the following conclusions can be drawn:

• The propulsion subsystem has the largest reliability impact of any subsystem whenmeasured, by.either the frequency of reported problem occurrence or the cost of repairparts standards and has the second highest impact in terms of the on-aircraft unscheduledmaintenance manhour requirement. This subsystem accounts for more than a third of

•• '• the reported malfunction occurrences, a quarter of the unscheduled maintenance man-hours, and two-thirds of the costs of repair materials and parts.

• The airframe subsystem, which had approximately one-fifth of the reported malfunctions,can be seen to have an almost negligible impact on the cost of repair materials and parts.However, the reported malfunctions of this subsystem cause 10 percent of the unschedul-ed maintenance manhours associated with the 30 major problems.

• The drive subsystem has the largest impact on unscheduled maintenance manhours ofany subsystem. The identified drive subsystem problems result in more than a third ofthe unscheduled maintenance manhours associated with the major problems. The driveis the second highest subsystem in terms of the relative cost of repair parts, resulting inover a fifth of the parts cost attributed to these major problems.

• The drive and rotor subsystems are nearly equal in reported failure rates, but the rotorsystem has approximately half the reliability impact of the drive in terms of unscheduledmaintenance manhours and associated cost of repair parts.

• The three subsystems, propulsion, drive, and rotor, constitute 98 percent of the reliabil-ity repair cost.

12

ALL REPORTED CIVIL HELICOPTER MALFUNCTIONS OR DEFECTS, 1971 THROUGH 1976

THIRTYRELIABILITY PROBLEMSANALYZED - 78%

RELIABILITY PROBLEMSNOT ANALYZED - 22%

Figure 7. Thirty analyzed reliability problems represent more than three-quartersof the malfunctions or defects reported to the FAA

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16

FUEL 1.1%

NOTE: MANHOURS BASED ON ON-AIRCRAFT REMOVAL,REPLACEMENT, OR REPAIR ASSOCIATED WITHMAJOR PROBLEMS

Figure 9. Major reliability problems of civil helicopters by relative unscheduledmaintenance manhours on subsystems

17

AIRFRAME \LANDING GEAR,FUEL !HYDRAULICS i

NOTE: COSTS BASED ON REPLACEMENT COMPONENTSAND REPAIR MATERIALS !

Figure 10. Major reliability problems of cavil helicopters by relative costsof repair parts for subsystems !

18

• . .The aggregate problems of the landing gear, hydraulic, and fuel subsystems which consti-tute nearly a fifth of the reported failures are nearly insignificant in terms of the relia-bility repair cost problem and are responsible for only a tenth of the associated majorproblem unscheduled maintenance manhours.

The subsystem reliability impact of these major problems is discussed in greater detailin the following section.

3.2 Subsystem Problem Discussion

This section gives a general discussion of the problems of each subsystem. A more de-tailed discussion of each problem on which this discussion is based is contained in the appendix.

3,2.1 Propulsion Subsystem. - The propulsion subsystem is the primary reliabilityproblem of the turbine-engine-powered civil helicopter operator.

As has been shown in Figures 8, 9, and 10, the propulsion subsystem has the largestreliability impact of any subsystem in terms of the frequency of reported malfunctions andthe associated cost of repair materials and replacement components and has the secondlargest impact on unscheduled on-aircraft maintenance manhours.

The major problems identified for this subsystem and the corresponding detailed techni-cal analysis reference number in the appendix are:

• Fuel control problems . Problem PI

• Fuel governor problem Problem P2

• Compressor failure Problem P3

• Engine and accessory lubrication system failures Problem P4

• Fuel pump spline wear Problem P5

• Turbine failure Problem P6

Figure 11 shows the contribution of each propulsion subsystem major problem to theentire subsystem reliability problem for each of the three reliability parameters of this study(relative frequency of occurrence, unscheduled maintenance manhours, and repair parts cost).In addition, the propulsion subsystem impact on the major problems of the entire civil heli-copter for each reliability parameter is given for reference.

Malfunctions of the fuel control and the fuel governor can be seen to be the two mostfrequently reported problems. Fuel control problems require the largest percentage of un-scheduled maintenance manhours, closely followed by compressor failures. Costs of repaircomponents for compressor failures are the dominant factor in the parts costs of the propulsionsubsystem, but repair costs associated with the fuel control, turbine failure, and fuel gover-nors are also significant as the propulsion subsystem costs of repairs are two-thirds thatgenerated by all civil helicopter major problems.

19,

- -The major propulsion system problems are solved by a remove-and-replace operation.The appropriate accessory or the basic engine is then returned to the manufacturer or overhaulfacility, since very few civil helicopter operators will do this type of specialized repair work.

Fuel controls and fuel governors are frequently removed and replaced as part of a diag-nostic technique. Often the unit is subsequently found to be fully functional when examinedby the overhaul facility. This may be an expensive means of troubleshooting as the costs ofdisassembly and examination of a functional unit can be very nearly as costly as examininga defective unit; however, this may be the most practical means of troubleshooting availableto the small helicopter operator.

The costs incurred by a compressor or turbine failure are those of the return of theengine to the manufacturer and the necessity for a major overhaul. The cost per malfunctionof this category of problem is clearly very high.

Engine and accessory lubrication system malfunctions and fuel pump spline wear prob-lems are each responsible for approximately 5 percent of the propulsion subsystem majorproblem failure rate but aggregate less than 1 percent of system repair cost. This results fromthe ease of diagnosis and correction of these problems relative to the balance of the propulsionsystem.

The causes"of the "propulsion subsystem problems are very poorly defined. As the civilhelicopter reliability problem is, to a large degree, a reflection of the propulsion subsystemproblems, it is important to determine the causes of malfunctions.

A rather comprehensive study of military aircraft turbine engine R and M problems(ref. 8)~gives some insight as to probable causes of malfunction of the civil helicopter pro-pulsion subsystem, as follows:

• Fuel controls. Observed failure modes were:— Contamination from the actuating media or If om the fuel itself causiny sticking

and binding of spools, leakage of valves, and clogged orifices. j

— Wear on moving or contacting elements.

— Fatigue failures of springs, bellows, and retention devices.

- Misadjustment, erroneous troubleshooting, etc.

• Compressors. FOD to the compressor vanes and blades was responsible for 27 percentof unscheduled engine removals.

• Turbine problems are principally with the nozzles as the result of thermal stresses,thermal weakening, and hot gas erosion.

A detailed study of civil helicopter turbine engine failure causes is in order in view ofthe magnitude of the propulsion subsystem reliability problem. For example, the civil

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helicopter FAA_M_or D data provides very little evidence of an FOD-related compressor failureproblem. This may be a true situation, or it may be due to the operator's inability to deter-mine FOD damage in the field. The review of military helicopter unscheduled engine removalcauses of reference 8 indicates that the magnitude of the compressor problem on militaryhelicopters is comparable to that of civil helicopters only if the FOD-related compressorproblems are included in the comparison. As such, it is inferred that the civil operator mayencounter FOD problems of a magnitude comparable to military users, although this issuedoes deserve additional research.

With regard to accessory/lube failures, again a comment on the civil helicopter data isrequired. Most of the failures included in this category are reported as lube system metalliccontamination probably related to oil-lubricated bearing failures, although it is impossible todetermine this without shop/overhaul data.

It is evident that the propulsion subsystem problem will not be alleviated unless thecorrect malfunction causes are known.

3.2.2 Airframe Subsystem. — The airframe subsystem ranks next to the powerplantsubsystem as a reliability problem from the standpoint of reported malfunctions, as wasshown in Figure 3.

Figures 8, 9, and 10 showed that, although the airframe subsystem accounts for nearly20 percent of the reported major problem malfunctions, the associated airframe unscheduledon-aircraft maintenance manhours account for only 10 percent of the major problem man-hour rate and the associated airframe cost of repair materials or components for these prob-lems is negligible.

The major problems identified for this subsystem and the corresponding detail technicalanalysis reference number of the appendix are:

• Airframe skin and structure cracking Problem Al

• Small hardware loss or failure Problem A2

• Skin unbending Problem A3

Figure 12 shows the contribution of each of the three airframe subsystem major prob-lems to the entire subsystem reliability problem for each of the three reliability parameters ofthis study~(relative frequency of occurrence, unscheduled maintenance manhours, and cost ofrepair parts). In addition, the airframe subsystem impact on the major problems of the entirecivil helicopter is given for each reliability parameter for reference.

The dominating reported problem of the airframe subsystem, as shown in Figure 12, isskin and structure cracking. This problem accounts for more than 80 percent of the airframereliability problems, whether measured against the frequency of problem occurrence, un-scheduled maintenance manhours, or cost of repair parameters.

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Skin,and.structure cracking has three principle causes:

• High localized vibration

• Load concentration at attachments of skin and structure

• Maintenance-oriented damage which results in a load concentration.

Small hardware and honeycomb panel skin unbending problems also have maintenance-oriented damage as a prime causal factor. Rough handling of the small hardware -hinges,latches, and attaching rivets or screws — will fail or deform parts while the skin of metal honey-comb panels is very vulnerable to puncture loads. Helicopter vibration is a significant partof the small-hardware problem, as well as improper design or manufacturing consideration forthe tolerance variations of a fastener installation. Metal honeycomb panels have had a historyof unbending problems which have been traced to improper processing procedures, as well asinternal corrosive attack caused by improper selection of bonding materials or improperlysealed panels.

Surprisingly, the FAA M or D airframe data shows little evidence of corrosion as a prob-lem, although information from civil operations led the investigators to believe that cor-rosion is a serious problem. A report on the Petroleum Helicopters, Inc., maintenancepractices (ref. 9) discusses corrosion as a significant problem and states that it is the PHIpractice "to get new equipment unpainted and then we do the job ourselves based onexperience." .

A~discussion of the airframe reliability problem is in order at this time. Airframe prob-lems tend to be rather easily detectable well before they become flight safety hazards. Re-pairs are relatively simple and rapid and have very low associated costs of repair materials.This has two results:

• Malfunctions such as fastener replacement, minor sheetmetal cracking, and corrosion canbe repaired before they progress to the hazardous condition requiring an FAA M or Dreport. Thus the reported airframe malfunction rate based on the FAA M or D reportsmay not be as representative of the civil helicopter airframe reliability problem as arethe dynamic systems, propulsion, drive, and rotor, in which a failure has a more directassociation with flight safety and has a greater probability of being reported on the FAAM or D report.

• The cost of repair of failure is low compared to a system involving removal and replace-ment of an expensive component such as the powerplant or drive. This is the reasonfor the low repair cost of the airframe subsystem relative to other major subsystems

"<•••. (<.!%),:as-previously shown in Figure 8. . ... •

In summary, the airframe failure occurrences are believed to occur at a higher rate than~ --:.. ~ reported on FAA M or D reports simply because of the nonsafety-critical nature of many0 -, malfunctions, and this ratio of malfunctions occurring to malfunctions reported is higher than

other dynamic systems. The associated maintenance manhours and costs of repair materials

. : - . . . - , . 24"i

would also be proportionally more than that shown in this report. Further study of airframereliability would be in order to prove or disprove this assumption, since airframe malfunctionrates on military helicopters with less safety-critical definitions of failure than the FAA M orD report have been reported at over 30 percent of the aircraft rate (ref. 10).

3.2.3 Drive Subsystem. — In terms of total failure rate, the drive subsystem ranks thirdbehind propulsion and airframe as a major civil helicopter problem. However, drive subsystemfailures rank first in terms of the requirement for on-aircraft unscheduled maintenance man-hours and second to the propulsion subsystem in terms of cost of repair components andmaterials associated with the failures.

The severest impact of a drive subsystem failure is caused by an internal failure of themain transmission. To remove a transmission it is necessary (on nearly every helicopter) toremove the rotor hub, blades, and rotor controls. After reassembly, the blade track must berechecked. Thus, there is a large manhour impact of a transmission malfunction.

The transmission overhaul is sufficiently complex and time-consuming that a small civiloperator will use a remove-and-replace technique for an internal malfunction, substitutinganother transmission while the defective unit is being repaired. This accounts for the highcost of repair components.

The major problems identified for this subsystem and the corresponding detailed techni-cal analysis reference number of the appendix are:

• Transmission bearing spalling Problem D1

• Tail rotor transmission mount cracking Problem D2

• Transmission housing cracking Problem D3

• Hanger bearing failure Problem D4

• Gear scuffing and spalling Problem D5

• Drive shaft spline wear Problem D6

Figure 13 shows the contribution of each of the above major problems to the drive sub-system reliability problem for each of the three reliability parameters of this study (relativefrequency of occurrence, unscheduled maintenance manhours, and cost of repair parts). Inaddition, the drive subsystem impact on the major problems of the entire civil helicopter isgiven for each reliability parameter for reference.

The dominating drive subsystem major problem is transmission bearing spalling. Thisproblem accounts for over 40 percent of the drive subsystem reported malfunctions, more than50 percent of the on-aircraft unscheduled manhours associated with drive subsystem reportedmalfunctions, and over 60 percent of the costs of drive subsystem repair components.

The .two other major drive subsystem problems requiring transmission removal andreplacement are cracking of the housing and scuffing or spalling of gearing. These problems,

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_when added_tq die bearing spall problem, account for 65 percent of drive system reportedmalfunctions, nearly 90 percent of the maintenance manhours, and 96 percent of the as-sociated repair costs. Clearly, main transmission reliability is a major driving factor in goodhelicopter reliability.

Investigation showed that bearing spalling is a major problem in the civil helicopter fleetonly because of one bearing installation in one model of helicopter and is not a major problemon all models. Cracking of the housing was found to be a similar problem, primarily occurringat one location on one model. These two problems appear to be typical of the state of the artof helicopter transmission design, where the emphasis is on very high power transmittal withvery low weight and good efficiencies. An unanticipated load or condition may result in acomponent malfunction and the correction of a condition of this type will often be technicallyfeasible but require a major component redesign.

Bearing spalling and gear scuffing and spalling are normal forms of deterioration of heli-copter transmissions, as is shown by Table 3 which displays detailed military helicopter trans-mission failure modes. Bearing problems account for 45 percent of the failures, with pittingand spalling accounting for nearly half of the bearing problems. Gear deterioration is respon-sible fo_r 23 percent of the.failures and pitting/scuffing/spall/wear is the largest contributingfailure mode. This deterioration is to be expected; however, a condition that exists primarilyat one location as reported in the civil helicopter FAA M or D data is a reflection of a detaildesign problem.

Of the remaining three high-frequency-of-occurrence drive system problems, two werealso found to be the result of detail design execution; the third reflects the state of the art.

The tail rotor transmission mount failure is predominantly a one-model-helicopter prob-lem. Hanger bearing failure is also reported as a major problem because of the failure rateoccurring with one configuration of tail rotor drive shaft and supporting bearings.

The spline wear problem has been a typical drive system problem and becomes moreprevalent when relubrication is difficult and the spline is forced to. accommodate misalign-ment between the coupled members. An improvement in the state of the art of splines,flexible couplings, and lubricants would be of benefit to future helicopter drive systems.

3.2;4 Rotor Subsystem. - The rotor subsystem ranks a close fourth to the drive sub-system in .terms of reported failure rate for the major problems. The rotor major problemfrequency of occurrence is 12 percent of all aircraft major problems. The associated man-hours tb\epair rank the rotor as the third highest subsystem at 20 percent of the total; thecost ofTepair components is also third at 11 percent of total parts cost. Furthermore, althoughnot quantitatively apparent from the civil helicopter data, troubleshooting rotor-system-related vibrations (1 per rev, n per rev) is a major erroneous removal and excessive aircraftdowntime problem of military helicopters.

27'

TABLE 3. CHARACTERISTICS OF THE MILITARY DRIVE SYSTEM

ComponentElement Distribution

Failure Mode (%)

Drive System

Transmissions

BearingsPitted/spalledCorrosionFracture/broken cagesSeizure (no lube) 45>21

Worn races and rollingelements ''

19.1721.50

0.910.176

fc 3.45

GearsPit/scuff/spall/wearCorrosionMount flange crackTeeth fracture 23-23

FrettingSpline wear

10.635.720.445.640.440.40

Lube System(Coolers/fanSjlubricators)CorrosionLeakingWear 3.24 •Broken studs, lines, etcSeizure of pump/fan

0.110.521.41.10.11

Retention and Mounting HdwPitted/brokenCorrosion 16.00 *Wear

7.10.588.318

Nonrotating StructureCorrosionWearBroken/chipped 5'08

Cracked

1.811.7140.750.81

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(hr)

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TABLE 3 - Continued

ComponentElement Distribution

Failure Mode (%)

Clutches ;.CorrosionSeizureWearSpline wear 3.25Broken clipsCracked

' .0.33' 0.002; 1.5

0.480.790.15

.. . i.Shafts (quill and gear)

Pitted/spalled splinesCorrosionFatigue crackingWorn splines '

0.750.490.202.55

Shafting

Sync Shafts/T/R ShaftsCrackedMisalignedDented 6

Scored

2.02.01.01.0

Hanger Assembly;Bearing wear/spall f 24.0Leaking 36 -1 8.0Lube starved I 4.0

Mounts fBroken 1 22.0Cracked 25 | 3.0

AdaptersLug failures f 3.0Improper alignment 16 "j 10.0Spline wear I 3.0

CouplingsBroken/crackedBolt failures 17BentWorn

12.01.02.02.0

Transmission data from USAAMRDL TR73-58Shafting data from Boeing document D21 0-1 0846-1

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1050010025

20020020

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The subject of fault-isolating the source of unusual vibration is addressed in the mainte-nance report (ref. 11) generated by this study.

The major problems identified for this subsystem and the corresponding detailed techni-cal analysis reference numbers of the appendix are:

• Blade cracks and corrosion Problem Rl

• Tension-torsion assembly failure Problem R2

• Swashplate support cracking Problem R3

• Rotpr_hub bearing failure Problem R4

• Main rotor retention nut failure Problem R5

• Tail rotor grip bearing failure Problem R6

• Swashplate bearing failure Problem R7

• Main rotor mast corrosion Problem R8

• Rotor hub seal leaks Problem R9

• Rotor hub corrosion Problem RIOU—

Figure 14 shows the contribution of each of the above major problems to the rotorsubsystem reliability problem for each of the three reliability parameters of this study (rela-tive frequency of occurrence, unscheduled maintenance manhours, and cost of repair parts).In addition, the rotor subsystem impact on the major problems of the entire civil helicopteris given for reference.

Cracking and corrosion of blades and failures of the tension-torsion assembly constitutethe two highest ranking rotor subsystem problems in terms of relative failure frequency andthe cost of repair/replace components. The tension- i orsion assembly problem as the highestunscheduled maintenance manhour requirement of the rotor subsystem.

The remaining problems individually are of relatively low failure rate, associated main-tenance manhours, and cost of repair parts but in aggregate constitute a significant problem.As discussed in more detail in the individual writeups, these problems are primarily due tothe state of the art in design execution for an area exposed to loads of a variable magnitudeand vibratory nature, with sand, dust, and rain environment, and subject to frequent mainte-nance operations which increase the chance of Murphy-type errors.

3.2.5 Landing Gear Subsystem. - There is a great difference in the landing gear prob-lems encountered by civil and military helicopter operators. A comparison of failure modesexperienced by the two operators is shown below:

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• Popout/fixed flotation gear leakage - Wheel bearings rough, broken"" t J

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• Broken skid - Strut worn, leaks

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Float leakage is the only landing gear problem on the civil helicopter major reliabilityproblem list but is the third largest reliability problem in terms of the frequency of reportedfailures for the period monitored, 1971 through 1976.

The extensive use of flotation gear by helicopters performing offshore oil-drilling sup-port raised this problem to a magnitude unparalleled by military landing gear problems.Simultaneously, the civil operators' general preference for skid-type gear rather than wheels(especially for oil platform operation) negated the major military problems related to wheels,brakes, and master cylinders. The impact of this problem on the civil helicopter reliabilityproblem in terms of the three reliability parameters of this study (relative frequency of oc-currence, unscheduled maintenance manhours, and the cost of repair parts) is given in Figure15. This problem-is discussed in more detail in the appendix, reference number LI.

Further investigation of the float leakage problem showed that an aggressive productimprovement program had been conducted by a major float manufacturer midway throughthe reporting period in response to the magnitude of the reported problem. After these .changes had been implemented, the reported failure rate dropped by a factor of 10 to 1; withthe lower malfunction rate, float leakage was no longer on the major reliability problem' list.

This was not apparent from the data review until a year-by-year study of reported failureswas made. When this was done, a rather dramatic drop in reported failures could be observedin the period following the introduction of the float improvements.

Following examination of the FAA M or D data and the reported failure modes, a pre-liminary report of this problem was discussed with a representative of the Air CruisersCompany, Division of the Garrett Corporation, a major float manufacturer. In brief, the AirCruisers Company stated that they had been alerted by the Helicopter Association of America ,and two helicopter manufacturers of reported problems. They took the following steps:

— Identified the problem modes and causes by:

• Discussion with the helicopter manufacturers.

• Discussion with the float-using operators.

• Review of the FAA M or D reports submitted by the HAA.

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— Instituted, an immediate short-range program to reduce problems, primarily to assurethe proper inspection, maintenance, and operational procedures since floats are ex-tremely sensitive to any deviations from anticipated design conditions in these areas.This is particularly true in terms of inflation practices and prevention of chafing andabrasion.

— Instituted a longer-term program to increase float performance in the present operationalenvironment. This includes the following:

• -Increased thickness of coatings on the float fabric.

• Reinforcement to wear areas.

• -Testing and making available to operators a top dressing to improve weather resistance.

• Reviewed and clarified maintenance manuals which define proper care of floats.

• An in-house review of manufacturing practices to assure that this was not contributingto the reported failures.

• Other detailed investigations and changes in the areas of fungus resistance, adhesiveand solvent compatibility of float materials, and structural improvement.

That this program was effective is indicated by the HAA records of FAA M or D reportswhich show, on one model helicopter, 105 float reports submitted in the 2-year period 1973and 1974, and only 5 in 1975.

It is significant that the original floats were reported to meet the helicopter manufacturers'specifications in all respects. The floats incorporating the long-range improvements carried aweight penalty of approximately 20 percent in achieving the improved reliability.

The history gives an indication of the reliability improvement possible in some areas ifproblems are vigorously attacked and realistic weight/reliability trades are adopted.

3.2.6 Hydraulics Subsystem. - In the military environment, line and hose leaks dominatethe hydraulic system failure rate. This has led the industry to design helicopters with modularhydraulic systems wherein the number of potential leak points is greatly reduced. Line/hoseleaks are also a problem to the civil operator, but not to as great an extent as the military.

Flight control actuator/servo leakage is the only hydraulic subsystem problem in thecivil helicopter major reliability problem list. Figure 16 shows the impact of the hydraulicsubsystem, and thus this problem, on the major problems of the civil helicopter in terms ofthe three reliability parameters of this study (relative frequency of occurrence, unscheduledmaintenance manhours, and cost of repair parts). This problem accounts for approximately4 percent of the reported failures, less than 3 percent of the associated manhours for an on-aircraft repair, and approximately one-tenth of one percent of the cost of repair/replacementcomponents. The problem is fundamentally one of seal technology (ref. 12). It appears that

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the reported problem is to a large degree solvable by existing technology, as the frequency ofproblem occurrence on one model of helicopter has driven this problem to the major reliabilityproblem list. The problem is discussed in greater detail in the appendix, reference number HI.

3.2.7 Fuel Subsystem. — The fact that three fuel subsystem problems are members ofthe top reliability problem list is of some surprise to a reliability engineer accustomed to re-viewing military helicopter data. These three problems and the corresponding detailed techni-cal analysis reference number of the appendix are: i

• Fuel pressure switch problems Problem Fl

• Fuel pump failure Problem F2

• Fuel cell leakage Problem F3

The fuel subsystem major problems account for 5 percent ofthe civil helicopter reliabilityproblem in terms of the frequency of occurrence of a problem, but account for only 1 percentof the unscheduled maintenance manhours and less than one-half percent of the cost of repaircomponents associated with these failures.

Figure 17 shows the contribution of each of these three problems to the entire fuel sub-system problem in terms of each of the three reliability parameters of this study (relativefrequency of occurrence, unscheduled maintenance manhours, and the cost of repair parts).In addition, the fuel subsystem impact on the major helicopter reliability problems is givenfor each parameter for reference.

Failure of fuel pressure switches is the most frequently occurring problem and responsiblefor the largest maintenance manhour expenditure on this subsystem. Fuel cell leakages causethe largest impact on repair parts costs. Fuel pump problems have the second highest frequencyof malfunction and are second highest in terms of maintenance manhours.

The solution of all fuel system problems would benefit from a better definition of failuremodes and causal factors. A review of problems to date has indicated that all three problemscan be alleviated by better application of current technology:

• Fuel pressure switch failures are partially due to the lack of design consideration for thehelicopter maintenance and vibratory environment.

• The fuel boost pump failures are believed to be predominantly direct-current-poweredunits; Boleirig Vertol experience has been that the newer ac-powered motor pumps havean appreciably improved reliability.

• Fuel cell "problems can be alleviated by better design for the aircraft maintenance andoperational environment.

More detail on these problems is given in the detailed technical discussion in the appendix.

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5.2.8JRemaining Systems. - None of the flight control, instrument, or electrical subsys-tem problems were of a sufficient magnitude to be members of the civil helicopter top re-liability and maintenance cost problem lists. These systems represent only 5 percent of thetotal problems and are nearly equal in occurrence. Some of the more significant problems inthese subsystems are listed below.

Instrumentation/Avionics (1-2/3% of total)

Engine gas temperature indicator (32% of this subsystem's problems)Bulbs (29% of this subsystem's problems)Transducers (23% of this subsystem's problems)Tachometers (16% of this subsystem's problems)

1 Flight Controls (1-2/3% of total)

Tail rotor chain (42% of this subsystem's problems)Binding or loose linkage (39% of this subsystem's problems)

Electrical (i-2/3% of total)

Generator (40% of this subsystem's problems)Battery (charge and overheating) (33% of this subsystem's problems)Anticollision light (13% of this subsystem's problems)

All three of these systems are significant contributors to the mission abort and trouble-shooting problems associated with helicopters. However, the quantification of these problemsfrom the FAA M or D reports is impossible as the abort and troubleshooting data is not in-cluded. The problem of mission aborts is discussed in the following section.

3.3 Mission Reliability

The FAA M or D report provides no information with regard to mission reliability, andso neither the magnitude nor the source of the mission abort problem could be determinedfor the civil helicopter fleet.

Data on mission aborts for the OH-58 helicopter is available (ref. 7). This is the militarycounterpart of one of the most widely used civil helicopters, the Bell Model 206. The magni-tude and source of the problem experienced by the military are an indication of the impor-tance of mission reliability to civil helicopter fleets.

Table 4 shows the magnitude of the military helicopter abort problem. Only three ofevery tO.OOO flights are not completed because of a hardware-oriented problem.

Table 5 gives the source (responsible subsystem or component) of the mission aborts andalso shows the comparative hazard of the failure in terms of a precautionary landing incident,forced landing, or major or minor accident. The dominant source of mission aborts is warninglights. This subject requires additional research to determine if the civil operator has this

38

problem. The second major source of mission aborts is engine failures. Engine failures havethe highest percentage of hazardous aborts, resulting in one-third of the major accidents, over40 percent of the incidents, and nearly three-quarters of the precuationary landings.

Figure 18 displays the causes of engine aborts for two turbine engines used in fixed-wingapplications. The dominant mode of failure is (as it is for civil helicopter engines) fuel controland governing devices.

Reference 7 states that 13 percent of mission aborts are caused by false indications ofthe warning light system. However, three quarters of the aborts which .resulted in a major orminor accident, forced landing, or incident originated from a malfunction of one of the majorproblem (area subsystems covered in this report. It is believed by the authors that the entiresubject of civil helicopter mission reliability requires additional research. The recommendedproduct improvements and research and development of this report will, however, have asubstantial impact in reducing civil helicopter mission aborts.

TABLE 4. FLIGHT SUCCESS OF THE OH-58A HELICOPTER,AUGUST 1970 THROUGH JUNE 1972

Statistic

Number of Flights

Average Flight Duration(minutes) i

Number of Flight Aborts

Flight Success (percent) '

Accrued Flight Hours

MTB Flight Aborts(flight hours)

NOTES:

Location .WithinCONUS1

681,572 .

16.037

188

99.973

182,173

969.005

OutsideCONUS1

1,000,275

21.023

318

99.968

, 350,487

1,102.16

Totals

1,681,847

19.003

506

99.970

532,660

1,052.668

1. CONUS = Continental United States2. Number of flights, number of landings, accrued

hours reported on DA Form 1352

3. Flight duration (minutes) -

flight hours, and total flight

accrued flight hoursno. of flights

no. of flight aborts4. Fhyh 1 success % - ( 1 - no of mghts ) x5. From reference 7

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Figure 18. Distribution of engine subsystem reliability problemson U.S. Navy aircraft

41

. . . . - . .

The major subsystem reliability problems were reviewed to see which could be alleviatedby application of present technology and which required additional research and developmentfor any substantial reliability improvement. Table 6 gives an estimate of the percentage ofthe reliability problems of each subsystem that require each of the two categories of action.As shown in Table 6, solution of propulsion subsystem reliability problems is highly dependenton additional R&D, while the other major subsystems - airframe, drive, and rotor — have aslightly higher percentage of problems that are solvable by product-improvement-type changesthan those requiring additional R&D for solution. The landing gear, fuel, and to a lesser degree,the hydraulic subsystem problems are largely solvable with product-improvement-type changes.

Table 7 gives a brief summary of the failure modes and the nature of the recommendedaction to improve the reliability for each major problem, either in terms of a product-improve-ment-type change or the recommended R&D program. The backup discussion of each problemis contained in the problem writeup in the appendix.

As can be seen from the detailed problem writeups and the recommended action, thisstudy defined product-improvement-type changes using present technology as having a greatpotential for alleviating the civil helicopter reliability problem. The difficulties of implement-ing these changes should not be underemphasized. Solution of some problems requires ex-tensive Redesign which affects other interfacing components. Requalification of componentsis needed, which is both expensive and time-consuming. Component weight and cost may beadversely affected. Service manuals, spares costs, and similar factors may add up to such ahigh price that retrofit incorporation of a change just for increased reliability may not be cost-effective in the existing civil helicopter fleet. It may well be the subject of a study to determinehow to establish incentives for incorporation of product-improvement reliability changes;perhaps a reliability warranty program could be examined.

42

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1 _^0 INCLUDINGj*EM ARKSg1. r

Table 8 is a summary of the research and development recommended for increased reli-ability, including an estimation of the impact on size, configuration, and mission applicability.All items considered are listed, even those having a low priority and/or low payoff. Estimated

. nonrecurring costs and timing are also included in Table 8.

; Table 8 gives an assessment of the priority and payoff for each recommended R&D pro-1 gram as well as the applicability to each size helicopter in the civil fleet. As is shown, it is; recommended that a high priority should be directed at R&D programs to improve the pro-pulsion subsystem reliability. This subsystem has the largest frequency of problem occurrence,

; the largest costs of associated repairs, and requires substantial on-aircraft unscheduled main-tenance manhours to correct reliability problems. The payoff from propulsion subsystem R&Dis expected to be high. Initial studies are directed at detailed problem identification.

The following areas of research defined in Table 8 are of benefit to more than onesubsystem:

• Vibration and load reduction are of benefit to the airframe, rotor, and drive subsystemas many of these subsystem problems are known to be related to high vibration levels orunanticipated loads.

• Spline lubrication improvements should alleviate propulsion subsystem problems withfuel splines and drive subsystem problems with drive shaft splines.

• Improvements in filtration will benefit the engine lubrication system and will also giveimproved seal life in the hydraulic actuators.

Airframe subsystem problems originate from vibration and also from inability to with-stand maintenance and operational loads. The requirements of the user's real environment needa better definition. Use of composite structures will alleviate many airframe skin and structureproblems, but a development program for compatible hardware is suggested to avoid introduc-tion of new problems.

Drive subsystem materials and lubricant development programs are recommended toimprove the overall reliability level of the next generation of helicopters (ref. 1).

i Many rotor problems will be alleviated if further development of composite and advancedcomposites structures results in the widespread use of bearingless main and tail rotor hubs.Corrosion-resistant, easily maintained blades with judicious use of advanced composites will

. give optimum structural characteristics while keeping the reliability gains associated with thecomposite blade. At the same time development of design parameters for oscillating seals andlubrication-free bearings such as elastomeric and TEFLON fabric is recommended.

-Development of improved coatings for metallic dynamic system components is recom-mended'to protect components from corrosion and maintenance damage.

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Materials jind^structures developments in float materials and for fuel cells will improvethe damage tolerance without the adverse weight impact now associated with chafe- andpuncture-resistant fabric structure.

A final recommended program is for the development of improved actuator seals, asflight control actuator leakage is a generic problem that has been increasing in magnitude asthe civil helicopter size has been increasing and the use of power- and stability-assisted flightcontrols becomes more widespread.

The estimated costs and elapsed time for each R&D program have been given in Table 8.Additional funding will be required for follow-on action after completion of the study pro-grams such as the three propulsion subsystem studies to determine the detailed failure modesand problem causal factors. It is anticipated that the studies will define any additionalrequirements.

All program costs shown in Table 8 are not additive as several problems call out the sameresearch program as a solution and the entire program cost was repeated for each problementry. No attempt was made to allocate the costs of a program among the several problemsthat it would alleviate.

This study has shown that a small number of reliability problems have a major impact onunscheduled maintenance manhours and repair parts cost. The propulsion, drive, and rotorsubsystems represent over 80 percent of these manhours and parts cost and additionally havea substantial impact on safety and mission reliability (aborts). Therefore the highest priorityefforts must be directed at improving reliability of these subsystems. The airframe subsystemcontributes significantly to unreliability because of a large number of failures and maintenancemanhours expended.

The research and development identified herein will have a long-term impact on reliability.

The importance of aggressive product-improvement programs using known technologymust be emphasized because these will have a more immediate impact on new helicopters nowbeing developed for the civil market.

60

APPENDIX

CIVIL HELICOPTER RELIABILITY PROBLEMS

Included in this appehdijfia^eVdetailed technical discussions'of the individual civil helicop-ter reliability-problems-for: -—-

1. The top 20 most:frequently reported problems,

2. The top 20 maintenance: mahhour problems,

3. The-top-iJTrepair-parts-cost problems:

For convenience an-_ j__ ---——t ——

Component/Subsystem

index of the problems is listed by subsystem.

Reference Number

Propulsion SubsystemFuel Control;- :Fuel Governpr....._Compressor FailureEngine LubricationFuel PulnFSpTinTWearTurbine Failures

Structure Cracks';or;

'Airframe Subsystem""T"Air frame Skin andSmallllardware. LossSkin iUnbonding

Drive Subsystem:Transmission Bearing SpallihgTail Rotor Transmission Mount CracksTransmission'Housi^g'CrackingHanger Bearing. Failure .Gear Scuffing and SpallingDrive Shaft Spline Wear ;

Rotor Subsysjernh___ i. _ :

Blade Cracks; and Corrosion :Tension-Torsion rAssismbly Failure ;Swashplate^Support-Gracking- '—Rotor Hub Bearing Failure -Main Rotor RetentionTail Rotor Grip Bearing FailureSwashplate Bearing Failure T"

61!

PIP2P3P4P5P6

Al,A2A3

DlD2D3D4D5D6

RlR2

-R3R4R5R6

~R7"

f c - S L A C KIC *-. DDEO

.Component/Subsystem

Main'Rotor Mast CorrosionRotor Hub Seal LeaksRotor Hub Corrosion

Landing Gear Subsystem ;Float; Leaks ; j

Hydraulic jSubsystem ___i__^; :: ;;_Flight Control Actuator Leaks

Fuel Subsystem^;Fuel Pressure Switch] ProblemsFuel Pump Failure7^ l:.Fuel bell-Leakage—r

Reference Number

R8R9RIO

LI

HI

FlF2F3

-'D' ZZ • :•-•• f".

62

.--. FUEL CONTROL

.' ' . : • ; • ' . ' . Problem

The~engines~widely used;in'civil-helicopters-nave-a gas generator-turbine-compressor unitand a power tiirbine.^ThekueLcontJol portion of the. fuel system providesi the fuel manage-ment during engine sitartirig arid up to the; flight range of the power turbine, approximately 85to 100 percent of the normal'flight;rpm. ,In the upper speed range, the governor controls thespeed. Common malfunctions of -the fuel control are improperstarting and fluctuations ofspeed in the-loweFiranges:-: T-hereis-little-.corrective action .possiblelor fuel controls in thefield, so the most cpmmorj correction of a malfunctioning fuel control is to remove and re-place the unit. \ ' ' • :

Often a unit is removed as part of a troubleshooting operation and subsequent test andwith no defect.

A study of the turbine engine reliability problems of Army helicopters (ref. 8) reportsthe following"failureTfiOdfsof fuel contrpi units: ~

''"'. ' • ; '. \ • I ' ' :,. : • 1 \ •

>_:',; • Conj;aminaônjfrpm|âctuatingmedia or from the fuel iteelf causing sticking andbinding of spook, leakage of valves, and clogged orifices.

• Wear on moving or contacting elements.

• Fatigue failure of:springs, bellows, and retention devices. ,

• Misacijustment", errorieoM^txpubleshboting, etc. (estas50l%). : • \- -

Problem Solution

Present TechnologyT—The-problems are not defined in detail in the civil helicopter database. The?prime engine manufacturer does not usually perform the detailed design, manufac-ture, or overhaul of the fuel control and seldom has detailed awareness of failure mode aridfrequenc^yJoilhis issembiyV The> area "needs considerable study to define the problem prior toestablishing a:solution. ^Records do indicate substantial differences between failure rates onthe fuelôntrols-6f-different-modelJielicbpters, indicating some technology is present for al-leviating the prpblern. _ : _';_

' . - . " ; . . ' : ' : . : : . . i .

' ' ' • A fleeTevaluôirplôgfamloflKeiro^ fuel controlswith similar units~protecfed by better filtration is recommended. This will document the

• magnitude of the possible reliability improvement and cost, weight, and complexity penalties.'. ' ' • ' of unproy.ed filtration.

63

_ItJsLalso recommended that a joint airframe/engine/controj manufacturer effort deter-mine causes and make recommendations for correcting fuel control problems.

Advanced Technology; —-This item-is one of the major erroneous-removal problems en-countered in turbine helicopters. .•As-many^as 65 percent of the military helicopter fuel controlsremoved in the field exhibit no defect when tested in the shop. This problem appears to beinstallatiori/interface-related and is a candidate for additional research prior to complete prob-lem resolution^ T! •": ;;

diagnostic/inspection; technique is required so that erroneous removals can bebe considered is the development of a field-level GSE item for

A field diagnostii; minimized! One option tooff-aircraft ftiercontrol~checfcout

•54;

CIVIL HELICOPTER RELIABILITY PROBLEM"P2

; ' FUEL GOVERNOR

Problem

The'functiomof"the-governor is-to'maintain nearly constant power turbine rpm in responseto wide variations-in power-demand by-the pilot's collective setting, while the turbine speed isin the 9(> to 10ftpercent:^ge. rGprnmon symptoms of a malfunctioning governor are speedvariations, torque: arid turbine: temperature variations, and a drop in rpm occurring with ademand for power?'The slplutioWfoFa malfunctioning governor is to replace the component.Very Uttle lorâireraft-m^ of atroubleshoioting operation and[;subsequ_enit tes.t and disassembly show the unit is fully functionalwith no defects. ; ' . \ " . ' . ' . ' . - " : " • • "

' I '.• '• \ ' • .. 'The governor of a twin-turbine engine helicopter has more demanding requirements. The

engine torques-and-turbmeJnlet-temperatures-should be matched throughout the flight range,and it is apparent to the pilot if the governor actions result in a variance.

" •'" \ ' ' P rob lem Solutioni . \ . I

^ên^^^nojogy.J-Âswith the fuel control system, the problemsare not determinablein detail. The prime engine manufacturer does not perform the detailed design, manufacture,or overhauToTthe fuefgovernor and seldom has a detailed awareness of failure modes. Thearea-needs)additional-study to define the problem prior to establishing a solution. It is recom-mended that.a joint airframe/engihe/goverrior manufacturer effort determine causes and makerecommendations :for £orrectiag_fuel^governor problems.

Advanced Technology. - As with the fuel control, a large percentage of the fuel governorremovals has-no defeet-when-checked at- the factory/overhaul facility. This item requires a fielddiagnostic/inspection technique so that erroneous removals can be minimized. One option tobe considered is the develppment of a field-level GSE item for off-aircraft fuel governorcheckout!

.:65

1 2 INCH -i=?'. C ADDED"> A T IN A

_ CIVIL HELICOPTER RELIABILITY PROBLEM P5; COMPRESSOR FAILURES

; . Problem

Civil heHcopter-reUability-data-lists only "failure" as the principle malfunction of thecompressor.^VirtuaUy;eyery intCTjiai.engine failure is:cause for removal; therefore, to anoperator failure description: is of little importance. Also, the real failure causes and modes canusually be determined only after disassembly so they are not known to the operator.

A study-of-m;any^of4he-same-engines used in-the civil fleet but installed in Army helicopters(ref. 8) reported the following as the major compressor failure modes:

• Corrosion/erosion-induced vane failures. . . . . . . . . . . n

• Blade/disk fatigue failures

• Diffuser cracking and leakage

• Compressor lining wear and cracking (this was a problem peculiar to one engine design)

• Variable stator and bleed problems (looseness and mechanism failure)

A tabulation :of the reasons for military helicopter unscheduled engine removals givenin the reference 8 report showed that a total of 41.6 percent of all engines removed was dueto compressor problems (compressor failures, FOD, and erosion, with FOD the predominantfailure cause). : _ [ _ _ :_

~ "" Problem Solution

Present Technology, — The; compressor failures must be examined in more detail to see ifthe civil helicopter problems are due to detail design execution or require an advancement intechnology. The problems may be solvable with existing technology once the details of thefailure modes are-determinedr- ItTS~possible that solution requires a means of establishing acost incentive to justify the corrective action.

If, for example, FOD is the predominant cause of failure, the cost of the present poorreliability and engine repair may outweigh the adverse cost and weight of inlet screens.

It is recommended .that.a. joint.airframe/engine manufacturer effort determine causes andmake recommendations for correcting compressor problems. To aid in this investigation, it isrecommended that the FAA M orT) report be revised to request that additional data on failurecauses be supplied when this form is submitted by the engine overhaul facility.

Advanced Technology. --Srnce-thecivil-helicopter data base does-not.contain depot tear-down/overhaul-results,-this-discussion-is-based exclusively on military helicopter experience.

The development of eroapn-resistant blade vane materials and corrosion-preventivecoatings, coupled with the development/application of effective air particle separators, isrequired to reduce the magnitude of this problem. The GE T700 engine has addressed all ofthese problems ami a study-of the effectiveness of"R&M improvements on the T700 is beingconducted by .GE for USAAMRD'L. This-may identify the areas of additional R&D required.

67

I «•' C. *v -i6,.v2! 3 71 > 'Et-i-U.^ i . '<'"« £s U A C K< 'I t r v t C H Crt ' l i

.£ •: '3

.CIVlUiELLCQPTER RELIABILITY. PROBLEM P4.. , . ,

ENGINE LUBRICATION SYSTEM MALFUNCTION

• . • : . ' • ! ' i . • . Problem

_ j

The" civil^eUcopter'enginelubrication-^system component failures and failure modes arenot well-defined. --Many lubriGation system failures jeopardize the engine and so the entireengine withitelubrication_pumps, filters,; and valvesjs sent Jp^ distribution center for reworkor checkout. ' . - . - . ' . ' ' • : • : ,

A study of military-nelicopter engine R&M-factors-(ref-.~8)-,- reporting on many of the sameengines used in civil helicopters, gave..two major categories of lubrication system malfunctions:

t ' ' ' ; : • ; . . ' . ' . : ' : .1. Pumps-and-associated-valving--

2. Coolers and filters • : " - " ' :

Tubing and fittings, leakage, and chafing made up the balance of the problem.; . - - • ; • i \

I Problem Solutioni ' " '" 7

!

Present-TeGhnalQgy.i-^-The-prQblem can be somewhat-alleviated -by additional filtrationto restrict any debris generated from contaminating the entire system and requiring the re-placement; of both engine: and helicopter-mounted components. Failure to remove the entiresystem has relwlted ifffesidual containinatioh causing secondary failures.

, A fieid_eyaliAation.pjbgrajmjjf Jh5_cpjrip.arative.reKabiUty,pfpxesent engine lubricationsystems with similar systems protected by better filtration is recommended. This will documentthe magnitude of the possible reliability improvement arid the cost, weight, and complexitypenalties'of improved^ffltratipn.

; , ... . . j. '_ . . ' .; ' ' ' ' "

It is^^ojvec^jnjnendejijthal^ ajoint ajrframeyfengine manufacturer effort be made to, determine' other causes arid make recomriiendations for correcting engine lubrication system

problems, i"

Advanced Technololgy. — Redundancy of the lubrication system may slightly increasethe frequency of rrialfuriction of these components, but the consequence of a failure in termsof safety ejslveil as iri unscheduled maintenance manhours and repair parts costs becomesrelatively benign.

681

P5

FUEL PUMP SPLINE WEAR

Problem

Wear pf'gas^iorbine^driyen; fuel-pumpi splines-has been a significant-problem for over adecade. The-spline-drive-is generally lubricated only at assembly- and operates at high speedand withjsome^misaUgimwjitjari^^ ;

i - - . . . . . . . • - . * - , . .

The motion of splineisurface¥:with conditions of marginal lubrication generates weardebris wWbh-acte~M-an~abrasive-td-generates^ environment,fuel contartnnatipn_may^ wasli away any grease. The emphasis .on.low weight results in highspline tooth-contact stresses and further aggravates the problem.

, - . - - : - I - - . - • • Problem Solving '

Present Technology. .-The problem can be alleviated with present technology but withan adverse! impact on weight and cost. Spline wear is reduced by using a slower-speed drive,keeping coirtact"stresseslow in t^e~splineiteeth, and minimizing misalignment both by configura-tion and by closer control of tolerances.

Design changes so that splines of this type can be relubricated without disassembly wouldgive considerable improvement. This is a product-improyement-type change rather than anR&D-program;--T— --> : - -

Advanced Technology. —' Design for longer spline life is an area in which R&D will showa payoff and suchia program to include materials, configuration, and lubricants is recommended.Considerable work is being done by the military organizations and any additional researchshould be interfaeed-with-their-effort;- A recent development of-the Naval Air Systems Com-mand, for example,. is_a:Rolyimid [sleeve between male and female spline portions which pre-vents metal-to-metail contact and accommodates misalignment.

69!

TURBINE FAILURES

. ' - . - _ ' . Problem

Civil ^heH^p'tiBTiÛabili^'daârlists"' -f ailure" as the principle H&M impacting malfunctionof the engihe>— Virtually; ever:yrinternal engine: failure is cause for removal, so to an operatorfailure description ;is of little importance. ______ _.'...'. _____ ____

The real failure causes i and modes can usually be determined only after disassembly, sothey are noHmown-tb-liie-operatpr; — -—' ---------- ....... ------ ---. ..... -

A study of Army turbine engines made to determine R&M factors (ref. 8) covered manyof the samb engines used in the civil helicopter fleet. This report gave the following as majorturbine failure modes: :

• Nozzle band cracking

• Nozzle support structure wear/cracking

• Nozzle erosion,, burning, sulfidation'

Blade/wheel cracking is an infrequent failure mode. This area is safety-impacting and isgiven a proportionally larger amount of design attention.

The major R&M problem, then, is caused by thermal stresses, thermal weakening, and theerosive effects of the hot gases. :

Problem Solution

Present Technology. — The civil helicopter turbine failure data base should be expandedto allow examination in greater detail. While some problems appear to be alleviated by existingdesign te£hnlqu~e^th"6~iniev^^ pressure ratios and operating tempera-tures and the trend to increase power-to-weight ratios make this an area for continued research.A joint airframe/engine manufacturer effort is recommended to determine causes and resolu-tion of turbine reliability problems.

Advanced-TechnblogyT—Since-the civilhelicopter-data base does not contain depotteardown/oyerhaulresult5y.th^^discu.ssion.is based exclusively on military helicopter experience.

The use of cast nozzles and slotted inner/outer nozzle bands, along "with" the developmentand application of improved materials, directional solidified alloys, and improved coatings toresist sulfidation, would be a great step toward reducing the magnitude of this problem.Analogouslyj-development/application-of .turbine engine fuels, with.lo.wer. sulphur content wouldhave-a-higWy-beneficial^êet-upon-tur-bine-FdUabaity. A materials/coatings development pro-

TO~a^^ problems:

' "7(3!

• I T - .

5 R I O

-CLVIL.HELICOPTER RELIABILITY PROBLEM,Al _

• AIRFRAME VIBRATION AND CRACKING

t : . Problem_ . ~ _ . _ * _ .... , , , _ . _ _ . . -.. _ . . . . . , .

. . . j . . .

i ' ~ . ' . ' ' ' , . ' . ' - ' .

Tcraeks'fesult'fronr three major factors: -

1. High localized vibratijori^(particularly^ in the vicinity of the rotors and engine) and pulsating~rQn~tpp~stiriaces.

2. Load concentration typically in the areas of;

Door attachments "•':•'.'•V-- ......... ; ..... — ---- | ......... - . . . . . _

Landing! gear attachments

• Engine attachments ;

3. Maintenah^-oiagiMt.ecT^amagTpn^

• Doors, accessrpahels loaded abnormally (wind load on open panel; vertical loads-- on open-doors; doors opened-too fary thus cracking structure at hinges;- bent

structure ; whichjresults.in cracking due to load concentration in operation)• f ;

• ~- Stringers-and~longerens^dented-from-maintenance, which then crack due to loadconcentrations ' -

i • . . . i . . .

.. « Damage- from -tools and-toolboxes resting on horizontal surfaces which are pri-imrUy fairings^ !

• pentingrdamag^:from-5tepping-onnonwalkway areas ....... .......

• Cargo, passenger-loading damage of unreinforced areas

; Problem Solution

Present' Technology^ -'The'pfoblem can be gre'atly alleviated with existing technology butwith a weight penalty. The design approach is twofold:

1. Design fbr'the real rnainferiance' and -user environment, that is:

• Heavier ;skin on horizontal surf aces

• Better wor}C;platfpnns. _

« _ 'Gompohents-de'signed-to-be-resistant-to maintenance damage. -Avoid_flimsy edges-on formers and-stringers that may be1 exposed to maintenance damage

2. Design for higher load factors, that is:

Reinforce structure at points of load transfer

.j 7i) 'ELLU .< i- IT" 6LAC'<• •'2 'NCH GRID ADDtTO'

.. _ • Design_tp_bje,.mQrje_resi5JtantjtO-vibration in_areas.dernQnsjTated to be vulnerableto this source of problems

Advanced Technology. — The following areas of technology offer considerable promise:

1. Advanced materials, which promise weight reductions of 12 percent with present designprinciplesroffera"means;of designto^the-real maintenance and vibration environmentwith equivalent weight as compared to today's technology (ref. 3).

2. Reduction pf| the^ yibfatiprrlevelsr"OneTnethod is'to use isolation systems between thefuselage and the rotor system. The airframe will then be less susceptible to vibration-induced cracks (ref, 2).

Additional References. -^ A further discussion of the skin arid structure cracking problemof two military helicopters is-given-in-reference 10. An excellent summary of the effects ofmaintenance/operational damage on the U.S. civil helicopter airframe is given in reference 16. •• -• - -

72;

3 • D

Oi :

::SMALL HARD WARE

::: :Problem

Hingesrktehes^rivetSrserewsrand-otKer-hardware fail-at such a-rate-as to cause a signifi-cant R&M problem.. The. problem is usually rninor.p.er. event, but.the. frequency of occurrenceis very high,; One:study;of military helicopters (ref, 10) reported that hardware problems ac-counted for 15 percent of the total ffiatfunctions on two models of helicopter. There is somesafety implication as panel loss resulting in rotor contact has occurred. FOD to enginesfrom Iost4a«iwar-6ris-ano^

, copter hardware failures'hiaye^been studied by:several investigators (ref. 10, 13^17, 18). Majorcauses from reference 13 are given below: An examination of civil helicopter hardware failuredata indicafe¥tW]3ame~causff

• Vibration-induced failures caused by loads imposed by adjacent structure

• Maintenance damage] caused ;by excessive loads applied while securing the fastener or byrougH handling of haitLware or attached structure

• Manufacturing-tolerance variations that cannot be accommodated without damage tot h e fastener • ! ' . . ' . ' ' . . .

The c'ivil helicopter problenvis nicely summed up in a recent statement by Frank McGuire,Editor, Helicopter News (ref. 16):i ."In.many ways_the helicopter is just out and out tooflimsy." ; : ; ] ^Jfa \ ' -

' ' ' ' " ] : '._ i . , —. "Problem Solution • •

Present Technology.;— The frequency and severity of the small hardware problem canbe greatly 'alleviated with present technology once it is acknowledged that there is a require-ment for a rugged'desigri to survive in the user's environment; that is, design for the following:

• Resistance to vibration

• Tolerance to^ maintenance damage

• Crew> passenger,:baggage, and cargo-damage tolerance

A program for field evaluation of the reliability of an airframe reinforced locally to-. withstandMgli.\n^i^iion^v^]s,'nofm^m^tenance and operational loads, and with the

- (; hardware installation:-inspeeted-to;-assure-all aspects-are-according to-manufacturer-'s require-' ments. This will enable a-quantitative determination of t-he reliability improvement and as-f sociated weight/cost penalties available with the present small hardware technology.

73!

r 0 ~ * • c ?£" 2 7; '• 'Ei,1 / 2 I N C H G R I D A O D E U

f ' > ~ v .',f ^22 "> 7t l * ^ A " ; - v ' A

Adyanced_Technglggy1j-; • . ..

More study and definition of the operational design criteria will permit a weight- andcosteffective approach for hardware survival.

Fastener research-is required1 to.improve the tolerance to maintenance and manufacturingdiscrepancies.

Hardware must be developed .to be more compatible, with the use of composite and honey-comb construction. Present'hardware is based on decades of use with solid metal struc-ture and some proves inadequate when used with composite or honeycomb.

Vibration levels should be reduced. One method would be to develop and field-test avibration-isolation system. :

74,

CIVIL HELICOPTER RELIABILITY PROBLEM A3

SKIN UNBONDING

Problem

Civil helicopters makerwideruse-of-hpneycomb panels; they are highly efficient from astrength-tO:weight;.ratio,._Both:.metal.and'composites are used for skin and core, although use

,, of metal is moreiprevaleht.:. Unbending of the skin and core is a widely experienced problem.

! . : • : : : • - ; . 1 . : . - : . . ; . . . : - . . ; . . - - . •Honeyconib panels are ffequently built into the primary aircraft structure and replace-

ment is a majpr-pp>Fationi.--Repairrof-a-jarge-unbonded-area is-also difficult and time-consuming.

There are many reasons which have been found for this problem which is most predominant. ..; with thelifetal honeycomfc' '

1. Manufacturing/processing methods have been at fault. Proper cleaning and surfacepreparatiolfrof mat¥fjiaTs ih^vTb^eli found (essential to good performance.

2. Some hygroscopic bonding materials have been used; these transmit moisture whichcorrodes the metallic surfaces arid "causes the bond failure.

i I . ; i .

3. Attachments, such as fasteners and other hardware, are not sealed which results in

4. Mechanical damage, such as punctures, allows moisture and other fluids to enter thehoneycbmb~cofeT Aluminum ! skin oh "the order of 0.010 inch thick is often used whichis structurally adequate, but nearly intolerant of maintenance, crew, or cargo damage.

Many panels ;now in use in the civil helicopter fleet, although quite new in chronologicalage, represent somewhat obsolete design and fabrication techniques because of the rapidchanges made urthis area of structures technology.

Problem Solution

Present Technology. —

• Considerable research has been done in the areas of improved processing and bondingagents,"so the problem will be alleviated as these procedures are implemented.

• Use of composites has resulted in fewer bonding problems than the metal honeycomb;:. stnicturel: V" r ~ 7 [ : ~ ~ ' T '.' ' ";' ." '.

, - • Better fastening methods to_the honeycomb panel have been developed so that bettersealing results against external moisture and fluid contamination.

z c

o.> ' -Afield-demonstration program-eould;evaluate the reliability-improvement possible for'_ a historically-troublesome"^^^

:': l .1-'1:....'-.'I--..;>r"'t' -.75

• .•> ,X :*. f. C . > ' 3 . ' I /EL' .'2

f^=!'/ i6'"?? '-3 71' '3> "

'

materials, manufacturing and process controls. and.Eanel.desigilLCorrfiguratipn were combinedto make a replacement pahel. The field reliability problems experienced by this present-technology panel would enable a cosf-weight-reliability trade to determine the effectivenessof fleet retrofit~of, present technolpgjrto replace that now in use in the civil helicopter fleet.

Advanced Technology. - Work is needed in the following areas: _

• Better-definition: oLthe requirements of .the operational environment is needed, par-ticularly in the areasfof maintenance, crew, and cargo-damage potential.

Better definition (supported:by testing) of the damage-tolerant performance of honey-comb skins: qf different thicknesses and materials^

Better fastener ..and attachment .procedures and hardware are required to assure fewerload-transfer;discontiriuities:at these points. Design criteria should be better defined; forexample, a desirable feature would be to require fastener attachments to exceed thestrength of the attached hardware, so that any maintenance damage results in failure ofthe-low-cost-fastene^j-rather-than-the-iiigh-cost panel.- This is particularly necessary forcomposite panels sinjce there is a lack of technology in this area.

...76,

CIVIL HELICOPTER RELIABILITY PROBLEM D1 [' . ' . ' . ' . ' • ' . ' ,

'- TRANSMISSION BEARING SPALLING

; ' : ' . • • ' : : Problem

Spoiling (or €lafcing)-of-bearirigs-is-the-result of^fatigue-failure of-thecontaeting surfaces.If the bearing is^;p4Qperiy_.made^impiMitedi,arid lubricated and the loads are not excessive, bear-ing spalling shouldoccur; only; infrequently.

Common problems found to contribute to transmission bearing spalling are the following:i:.J ^ K- -::;::f::;:-.I:::!. T;::::.:: __ _.:

\ 1. Unanticipated loads* Jin excess of the design load spectrum.: i -_ _ ; __ • ' _ : ; . ' _ > _ .;' ' • _ '

i 2. Lack ipf deaiiiiness ojf the bearing material. Inclusions or flaws act as origins forincipient failure. 1

3. Misalignment or other assembly conditions such as to cause an unanticipated distributiono f load. • ; • . ; . .

4. Improper surface finish of the ball, roller, or raceway paths such that lubrication break-down occurs. [ ; ;""'""

5. Contamination of this lubricant.

In the civil helicopter data base, this problem is associated predominantly with thespalling of the upper-shaft support bearing-in one model of helicopter and, as such, is not ageneric problem to the civil helicopter of the magnitude indicated but rather a particularproblem in design execution. Although the magnitude of this problem may be biased by thisone specffic~bliaSng, as^TfevTolisIy^Kown-in'Table 3, bearing* spalls arFthe "cause of more than19 percent of all military helicopter transmission unscheduled removals.

i • ; Problem Solution

Preseht-TechnoIogyr^-The-particular problem in the one helicopter model that drovethis failure cause.to: the predominant problem list appears to be a matter of design execution.Correction is probably impaired by cost and weight changes and the necessity to change inter-facing components. . : ; ' .. i ' . . ' ;

The- generic-problem of transmission drive-system bearing spalling can be alleviatedwithin the bounds of present bearing technology. Incorporation of the following product-improvement-type design (approaches will reduce the problem magnitude:

1. Bearings of VIM-VAR processed steel (M50-or 9310, vacuum-induction-melted orvacuum-arc-remelted) with the forged billet ultrasonically inspected. This material givesfewer and smaller inclusions for possible fatigue failure initiation than single vacuum-arc-rneltecTsteels with ^olSvelilfionallmpectio^tecHniques.

77.

2. IniprjDyjdjguali;^.assuraj^^ such as surface finish andinternal clearances, assuring better lubrication and more predictable load-carrying ability.

3. Since (sipallingiis_a.loidTseJisiêj:haracteristicI. increasing the size of the bearing will reducethe surface contact stresses. The bearing life increases as the inverse of the 7th or 9thpower of the pontact;Stress ratios if all other factors remain unchanged. This approachdoes" qarry~a~weighrpen^ is only 7percent of the transmission assembly weight (ref. 1, Figure 18), a trade of this sort (weightincrea^tc^mcrea^rÛabil^)_niay be in orderThe problem could then be seen in properperspective. ; J:. : : ' . ' - • . • ' . . ' • • . . . '

Advanced T^chnolbgyv~--T%e following approaches may fur-ther-reduce the transmissionbearing spalling^problem:. j • _ . ! : • . . . ' L - _ . . : . .

t '. . \ '. . . . I ' . ' ' . ' . ' •

1. The civil-helicopter fleet has-more freedom to use lubricants more suited to the specificapplication than does the military, which relies on a universal lubricant, suitable for all

! applications and environments (but possibly optimum for none). Research shows thatfor many helicopter applications, straight mjheraToU gives about twice the life of themilitary lubribants; One concern is that introducing more lubricant types could createadditional logistics (supply)-problems.... ._

i ' . " ' [ ' ' ? _ ' ' , ; ] . . :

12. Continued materials research to develop stiffer transmission housings and bearing ma-terials-wi^_betterJoidârjyJng^chariacteristics.J A program outlined in reference 1 isrecommended. ; :1 . . . . . . ^ - . -

.3... - Vibration reduction to-reduce unanticipated loads upon the .bearings. The program out-lined in reference 2 is recommended.

EN D

.:178t

-_CiyjL.HELICOPTER,RELIABILITY PROBLEM D2: TAIL ROTOR TRANSMISSION MOUNTING

~~.":~~: Problem

a^problem._The_details of the failureare not reported; However^ the failures are principally a problem in one model helicopter. Thisis evidence; that this is:notjaprpbiem indigenous to the civil helicopter, but rather a problemof desig^Vxeculipn~on"thie~one"inodel~that is the principle source~of~these reported failures.

Failures in the tail rotor area of helicopters are not unusual due to the high vibration anddifficulty of accurately predicting loads.

- )- — : '- : -Problem Solutioni

Preseht Technology. — Better detailed design execution would reduce the magnitude ofthis problem greatly. Reliability warranties might increase the cost-effectiveness of designchanges to. correct this type of problem.

Advanced Technology. - A vibration reduction program, oriented to the tail rotor area,would reduce the frequency of this type of failure which occurs primarily due to unanticipatedloads. This~pT6^am^houId~address"tHe^^ vibration reduction approaches of reference 2.

79,

CiVIL'HELICOPTER RELIABILITY PROBLEM D3


;: Problem

Cracks in-therattachment/lifHugs-of the-transmission-of one helicopter model caused thisproblem t6:make:the_top problem list. _ The .problem is believed to be one of design executionas other helicopters have a :much lower frequency of malfunction in this area. The transmis-sion attachment liftlug arrangement in question is designed to isolate the" fuselage from rotor-induced vibration. There~!are six transmission-to-fuselage attachments. One lift link is attachedto the fomaFd^dge. ofê'-tt^snussionând-transmits -verticaLforces-with.no. lost-motion; theother five lugsjittach tpjtnounts having a degree of resilience. The two aft outboard attach-ments have friction dampers as well.

r • • . i • •! •i . - 4

• \ Problem Solution

Present Technology.;- The particular problem of the helicopter in question appearssolvable with present technology with probable adverse impact on cost and weight. The lugattachment cfacMngpTdblem does not appear to be generic to the civil helicopter.

i

Advanced Te^nojo^gy^—jêductigii^f rotor loads and vibration should alleviate this andother unanticipated helicopter component cracking problems. Research requirements forsuch a program are outlined in reference 2.

so;

\' £. t. l_u;-: ' .MTH S1_>*CKi '2 INCH ,;RIC <..ooEr.

C.IVIiaiELICQPTER .RELIABILITY PROBLEM D4

HANGER BEARING FAILURE

• Problem

The drive-shafting-between-the;main"rotor transmission and the tail-rotor transmission issupported i-by-a number, of -hanger bearings. -The shaft weight is generally negligible in termsof the bear^g aUgwable^oads, so: the principle loads are those caused by structural deflectionsand vibration. The calculated lives of these bearings are so high that the bearing should notconstituted problem. Yet," the!problem of providing axial and angular freedom for shaft de-flection meikes-this-a-diffieult-bearing- application- and- the frequency of-failure drove thisproblem to the major reliability problem list.

Problem Solution

Present-TechnQlQgy.-r^-This-problem is reported to be-more prevalent in one model ofhelicopter'than in; the overall civil fleet, leading to the conclusion that the problem is one ofdesign execution and not a generic problem.

In reviewing the history of shaft hanger bearings, it was found that many design ap-proaches hambeentakenlto alleviate this problem with considerable success. These includesplined shaft sections to allow axial and angular freedom, flexible couplings between shaftsections, flexibly mounted hanger assemblies, damped hanger assemblies, elastomeric mounting

-of the bearing, and the requirement for periodic lubrication of the bearing. In general, themore complex and expensive approaches have resulted in more reliable assemblies.

Advanced Technology. — There is a need to determine the life-cycle cost impact ofhigher initial investment to alleviate this R&M problem.

HELICOPTER RELIABILITY PROBLEM D5

GEAR SCUFFING/SPALLING

: Problem

Scuffîng-and-spalling;-of-gear-teeth-are surface distress phenomena.- Scuffing is a verylocalized surface;welding and tearing loose caused by sliding of contacting surfaces. Scuffingis an indication of lubficatipn breakdown at the tooth contact point and an important causehas been found to;be improper surface finish. Scuffing is generally initiated during run-in of atransmission andêxhibits a decreasing failure rate with time. Spalling results from high contactstresses and is-a surface fatigue-condition.-Spalling generates small -metallic particles which aredetectable ;by debris-detectiph systems. Gear spalling is potentially a nucleus for a more serioustooth fatigue failure but is not safety-critical by itself. Debris generated by spalling and de-tected by a~debns?detecfibn "systehY would be cause for transmission removal and so severelyimpacts the maintenance gost-of a:heKcopter.

: Problem Solution

Present Technology. — Some of the spalling and scuffing problem can be alleviated byimproved control of quality factors as discussed under transmission bearings. This includes:

1. Impr ovedTnatefisl~cleanlin~ess by"use~df "doubie-vacuum-melrsteels with ultrasonicinspection of the billet.

2. ~ Impr6ved~sur:face finish of the contacting areas.

3. Better distribution of lubricant on the contacting surface.

4. A weight/reliability trade to reduce the load-related failure phenomena.

Advanced Technology. — The following areas of potential R&M improvement in the areaof scuffing and/or spalling were identified in reference 1:

1. Improved^îoad~sharing"and"redticed sliding ofnoninvolute,"iiigh-contact-Tatio spur-gearteeth'provide reduced noise and vibration levels and increased load capacity and life forplanetary systems.

2. Increased contact area, reduced misalignment sensitivity, and sliding of nonoctoid, high-conjtactjraj^.bejejkeeth jroyidejreduced noise and vibration levels and increased loadcapacity and'life.

3. Advanced hof-hardriess gear materials provide increasedjoad capacity and life and in-creased tolerance to emergency oil-off operation.

82-

• E _ U l_ •-• '. ! f i- & t. A C K> l:-'CH CK:^ A D D E - C

•» A T ',..(.

4. Advanced finite-element andysj£0^planeta^ (spur and helical) and spiral-bevel-gearrim thickness will preclude a catastrophic failure through the gear rim or web and animproved load distribution can be obtained.

In addition to_ these, improvements in lubricants for gearing offer both short- and long-termpossibilities to increase load capacity.

83.

DRIVE SHAFT SPLINE WEAR

Problem

This problem-relates:to wear-of a spherical-spline used in each of-the two flexible couplingsof the driv.e,shaf^tihg.fromLthelehgirielto themain transmission. The splines accommodate axial

,, motion an.d misalignmentj l:TJiey:are:paeked at assembly with an extreme-pressure grease. The^assembly is sealed] ; I:!: : : I-' ' . ' - . ' " . - .

iat^system and an:airframe:mjoj^ requires couplings to accommodate large axialmotions and angular misalignment. Sealing to retain the lubricant and exclude contaminants :is difficulttSeaifaflurefe;sults;in lubricant loss and subsequent spline wear]

Maintenance:of-the-xjrive-shaft-spjined.couplings is.timerconsummg as .the shafting mustbe removed from the helicopter, the couplings disassembled and then inspected for wear, re-grease-packed, and then reassembled.

There-is some indicajtion of lubrication breakdown in high-ambient-temperature opera-tion of tbJsjyriHythjrfy^

Problem Solution

Present,Technology.{ —. The flexible coupling configuration which employs a sphericalspline was forced by the design configuration of the flexibly mounted transmission with theengine separatelyhidunted on the aMrame. With this configuration, the most promising areaof present technology is the development of a grease lubrication system that does not requireremoval- from the-helicopter-and subsequent disassembly .With more frequent lubrication ofthe splines, the spline wear problem would be alleviated and the R&M impact improved.

AdvancedTeHmblogy.T^^De'velopmeht of improved flexibreTcbuplings would improvethe helicopter- R&M. The^re:is increasing need for shafting to accommodate relative motion

,:; • betweenjcomponents -as-rptor-isolation systems are used to reduce the helicopter vibrationlevels. i ;. ; . ; ;

- • - ' Further.research,on-highr;pressure.spline-lubricants, particularly for operation in highambient temperatures, may also alleviate this problem. Use of dry-lubricant coatings should

' ' be cor^deredrm any spline lubricantprogram.

84

... _CLVJL.HELIC0Fr.ERRELIABILITYPRQB_LEMR1

METAL BLADE CRACKING AND CORROSION

. ' : Problem

The~metd~blades-consist~pf-airaluminumextruded spar with analuminum sheetmetalfairing bonded-to-the trailing edge;--Common failure modes are cracking (predominantly in the

,bond areas) and corrosion. These-are not safety-of-flight problems but must be corrected before.they progress to that stage. This blade concept, while initially inexpensive, is very difficult torepair and so a large proportion'of "damaged blades are scrapped. The blade is susceptible toFQD. Repair of~dents-makes~the-blade susceptible to further 'cr-aeking-of the dented area, re-sulting in subsequent scrapping.^ .

: Problem Solution

Present-Technology.--^-The fiberglass composite -blade is much less susceptible to bondingcracks, denting, FOD, or corrosion. Although the initial cost of this blade is presently high,)with increased production; quantities the initial costs are projected to be similar to metal blades.Repair has|been simple'alnd inexpensive, resulting in low life-cycle costs.

Advahcedjr.echnology.^:.The,use.of .advanced, composites .giyesj)etter weight-to-strengthratios with R&M characteristics similar to fiberglass and offers overall helicopter performancegairis. Thus, the value of advanced composite material application to blades will be in the areaof providing R&M:characteristics comparable to fiberglass, while simultaneously providing aweight/performance improvement which can be applied for reliability improvement elsewherein the hejicqpter. A program is^recommended to develop such an advanced composite blade.

85;

1 FOR.: Su021 ; S ' 7 : > •£I_;_C.V. '"1 TH BLACK' '2 IriCH 3R!O »-DCED

! ~O-~ *• - f "? ; •' "'• "•« TI••,'-.

CIVIL HELICOPTER RELIABILITY PROBLEM R2

TENSION-TORSION ASSEMBLY FAILURES

' i Problem

The-tensiori-tprsiQn-assembly-retains the blade against centrifugal-forces while permittingthe. torsional^^ moyement;re!qu^ed fpirbjade contt Catastrophic blade loss has resulted fromwirepack tension-torsion assembly failures in current turbine helicopters. As the function ofthe pack~assembly jis critical" tasafeToperafioh'arid any paclTdeterioratioh cannot be observedin the aircraft installation^the packs are replaced when any deterioration is found at overhaul,regardless.of,how_serious._: _: _•_ ___:.__ . _ . ' . _ .

Two forms of deterioration have been observed:

1. The pblyurethane cover deteriorates from hydraulic oil or other fluid contamination.This becomes evident from the blistered appearance. (Cover deterioration is a possiblecause ofp'aclrfailure".)

2. Broken wires protrude through the cover.

Although the pack consists of multiple wire windings, the assembly shows little sign ofredundancy and acts as a single load path for blade retention.

~" - • j ; "• '"" """

' ' [1 ' | . ; Problem Solution

Present Technology.^- It is probable that continued effort will soon result in completeunderstanding of problem|cjusalJFactors and solutions of the wirepack problems. Other ap-proaches for the blade retention problem which have been successfully used are the following:

1. A series of angular-contact baU bearings sharing the centrifugal load as a thrust load oneach bearing. ' . ; ' • "

2. Many parallel stainless-steel sttaps_carrying the centrifugal load as a shared tension loadand twisting to provide blade pitch freedom.

3. Many parallel metaJTbars carrying the centrifugal load as a shared tension load and twistingto provide blade pitch freedom.

4. Elastpmeric bearings in compression.

These approaches are in use, land as far as is known, none has resulted in a safety-of-flightproblem,- Some-o^fier-positive-redundanGy-in-load-path-ahd-better- inspectability of condition.Some require a configuration change to the rotor head. Weight and cost may be slightly affectedadversely.

86.

Advanced Technology. - Bearingless rotor hubs having a tprsionally flexible compositeelement to' retain the blade are under development and offer promise of weight and cost savingsand reliability and: maintainability improvement. Continued development of bearinglesscomposite'hubs similar to,that discussed in-reference 14 is recommended.

.187;

:• 1 ) »• 6 i_ l_ u •• '.1.2 i ;*' C M

-i 3 i. -'. c :<i O A D D t O

CIVIL HELICOPTER RELIABILITY PROBLEM R3

HUB AND SWASHPLATE SUPPORT CRACKING

: . Problem

The problem refers-to-cracks in dynamically loaded components-of the rotor hub andupper controls. The cracks appear repetitively in a few of many similar components and occuras a significant problem in only a few models of helicopters. These components have very littleredundancy, and since thesy are critical to safe flight, they are carefully inspected at frequentintervals once aproblem of this type materializes. This adds to the R&M burden.

Problem Solution

Present Technology. — Although the exact causes of this problem are not understood,many similar components; are functioning satisfactorily in the same design of helicopters, andmany other-helicopter-designs-have-components.that accomplish the same function as thecomponents in question and are not an R&M and safety problem. This appears to be a problemin detail design execution (that may be difficult to correct without a serious impact on cost andweight, and'the YedesigrTpf "the malfunctioningand interfacing components.

loads reduction program identified in refer-ence 2, if adapted, will result in better identification of loads. Unanticipated load levels mustbe a significant factor in this problem.

..CLVlL.HELiaQPXER.RELLABILI.TY PROBLEM R4.

HUB BEARINGS

Problem

This problemris^ bearings in-the main rotor hub. Thereare both oil- and grease4ubricated-bearings reflected in this problem. The loads have bothsteady and ^terna^g^cpjmponjente:andjtih.e bearings oscillate through small angular excursionsat rotor frequency. Proper -functioning of these bearings is essential to minimizing helicoptervibrations. A nialfunction,'.if "undetected, can progress to a safety-of-flight problem.

i

The bearings|includedâre..thefQllQwing:

1. Piteh-ehangerbearirigs—

2. Teetering-hihge-bearings: — -

3. Piteh-linfc-attachment-trunnion-assemblies -

Replacement of the pitch-change and teetering-hinge bearings generally requires removaland partial disassembly of the rotor hub, iafter which a check track and balance should beconducted. Replacement of the pitch link attachment trunnion can usually be done withoutimpact omblade-track;-ho!wever,-the-check-track-is.desirable,.^The^R&M and cost impact re-sults fromi the above operations. !

~" Problem Solution

îri-Tlie_prQblejm,.is .most, severe.Qiyhe smallest,.more lightly loadedbearing, the grease-lubricated pitch link attachment trunnion bearing. These particular prob-lems could be; alleviated by larger' bearings, but with some adverse impact on cost and weight.

~-;™;: r~' : ~~" ......... ...............Advanced Technology. ^-The bearingless main rotor,. with flexing elements rather than

lârings, is a_concepjt thatjpf fers R&M savings. Other promising areas forwhich technology must be developed to assure a favorable R&M impact are:

1. Elastlomeric bearings ___

2. TEFLON fabric hearings' ~ ........... ••; !

•

As discussed in Civil Helicopter Reliability Problem R6 (Tail Rotor Grip Bearings), thetechnology .of design for. these bearings under alternating loads and oscillating motion is notcurrently Well enough defined to; permit accurate life prediction.

89

' " • • EL-;;.' • 'n ~ L--L>*CK

^15aL_HELIjCLOPTER_RELIABILITY PROBLEM R5

MAIN ROTOR RETENTION NUT FAILURE

Problem

This problem^relates tDTnabfunction-and failure of essentially standard hardware and themaintenance impact-of-such a-failiire: Tlie failures involved are mainly cracking of nuts onblade retention: bolts.: :Malfuhctiohs involve failure of the nut retaining latch and loosening ofthe nut. AiS:the niits'in question perform .a critical function and have little redundancy, thereare safety-df-flight implieations from thisproblem.

The failure,causal factors were npt.repprted in the data reviewed. While there are manypossible causes for these failures, high-strength aircraft hardware has been found particularlysensitive~tb materials defects and deviations in manufacturing and processing. Brittle failuresof aircraft nuts have been' found toyhave resulted from deviations in the heat-treatmentprocesses. ; 1 -- . .

" " I Problem Solution

Present Technology. — Redesign to use material less sensitive to processing flaws would1atejthe._pr_oMejm»^ on component weight,and a redesign and

requalification required, i :1 " " • " . " i - - . - • • • • - • •"Use of "hardware; calling for more stringent control of critical processing characteristics

would alleviate the problem :

Advanced Technology. — Redundant means of attachment of safety-critical hardware,if developed, would reduce the criticality of this hardware and permit the use of lower heat-treat material, less sensitive to material and processing flaws.

. 90

EL Ul^ .' • .-: T-. e i _ > » C H'. i. c H o -• • c A r c, E; i

_CLVJL.HEL1CQPTER RELIABILITY PROBLEM R6

TAIL ROTOR GRIP BEARINGS

. : Problem

TEFLON"fa"bric-Uned-beîMgs"are;cornrnonly used for tail rotor blade mountings to pro-vide retention againstcentrifugd force while allowing pitch freedom, and also as teetering-axis beaj^gs^^Tlfeeibearmgs light in weight. They do not need lubricationand generally give excellent, maintenance-free service for a reasonable life. In some applica-tions, however,"l>b6rbearmg"perf6rmahce:results for no apparent reason. Two examples ofthis are-the-mdfunetion;of-teet«rîng-axis-bearings of one helicopter model arid the tail rotorbearings QJFâno.ther model, ;

Replacement of tail; rotor bearings is usually followed by a check track and balance; thisrepresents an undesirable^ burden on maintenance personnel.

The .problem is caused by a lack of design criteria for nonlubricated bearings. Very littleresearch has been conducted as to the required load levels and motions which will give satis-factory'lives throughout; the helicopter dynamic system. Changes in bearing methods of con-struction ;and bonding resins which improve performance under one condition are sometimesfound detrimenjtaljtQ^^fjacio^_Qêmtionjonder another condition... Most TEFLON fabricbearing testing has been directed at verification of performance for specific applications.

- - - - - - - Problem Solution

Presgnt Technology. -^^ of the specificihelicopters.could be greatlyalleviated by a change in component geometry to reduce the loads. This would involve thecosts of nearly complete rotor hub redesign and requalification, with probable adverse impacton component procurement-costs and weight.

Bearingless tail rotors using flexing elements of fiberglass-reinforced composite have ,demonstrated excellent performance in two recent helicopter designs. "These hubs eliminateall but the pitch-change linkage bearings! This concept should greatly alleviate R&M problemsof tail rotor assemblies.. This is a product-improvement-type change.

Advanced Technology. - Research is needed to determine the parametric relation offactorslrifluencing'TEFLON fabficrlined~arid other nonlubricated bearing wear.

, - - • • - i' " ' :T ' • . . .Elastomeric bearings have the potential for maintenancejjree^peration for many rotor

system applications. Again, considerable research is needed to fully understand the load/motion requirements to give long life in the many differing helicopter dynamic systemapplications.

91:

> ~ tf. L, ->• C K\'^ A O C C. «

SWASHPLATE BEARING FAILURES

Problem

The many •bearingsând-bushingffin-the swashplate and support system deteriorate.These are |mostly,dryJubricantr(TEFLON fabric) bearings, although rollingêlement bearingsare also involved:]_:'!: ..,;: ' . ' . ' : ' i :£'__'_;_;

i . : : . . . . ; . . . : : > ' ' . . . ' . . :

The reported problems involve malfunction of the following components:___ '__ _ ___4—-^-~-i_-_ •__!_ ._

• Bearings, bushings, and bolts for swashplate centering

• Uniball for swashplate centering

• Collective yoke bearing

• Swashplate bearing (rolling-element) spalling

In addition to the above; the military version of one model of these helicopters reportsa very high,detertQraliQnrateLO£ _)the_TEFLQN fabric sleeve bearings for swashplate centering.

The problem appeals to be associated with the large number of bearings in an installationand the'neicessiiy toTenipve blades,-hub,fand"rotorcontrols to replace~a malfunctioning bear-ing. Also!, deterioration of bearings in the rotor controls results in unacceptable vibrationlevels so that maintenance cannot be deferred.

r ;- - Problem Solution

Present Technology. - This is a problem in design execution. The rotor controls are verycompact for light weight and low drag. It is difficult to increase the capacity of any one ofthe bearings once thef configurafion has been established without impacting the size of theentire assembly. Thus, improving the R&M requires a weight-cost trade, but can be achievedwith existing-technology.™ ,.

Advanced Technology. — A new concept of using scissors-type linkages for swashplatecenteiingfwaT~devel6ped ,by""Boeing Vertol and used on the YUH-61A and the HLH rotor con-trols. This linkage was completely replaceable without removing the rotor system and thus,the R&MJimpafitorbearing replacement ismnimized.

Further devieloplne^f H theêchnblcgy of: TEFLON fabric and other dry-lubricantbearings, *asTvell: as"elast6meric-bearingsrwould aid-in-life-prediction of future bearings of thistype and so reduce the magnitude of this problem in future helicopters.

EL.-U.-'! ' - /1TH E. l_ ;i C K1 ."-> i , - : C M ;iR'C ttDDED

RSMAIM ROTOR MAST CORROSION

: i Problem

The definition-of; the specific areas of -corrosion was not giveivin~the data reviewed. It isnot clear whetherLthe:problerh:is:in-either of the splined areas, the.threaded areas, or inside or

" ' "

The main rotor mas'iis a "dynamic component whose integrity is critical to flight safety,and the -permissibledamage4evel-?rom niisfcs, dents, or coirosipn is low. The corrosive damagepermissible;on one area .of the rotor; shaft of a:military version of one of the more widely usedcivil helicopters is limited; to that amount that can be cleaned up with steel wool.

; Problem Solution

Present Technology.! — The technology exists to improve the corrosion resistance of eachof the areks of the rotor mast, either by damage-tolerant coatings over the plated exteriorsurfaces, additionals^al^corrosive-resistant oil-compatible; coatings for the shafting inside the transmission. Redesign,additional pro£ur.emMt^ R&Mimpact. The problem also could be alleviated by a redesign to reduce the criticality of corrosivedamage. This would adversely affect weight.

Advanced Technology.;^Development of improved corrosive-resistant, damage-tolerantcoatings would alleviate much of the helicopter R&M cost problem.

93.

. . . . . ' . ClVHiHELIC5PTER_RELIABILITY PROBLEM R9

ROTOR HUB LEAKS

Problem

Although-not^a-highffailures-ate item,-leaking of oscillating seals in the rotor hub is atraditional helicopter iproblem.:: These seals retain the oil for the main rotor hinge-bearing lub-rication^ The :problemis_that thejusage of jthe seals is small and so it has never been profitablefor a seal manufacturer to develop the;technology for this nearly unique application. Helicop-ter manufacturers have conducted thousands of hours of seal testing aimed at development ofa successful-seal-fpr-a^specifiG-application.-rather than development of parameters necessaryto insure a successful seal. The R&M impact of a seal failure is severe as it is necessary to re-move the blades and hub to replace the seal, then reassemble and retrack the blades.

Problem Solution

Present Technology. - The random nature of the seal leakage problem indicates a devia-tion in materials or manufacturing process control. Increased quality assurance that sealcomponents are meeting specified design requirements and that critical processes are properlycontrolled are the most effective areas of present technology to alleviate this R&M problem.

Advanced Technology. — The following areas of further research should be examined:

• A program, to determine[design parameters for oscillating seals.

• A program to evaluate:flexible boot-type seals.

• Development Welastom¥ric~^

• Development of bearingless rotor hubs.

94

• PROBLEM RIO

RÔTOR HUB CORROSION

i i : Problem

The cadnBum-platedlêelrand-tne aluminum surfaces of rotor hub-components corrodewhen the surface finish:isjscratched; worri, or otherwise damaged. As the major hub componentsare dynamically loadeciând jiaye: Uttle redundancy, corrosivejlamage_in a critical area in excessof 0.005 inch in depth is sufficient to cause the component to be rejected. Many areas cannotbe properly inspected: or êpau-ed:rOT the air craft. Consequently > if damage is suspected inthese areai^-tê-roitor-blades-and-hub-must-be-removed from the aircraft. This causes the R&Mimpact. _:....: _L:.. _-!.- .11: -1:-1:41: :._.:'.:."•.._

I . _ . . . . ;

\ . . . < .

" " [ ! " : • ; Problem Solutioni " ' - : • • • - • - - j - - - - - - . . . - ; • : • • • - :

Present-Technology.'-=_As-.the_damageJs maintenance-initiated,fpr,the,most part, a toughersurface costing such as a polyurethane base would alleviate the problem, although impactingadversely the procurement cost of components. The life-cycle cost would be reduced, however,by the re~duced''"'nîjitenMcje'fequiremeMts.'

i • . i . ' . " " . ; . ; ' _ . . . - . - '

Other corrosion-resistant materials (titanium and stainless steels) and coatings (nylonpowder and plasma spray) will reduce R&M problems with present design configurations.

Advanced Technology. - The composite rotor hub (bearingless main rotor) as discussedin reference 14 offers corrosion resistance as well as weight savings. ;

95,

EL.LO./•'z !HCH

"- A T i A

S L A C KADDED

CIVIL HELICOPTER RELIABILITY PROBLEM LI' i ;

. ; . . ! i - - - F L O A T LEAKS

; . ' • •; : j : .Problem

; . . : / ; . : : • • " ' . ( , ' . ' . : ' . . : I . : . : : . . : • ! . . 'Leaks in"floats"cjan-be~a"seriousâfety-impacting problem because, rin some operations,

, , floats are Used only-in- emergehxgphndings; : :Two types of float are involved : the permanent:,r installation arid the emergency type; The. latter float is carried collapsed and is inflated forI 1 . ' emergency: water|anding|: Reported failures are as follows:

, .;. 1. Perjnianentfloate Âbrasionrand chafing, both from landing and from passengers/crewr en tering:and leaving; the; aircraft,: cause leaks.•.., | .__.p_._r_._j_r_....: .__ . . ._ ._ . ....... . . . . . . .

2. Collapsible floats - Primarily, these leak at pinholes and seams; occasionally they are; founjd to have chafing damage. Chafing occurs in the packaged float when aircraft

deflections result ; in'relativeTmotibn between adjacent layers of fabrics. The floats must"' be~tested-at-regular-mtervalsr-T-hey fail-to-inflate and are found to leak.' '

Float materials are deteriorated by sunlight and contamination such as is caused bypetroleum-based fluids and other common chenucalsjsuch as cleaners. Abrasion is caused by jsand, dirt; and mechanical damage; Theifloats can be easily punctured. Both over- and under-mflation-can-be-stoictur^lly-damaging, -yet,the -.normal. operating pressure,range is only from1.5 to 2.0 PSIG. (A 30-degree increase in temperature will cause approximately 1 PSIG changein pressure, as will a 2,000-foot altitude change at standard atmospheric conditions.)

' . . . ' ' . . i . . . - . •* \ •Floats are designed to :meet: critical target weights and must be treated with consideration

for the inherejnt material IcharacteriMîf :tTOuble-free service, is^ desired.T—

As explained in the Landing Gear Subsystem Discussion, Section 3.2.5, the float reliabil-ity problem wasngreatly alleviated-when one of the major float manufacturers instituted anaggressive product-improvement program in 1974. Following this a dramatic reduction infloat failure rate(became; .ewdent; witib thisjower rate float leaks would no longer be rankedas a major problem. It is interesting that part of the reliability improvement came from theuse of heavier fabric coatings and additional fabric chafing strips.

: .„ L ; Problem Solution

Present Technology. - Float reEabillty can be improved by the judicious addition of: weight. Heavier materials and more reinforcement of materials are examples of this ap-• te' proach. Float procurement specifications should require abuse resistance more typical of / " /•"' the real operating environment and permit increased weight to achieve this reliability.

T v = , - Advanced Technology. - This problem appears unique to civil helicopter operation.P ;:„._- ., MUtary^heEcopteToperatiorrWithô^^^^ by the

military organizations.

//^ /

.. Research to deyelop new .materials.and new .construction methods is. needed so that theadverse weight impact of a reliability improvement can be minimized.

Development'ofweight-cost-reliability trade criteria would assist in the selection of anoptimum: design floatfor the desired application.

97.

-. . CHflL-HELICOETER.RELIABILITY PROBLEM.H1

ACTUATOR LEAKS

Problem

Flight-control-actuator-seal.leakage has been.a recurring problem in helicopters ever sincehydraulic;boost systems have been employed.

The fh'ght-c9ntrbl-actuator reacts rotor control forces. These forces alternate, coming toa peak each time a .blade_passes over the fuselage. As the hydraulic cylinder acts as a stiffspring, the piston moves in the cylinder N times per rotor revolution, where N is the number ofblades. "This motiblTwBarssealsT'TlTe^fe^ure~pu^es gen^lltell clefoTm the seal at the samefrequency; generating further wear. Since any generated metallic wear debris is abrasive, itresults in 'accelerated wear. J&ternal contamination of the fluid or seal causes further acceler-ation of wear. ; '.

Improvements in seal technology-have alleviated these problems by research in the fol-lowing areas: :

• Improved seal configurations

• Improved piston, piston rod, and cylinder materials and coatings

• ImprovemenHn-filtration-of-the-hydraulic fluid - -

• Boots and scraper rings to exclude contaminants

4 Multiple vented and unvented seal concepts

Actuator sMng-is-another-faetor-in seal wear. A large actuator is stiffer hydraulicallythan a small one, so that less motion results from the alternating loads. The reduced motionimproves the seal life.

The generic problem of actuator leakage in civil helicopters does not appear to be asmuch of a-problem-as-indicated -by-the-data. ..This is.because.the leakage in one model ofhelicopter drove this problem to the major R&M problem list.

, "". Problem Solution

Present/Te<chriQlogy,._— ..The.sexere.seal.leakage of ..the one model helicopter mentionedpreviously appears to be the result of design execution. It would appear that the problemcould be reduced to the acceptable level of other helicopters by using existing technology,although lmtfrTFr<5ss1bie''aidverse'impact on-initialcost and weight.———

Advanced Technology. — The entire area of sealing and filtration is a subject that deservesfurther research in order to improve the R&M factors of future civil helicopters.

98'/

CiyiLHELiCbPTER RELIABILITY PROBLEM Fl

FUEL PRESSURE SWITCH

: Problem

Fuel pressure-switches-are-commonly used to signify loss of fuel boost pressure. Theelectrical Signal.goes. to^a caution light. Some helicopters have another pressure switch tosignal that the fuel filter is blocked^ahd is about to bypass, the switch is usually a sealed as-sembly that isreplaced rather than repaired- Consequently, failure modes are not defined.Some of the problems reported are:"

• A susceptibility to -fuel contamination

• Mamtenance damagei _ _____

• Deterioration from the .vibratory environment (this is especially predominant inimported jwitches^ _J

: Problem Solution

Present Technology I — A better definition of failure modes is required and the modesmust be associated with design approaches.

Somje pressure switches have been examined that have good, sturdy construction, withwrench jflatsJxL avoid.maintenance damage. Others have been examined that have a highlycantileverjed suppprt, so that they are susceptible to vibratory or maintenance damage, haveno wrench flats so that vise; grips or pliers must be used for installation and removal, and theelectrical la^semblsris~susceptible to -overtorque: "Surprisingly, the iatterswitch carries aweight penalty in addition to other disadvantages.

These factors indicate that product-improvement-type changes will greatly alleviate thisproblem.

Advanced Technology. - It is probable that no new technology is required but merelya careful definition of requirements to survive in the real environment. This will then requirean investment to design and fabricate components to meet these more demandingspecifications. ' - r . '•'

"99'

L.:.' WITH B L A C K,.CH GHiO A C O E C

CIVIL HELICOPTER RELIABILITY PROBLEM F2

. .-• FUEL PUMP FAILURE

; : .Problem

The civil-helicopter-normally-has-fuel boost pumps to pump from the fuel tank to theengine and an engine-mounted.pump, to maintain controlled pressure to the fuel nozzles. Mal-function of both; qfrtltese|_pumps has been a historical problem in the civil helicopter fleet.The fuel boost pump is technically considered part of the fuel system, while the engine-mounted pump is technically a powerplaht component. While the problem is reported as thatof a fuel-system^ump4^ufe-(i7e^-the-fuel-boost pump),-the-par-t-number-of the malfunction-ing comp6nents.were.npt available,, and it is suspected that some of the reported problemsrepresent failure of the engine-mounted fuel pump.

sThe overwhelming proportion of maintenance actions for the two pumps is to replace

the unit. FaUure-jnodes-are-usually-not-determinable at the operating level. In an Army studyof R&M problems of the military version of one of the more widely used civil helicopters,the fuel boost pump was reported to have leaking and burned failure modes. The majorityof failure 'modes were undetermined ~, however.

Boeing Verto! experience has_been that these boost pumps, which are predominantlydirect-current, motor-driven units, have problems with the commutator brushes and tend todevelop leaks between the pump and motor. The engine-mounted pumps are susceptible tocontamination,~and-seizing is a common failure mode.

: Problem Solution

Present Technology. - A further identification of the problem is needed. The failuremodes mustbe determined in detail7 and-the relative performance of various design approachesshould be established to; determine if the problem exhibited reflects a unique design approach.A trade study should be made of the application of ac motor pumps for the fuel boost sys-tem. TEese mbtoTpumps are brusHless and operate completely submerged in fuel, therebyeliminating shaft seal problems. :

Advanced Technology. — None proposed.

100?

FUEL CELL LEAKAGE

:; : .. ProblemI • - . ; . • ' • : ' : . • ' . . ' . : ' . . ' •

Fuel ,cgjlte~afe~fHad"e~b"flayers~pf fabric'impregnated with an elastomer. The followingcausal factors have been experienced for leakage:

I • ... r - ... s . . . , " . . . . ' ' ; . • ' * . .

1. Melih"âWia"magB~dVing:shippiiig|-;storage, installation; and maintenance results inpunctures and tears.: - - - - -

2. (âfirajisrrivet^ and F0D such as loose-hardware, tools, andother debris wears and punctures fabric. (Strips for chafing protection are installed atassembly but become didpdgedjgrjinbqnded by fuel contamination.)

. . . j . .3. Delaminatipn of fabrics, particularly at seams.

4. SlfessôênWationarfittin'gs"and attachments weakens fabric.i . . . . t

5. Shelf life: materials' are age- and sunlight-sensitive."™ * ' j""* - - - - - - * - - - - - - - - - - • - - - - •

6. Cell materials stiffen in the cold and may crack if handled at low temperature.

The pT^bleWWy~witfrrthe;delaik of construction.

_... _ Problem Solution

; -:

Present Technology. - Many of the problems are caused by methods of design execution,manufacturing-processing;-and-the-desireto-minimize weight. Some examples of this are:

• Fuel cells are generally located in ah inaccessible area. It is difficult to remove or installthe tank without risking damage.

• Noncrashworthy tanks (believed to be the only type represented by this data) are madeof thin fabric very susceptible to mechanical damage and chafing.'

• The Hgh rate of delamination of some manufacturers' fuel cells indicates the existenceof a,processing problem.

• Crashworthy fuel cells are becoming available for the civil helicopter market. These aremuch thicker and mprentblelrarit of mechanical damage arid chafing.

The :problem-ofifuel-cell-leakage-cah-be alleviated.within. present technology by design tocope with-the maintenance and operational environment; specifically, design to give better accessto fuel cell area and use of sturdier crash-resistant-type cells. These approaches, however, willadversely impact procurement cost and weight.

.»ior

-MvancedJ.echnolagy^-,E^ processing,methods appearswarranted to develop crashwprthy, reliable fuel cells with a minimal adverse impact on costand weight. •, •

-,- -v-p

102|:

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103-

..l.l.._MJ!Bon, Da^l^_aVfa^s,_KsimetiiT..: JResearch Requirements to Reduce Main-tenance Cost of Civil Helicopters. Boeing document D210-11277-1, Boeing VertolCompany, Philadelphia, Pennsylvania; National Aeronautics and Space Administration,Langley Research-Center, Hampton, Virginia, November 1977.

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' - - - - - - - - - - - - • • - - - - - - - f - - ' , : "*16. Vertiftite, Volume 23, No. 1, American Helicopter Society, 1325 18th Street N.W.,

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1 - :

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