the path to condition based maintenance · changes to the maintenance practices have been made to...

AIAC12 Twelfth Australian International Aerospace Congress19 – 22 March 2007

The Path to Condition Based Maintenance

Dr. John Berry and Dr. Samuel T. CrewsUS Army Aviation and Missile Research, Development, and Engineering Center

Redstone Arsenal, Alabama 35898-5000, USA

Abstract

The US Army is actively transforming Aviation Maintenance from current practice to one where maintenance actions are based on better use of existing sensors, additional embedded sensors, modern signal processing algorithms hosted on modern airworthy-certifiable processors, where these technologies allow the measurement of the use and condition of aircraft components. Benefits expected from this transformation are improved readiness, reduced maintenance burden,and information and analysis necessary to optimize the maintenance and supply chains. Demonstrations of several health and usage monitoring systems’ capabilities have been conducted to identify the costs and risks of fleet wide implementation when these systems are adapted to the US Army environment.

Background - The Fielded Fleet, Qualification, and Baseline Risk:

Operating the world’s largest fleet of rotary wing aircraft, the US Army has faced significant challenges in safely and effectively maintaining these complex mechanical systems. The maintenance that is routinely performed by field maintainers, typically very young and inexperienced, is often complex and requires a degree of subjective assessment when faced with routine “leave-it-on or pull-it-off” decisions. To assure a level of baseline risk consistent with that achieved by the Development and Qualification (D&Q) program for a given aircraft, components are often replaced well before they are no longer safe to fly.

At the end of an aircraft D&Q program, we field the aircraft with a baseline risk which the Armyis willing to accept given all of the analysis, testing, and production planning accomplished during the D&Q program. At that point we have established acceptable risk in production, operations, and maintenance procedures reflected in frozen planning, the Operator’s Manuals, and the Maintenance Manuals for that aircraft. Maintenance procedures will include required condition inspections and intervals, retirement lives, and Time Based Overhauls (TBO's).

The U. S. Army’s Condition Based Maintenance Plus (CBM+)

The US Department of the Army1 defines CBM+ as “… a set of maintenance processes and capabilities derived primarily from real-time assessment of weapon system condition obtained from embedded sensors and/or external test and measurements using portable equipment.” For the military rotorcraft, this capability is associated with Health and Usage Monitoring Systems (HUMS). The soldier will perceive the effective implementation of CBM as a reduction in teardowns and subsequent inspections (e.g., visual, eddy current, or ‘feel for roughness’), a

5th DSTO International Conference on Health & Usage Monitoring1


reduction in replacement of parts that are fully airworthy as determined by these new criteria, and a better, earlier indication of flaws in components with poor or difficult inspect-ability. Successful implementation results in an overall improvement in readiness and safety. The logistician will see the implementation results in better part condition/projected need information, which will result in a reduction in procurement costs due to better procurement and stocking practices.

One of the tradeoffs considered in implementing a fleet-wide application of a new capability, is the determination of when added complexity and cost of the new capability exceed the perceivedvalue of the system. Cost and benefit studies for systems of embedded sensor and diagnostic systems have been conducted with significant variation in perceived benefit. Another significantconstraint is maintaining the assurance that baseline risk is not elevated when a burdensome inspection interval is extended or eliminated.

New, trend-able CBM allows improved automated inspections to determine the actual condition of critical components. Continuous quantitative evaluation of condition through Condition Indicators (CI's) allows specific parts to be ordered before they need to be removed from the aircraft. New parts will then be available at aircraft when a CI says the part needs to be replaced.This trend-ability (prognostics) enables additional streamlining of the logistics system as well as improved availability.

What is a “condition indicator” or CI? Condition Indicators (CIs) form the basic unit of information used to guide the transformation to condition based maintenance. Generally a CI is a single valued variable resulting from implementation of a mathematical algorithm that extracts a specific characteristic of a signal taken from a specific sensor in a specific location at a specific time. This variable may represent a level of abnormality, a degree of fault emergence, or an indication of the mechanical state of a specific component. CIs are trend-able, i.e., have a range of values. As a simple example, the first order harmonic (the CI) of a rotating component (sensed by an accelerometer) is a measure of out-of-balance of that component. The counter example is a chip detector that only indicates a presence or absence of metal fragments; there is no change in value prior to being in a faulted state.

There are 3 circumstances that justify fielding CBM technology to the fleet:(1) To augment and/or replace existing maintenance practices without increasing, and

perhaps reducing, baseline risk while reducing operations and sustainment costs and increasing availability.

(2) To rapidly recover from unanticipated material failures. Fielding eventually produces faults, some flight critical, that were missed in the original D&Q program. When one of these unexpected faults occurs, the assumed risk goes beyond baseline. By way of a risk assessment, upper level Army management may temporarily accept responsibility for the increased risk and must put into place a recovery plan to get back to the original assumed baseline risk. This recovery plan may include additional operational restrictions, reduced retirement lives, reduced TBO's, reduced inspection intervals, and/or additional and/or more frequent inspection of condition.



(3) To replace TBO’s with trend-able CBM. In this case certification of the CBM elements has to be equivalent to new aircraft qualification requirements. The certification requirements will demonstrate a tolerance to damages that are assessed continually by non-visualmeans.

The existing US Army aircraft fleets of Apache (AH-64), Black Hawk (UH-60), Chinook (CH-47), and Kiowa (OH-58D) have each accumulated more than a million (1,000,000) flight hours and consequently are very mature. Almost all field-produced faults have already occurred (sometimes we get a real surprise like the UH-60 Main Planetary Drive carrier plate) and changes to the maintenance practices have been made to return the aircraft to baseline risk. Mostof these changes have added inspections and/or shortened retirement lives, which increase both maintenance man hours and cost. Some have added periodic Condition Control (e.g., UH-60 high speed shaft health check) as a technique for preventing failure. New trend-able CBM technology that allows updating the specific monitoring characteristics through software can replace most of these labor intensive or expensive maintenance changes and can return operations to baseline risk at reduced cost.

The ability to implement maintenance changes with confidence is derived from the accumulationof data that represents the distribution of conditions in the fleet of aircraft. This accumulation is driven by the number of aircraft that have been instrumented to acquire relevant data. The US Army has over 200 aircraft currently instrumented and operating to accumulate this data. The data contain a representative number of components that are at various stages in the progression from a healthy to a faulted state. The availability of this data and specifically data that represent known faulted states is extremely valuable. We are interested in opportunities to exchange parts of our data sets with organizations and agencies where it is to our mutual benefit.

A Fishing-Net Approach to Implementation

The systems engineering approach to implementation of a mechanical diagnostic system would look for a specific value derived from each sensor and algorithm to determine if it should be implemented. However, most Health and Usage Monitoring Systems are implemented from a “fishing net” approach. Although there may not be a clear return on investment, business case for the implementation of a specific sensor and algorithm, the incremental cost of adding one additional sensor or a set of additional computations for specific faults is minimal with the potential for observing abnormal behavior of a component that may well develop into a fault thatcould cascade into a catastrophic system failure.

The ability to practically eliminate pesky false alarms is also easily implemented with this approach. No maintenance credit is assumed for the incremental installation of a system. There is no in-cockpit identifications of warnings derived from the processing of sensor data in flight. Data is collected over a significant number of flight hours on multiple aircraft before any attemptto isolate suspected faults or failures is made. With a statistically significant collection of data, outliers are identified in the data. The aircraft with these outliers are carefully monitored for significant changes in other inspection and maintenance criteria to correlate the elevated conditions indicated by the diagnostics. Selective removals of components for tear-down analysisare based on the clear, unambiguous indication of the existence of a flaw.



Maintenance Credits

Many of the potential benefits of embedded diagnostics depend on a change in the way maintenance is performed. The existing maintenance practice is primarily based on “safe” intervals for critical inspections, lubrications, adjustments, or replacements. These intervals are set to insure that normal operations cannot span two intervals and reach an unacceptable level of damage. The CBM transformation is based on monitoring the condition of specific components using embedded diagnostics. Maintenance can now be directed to improve known defective conditions. The basis for the acceptance of a ‘maintenance credit’ by the airworthiness authority is an assurance that “baseline risk” is not increased when that credit is put into place.

The issues associated with maintenance credits are well known. There are few validating cases where actual damage is found to correlate well with diagnostics indicators. Generally the number is well below the accepted requirement for assurance by a certification/qualification agency. The US Army is in the unique position of operating the largest fleet of rotorcraft in the world. The ability to capture the required number of correlation cases should occur sooner for the US Army than any other operating activity. Naturally occurring damage evidence can be supplemented with seeded-fault bench tests to determine relative magnitudes from diagnostic indicators for specific levels of damage. The seeded-fault testing will also give primary evidenceof the behavior of the indicators as the fault approaches unacceptable levels of damage.

There exists a hidden requirement to train diagnostic systems when first introduced on an aircraftseries. The threat of incurring false alarms is serious due to the poor confidence that users may develop. During training (or building a statistical basis) either no alarms should be enabled, or alarm level should be set very high. It is also normal to watch indicators carefully.

Vibration Based Mechanical Diagnostics

The suite of potential flaws that can be detected by the analysis of vibrations in mechanical systems is robust. The algorithms that have been devised for associating vibration signatures with specific flaws are well established2. New implementations of classic methods to improve robustness and reduce the reliance on case-by-case engineering interpretation continue to emerge. For complex mechanical systems, such as helicopters, there are many components that are well suited to vibration-based diagnostics.

The US Army has a wealth of experience in applying vibration checks to assure the on-going health of components that have experienced unacceptable failures. The fielded Aviation Vibration Analyzer (AVA) has provided the capability to field maintainers to make simple vibration measurements during periodic inspections. An example of a component that requires this measurement is the UH-60 high-speed shaft. These shafts require a check of the first harmonic, or out-of-balance, vibration amplitude every 120 hours of operation. This simple check drastically reduced the occurrence of shaft failures (and subsequent cascading damage) when it was introduced. These early Condition Indicators have been the basis for providing initial value to embedded diagnostic systems.



The state of the art in implementing data capturing and processing is at a point where the cost, weight, space, and power requirements needed to implement mechanical diagnostics in the aircraft, and especially for helicopters with little payload margin, is no longer excessive. Micro-processor systems are now capable of rapidly performing complex algorithm calculations that areat the heart of the vibration analysis of rotating components.

Demonstrated Effectiveness of Vibration-Based Diagnostics

A CI time history (or trend) is an indication of the progression of a specific fault on its way to failure. On the rotorcraft drive system CIs are based on well documented signal analysis of vibration sensors. The US Army has over 200 helicopters operating with embedded vibration monitoring systems to acquire CIs. Changes to the allowable maintenance practices (maintenance credits) have been allowed by the airworthiness authority based on observed evidence where the values of specific CIs have been correlated to specific material conditions that represent flaws.

One specific example is given for the AH-64, Apache main rotor swashplate3. This component must be overhauled after 2250 flight hours. But, after 1750 flight hours, a physical inspection of the bearings must be made every 50 hours. After accruing approximately 20,000 flight hours of CI data for the swashplate on several aircraft, we made the comparison of condition histories shown below. The yellow horizontal bar represents a statistically established value for attention. The trend of swashplate CI, based on spectral energy, for Apache Tail 460 (in light blue) clearly is an outlier to the total set of data collected.

Figure 1: Swashplate CI Trending Analysis

Spectral information was also collected that support the condition indicators. This spectral information for Apache Tail 460 and another, representative “normal” aircraft are shown below. A principal peak of energy is seen in data from the outlier swashplate that does not exist on a normal aircraft. In addition, the frequencies of sideband energy are related to the specific geometric configuration of the swashplate bearing assembly. It should be noted that the swashplate was not found to be faulted when the prescribed manual inspection (required after 1750 hours) was conducted at three separate intervals.



Figure 2: Spectra from Accelerometer on Apache Swashplate

The swashplate was removed from Tail 460 and disassembled. The results of the inspection revealed a material condition identified below. Evidence of corrosion, grease contamination, a broken cage, and surface degradation of the balls are seen. Based on this evidence, an allowance(maintenance credit) for those aircraft equipped with the monitoring system was issued to replace the requirement for the post 1750 hour manual inspection.

Figure 3: Balls, Cage, and Inner Race of Swashplate BearingFrom Apache Tail 460

A second example is given for a UH-60 oil cooler fan bearing. This example identifies faults that have been diagnosed by two systems, the Goodrich Integrated Mechanical Diagnostic (IMD)system and the Vibration Management Enhancement Program (VMEP) system. Multiple identifications of degraded bearings that support the oil cooler fan have been identified by these systems. The oil cooler fan is in line with the tail rotor drive shaft and, if failed, could potentially sever the tail driveshaft and cause catastrophic failure of the aircraft.

Early during fielding the Black Hawk system experienced oil cooler bearing failures. To continue fleet operation, a periodic vibration check of the oil cooler fan was implemented to determine if bearing degradation had progressed to the point of allowing a first harmonic imbalance of the shaft. This check is required every 120 hours of aircraft operation and requires the installation of a portable vibration analyzer and ground run-up to make the vibration measurement. Implementation of an airworthy embedded vibration diagnostic system eliminatesthe requirement for this check.



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Figure 4: Oil Cooler Bearing CI Trending Analysis

Figure 4 shows the condition indicators that are associated with the oil cooler bearing on UH-60A tail numbers 874 and 900 operating in Republic of Korea during the Fall 2004. The condition indicator for the oil cooler bearings appeared in the yellow (caution) range upon initial installation and stayed steady for approximately 8 months. It is interesting to note, however, thatthe 120-hr vibration check was in the normal range.

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Figure 5: Oil Cooler Bearing Spectrum Content

When the underlying spectral data for these oil cooler measurements is examined (Figure 5), the specific fault frequencies associated with the dimensional characteristics of this bearing are seen to be significantly elevated when compared with other aircraft. Working with the unit maintainers, a roughness was felt when the installed bearing was hand-rotated. With assistance



from the Utility Program Manager, the unit replaced these oil cooler assemblies for detailed analysis. [Resulting in a beer bet that the bearings were bad or not]

Figure 6: Oil Cooler Bearing Components From Tail 874

The tear-down analysis was performed on the oil cooler assembly from tail number 874 at the Army Depot at Corpus Christi, Texas. The physical state of the bearing is shown in figure 6, where the band of spalling of the ball element is highlighted. The tear-down findings were:

(1) Corrosion pitting on ball, race, and cages(2) Spalling and deformation of balls(3) “..opinion of the … investigators that it was a wise decision to remove the oil cooler fan from service.”

A third example comes from the UH-60, Black Hawk accessory gearbox assemblies. The accessory gearbox supports a generator and hydraulic pump. Since there are separate assemblieson each engine drive to the main transmission module, this redundancy mitigates the risk associated with failure of this component.



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Figure 7: Condition Indicator for Accessory Gearbox BearingFrom Tail 443

Figure 7 shows the condition indicator for the port side accessory gearbox on Black Hawk tail 443 operating at Fort Cambell, Kentucky, USA during the period covering summer of 2004 through the summer of 2005. This condition indicator is based on Root-Mean-Squared (RMS energy from a high-frequency band of a measurement on the accessory gearbox. Initial indications in this CI were detected in the summer of 2004.

Figure 8: Spectrum of Port Accessory Gearbox BearingOn Tail 443



Consensus of the engineers at US Army, Goodrich, and Sikorsky was reached to continue operation of this gearbox. Risks of continued operation were mitigated by chip detectors and system redundancy. At approximately 155 hours of operation after this initial determination, three (3) chip-lights occurred in a period of 25 flight hours. After the unit maintenance personnel removed and replaced the accessory and input module, it was sent to Sikorsky for teardown analysis.

Three spectra from the accelerometer mounted on the port accessory module of this aircraft are shown in figure 8. The times associated with these spectra are identified by color-coded circles in figure 7. The first spectrum (blue) is taken from signals captured during the early detection of this component as a candidate for investigation and is labeled as “early detection”. The second spectrum (cyan) is taken during the chip light event and is labeled “before removal”. The third spectrum (green) is taken after replacement of the accessory module and is labeled “after replacement”. Very clear differentiation is seen at higher frequencies. At lower frequencies (0 to 2500 Hz) the differences in tones associated with specific bearing defects are not as clearly differentiable.

Figure 9: Condition of the Accessory Gearbox Bearing From Tail 443

Results of the Sikorsky Laboratory tear down analysis are seen in the bearing components shownin figure 9. The Sikorsky Laboratory findings are reported as: “…Spalled bearing found in the Accessory Module and chips migrated to the Input Module. The bearing … was sent to MaterialsLab for examination and found heavily spalled balls and spalled inner raceway on one bearing. The raceway spalling is uneven around the circumference and many of the balls have "equatorial bands" of spalling.



Rotor Smoothing

The far and away top benefit derived from an embedded vibration diagnostic system is the simplification of maintaining a smooth flying rotor system. The current practice of reducing main and tail rotor induced vibrations is through a process often referred to as “rotor tracking andbalancing.” Under current maintenance practice, this process often requires an excessive numberof valuable flying hours and maintenance man-hours. The details of the current practice that leadto a potential for savings with embedded diagnostic systems are common: (1) Inadequate level oftraining for use of the current portable analyzer systems, (2) Multiple test components must be installed on the aircraft limiting operation to restrictive maintenance tests flights, (3) Potential fordamaged sensors and/or cables due to wear and tear experienced during routine installation and removal, (4) Potential for improper configuration for test flight, e. g., cables routed to wrong portof analyzer, sensor installed in improper orientation, etc., (5) Potential for applying the prescribed adjustment in the wrong direction or on the wrong blade, and (6) Lack of trust in the system results when any of the potentials above result in a failure to reduce vibration.

Embedding a modern rotor smoothing diagnostic system on the helicopter minimizes several of the most likely causes for ineffectiveness of the current portable analyzer systems. Sensors and cables are not subject to the same wear, tear, or damage that resulting from routine installation and removal. Sensor orientation is fixed at initial installation time. There is no need to modify the aircraft for restrictive maintenance test flights; vibration can be sensed during normal operations. Recent work has indicated that dedicated flight regime capture may not be required to determine adjustments that will reduce first harmonic vibrations4.

Structural Condition Monitoring

Health and Usage Monitoring Systems (HUMS) are often inclusive of a feature that is expected to extend the operation of expensive, fatigue-life-limited components. This feature is commonly referred to as a Structural Usage Monitoring System (SUMS). The heart of the SUMS is the accumulation of fractional operating time in well defined operational regimes. The amount of fatigue damage attributed to each regime is established during the Development and Qualification phase of aircraft acquisition program by conducting Dynamic Strain Surveys. Currently the time allowed on these components is determined from a spectrum of life usage in the operational regimes. With an on-board regime recognition system, the accumulation of operating time in damaging regimes for each life-limited component can be tracked.



Figure 6: Design Life CurveFor Critical Safety Items

The US Army peacetime demand data for the 96 Critical Safety Items (CSIs) indicate each is replaced for cause on average at 25% of its design fatigue life. This means that Army Aviation spends three times the cost per year more for CSIs than would be required if each achieved its design fatigue life. Remediation has the potential for achieving a larger % of CSI design life before retirement. If the life fraction is increased from 1/4 to 1/3, the savings would be significant.

Embedded diagnostic systems can collect the parameters needed for accurate regime recognition from aircraft state. Parts tracking with accumulated life reductions based on time-in-regime will be enabled with modernization of The Army Maintenance Management System - Aviation (TAMMS-A)5. Through accurate regime recognition and parts tracking, a strategy has been developed to combine life extension with a parts damage remediation process. This strategy willallow a larger percentage of fatigue-life-limited components to meet and exceed the current life limits.

Summary

The US Army has historically used Condition Indicators to get back to ‘baseline risk’ when faced with an unanticipated failure in the field which affects airworthiness. We are on a path to implementation of new condition based maintenance practices using matured condition evaluation technology. Finding methods to implement simplified and timely maintenance practices based on the evaluation of on-board vibration sensors signal output is a near-term CBM



payoff for drive system components. We are also interested in opportunities to exchange data sets with organizations and agencies where it is of mutual benefit.

Acknowledgements

Appreciation is extended for the assistance of those who turn the wrenches for us, and specifically Drs. Jon Keller, Robert Vaughan and Mr. Dan Wade. Encouragement and programmatic support from COL Frank Atkins and Ms. Gail Cruce were invaluable in getting CBM to this stage in the US Army.

References:


1 “Regulation 750-1, Army Material Maintenance Policy,” Department of the Army, Washington, DC, 5 September 2006.2 Stewart, R. M.: “The Development of Multi-Level Diagnostics for Rotating Machinery,” Machinery Health Monitoring Group, Institute of Sound and Vibration Research, University of Southhampton, Report MHM/R/08/79, Aug 19793 Branhof, Robert W.; Dunaway, Dennis; Grabill, Paul; Keller, Jonathan A.: “Examples of Condition Based Maintenance with the Vibration Management Enhancement Program,” Proceedings of the 61st Annual Forum of the American Helicopter Society, Grapvine, TX, 1-3 June, 2005.4 Branhof, R.; Grabill, P., Grant, L., and Keller, J.: “Application of Automated Rotor Smoothing Using Continuous Vibration Measurements,” Proceedings of the 61st Annual Forum of the American Helicopter Society, Grapvine, TX, 1-3 June, 2005.5 “Pamphlet 738-751. Functional Users Manual for the Army Maintenance Management System--Aviation (TAMMS-A),” Department of the Army, Washington, DC, 15 January 1988.

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