VOL. 4, NO. 3, JUL Y·SEPTEMBER 1977 S.Cond Pt lnting AUOUS-t 1982

COMPAHV. A OIVISIC)H OF l.OC!5l1EED AIRGIAFT CORflORATtON - --

A SERVICE PUBLICATION OFLOCKHEED-GEORGIA COMPANYA DIVISION OFLOCKHEED AIRCRAFT CORPORATION

EditorJay V. Roy

Associate EditorsCharles I. GaleDon H. HungateJames A. Loftin

Art Direction &ProductionAnne G. Anderson

Cover : An aerial view of theLockheed-Georgia Company facil-ity and Dobbins AFB, Marietta,Georgia.

Published by Lockheed-Georgia Company, a Division ofLockheed Aircraft Corporation. information contained inthis issue is considered by Lockheed-Georgia Company tobe accurate and authoritative: if should not be assumed,however, that this material has received approval from anygovernmental agency or military service unless it isspecifically noted. This publication is for planning andinformation purposes only, and it is not to be construedas authority for making changes on aircraft or equipment,or as superseding any established operational ormaintenance procedures or policies. The following marksare registered and owned by Lockheed AircraftCorporation: ‘I “Lockheed”, “Hercules”, and“JetStar”. Written permission must be obtained fromLockheed-Georgia Company before republishing anymaterial in this periodical. Address all communications toEditor, Service News, Department 64-22, Zone 278,LockheedGeorgia Company, Marietta, Georgia 30063.Copyright 1977 Lockheed Aircraft Corporation.

Vol. 4, No. 3, July - September 1977

CONTENTS

3 Tires4 Construction

4 Operating and Handling Tips

6 Inspections

10 Maintenance

16 Summary

17 Crew Entrance Door Cable

18 Hot Brakes

20 Precipitation Static Dissipation

21 Thinner Serrated Platesfor Hercules Main Landing Gear

21 Static Ground AssembliesDeleted from Landing Gear

22 Hercules Towbar Shear Bolts

StarTip17 Using the Tach Generator Tester

for an Auto-Tat

DIRECTOR

MM HODNETT

R.C. WEIHE

C.K. ALLEN

H.T. NISSLEY JR.

J.L. THURMOND

ForewordThis is a general article intended to stress the importance of proper care and maintenance of aircraft tires. Due to differencesbetween aircraft and between civil and military requirements, we have tried to avoid specific information where there couldbe a conflict. Procedures and practices approved and used by your organization take precedence over any informationcontained herein.

We wish to express our appreciation to the B. F. Goodrich Company for permitting us to use information in their booklet,Care and Maintenance of Aircraft Tires, from which this article was adapted, and for providing photographs as noted.

TIRESproper care and maintenance can prevent property loss

and serious injury

Most of us have had some experience with tires. In fact,we have become so used to them that we tend to takethem for granted, and do not readily acknowledge thesignificance and importance of the tire and its functions.Unfortunately, a “relaxed” attitude toward tires oftencarries with it some severe penalties. Careless main-tenance and operation of tires leads to accelerated wear,premature failure, and excessive aircraft downtime; thiscosts money. Still worse, serious property loss and injuryto personnel can result if aircraft tires are handledimproperly. Aircraft damage in tire-related accidents hasincluded everything from a few skin scratches to thedestruction of the entire airplane. Injury to personnel can

run the gamut from minor bruises to loss of sight, limb, oreven life. A basic review of the aircraft tire, including itsconstruction, handling, and maintenance, can go a longway in helping us toward safer, and more efficient, tireoperation.

Aircraft tires provide a cushion of air that helps absorbthe shocks and roughness of landings and takeoffs. Theysupport the weight of the aircraft while on the ground andprovide the necessary traction for braking and stoppingaircraft on landing. Thus, aircraft tires must be carefullymaintained to accept a variety of static and dynamicstresses over a wide range of operating conditions.

The modern aircraft tire is a carefully engineered productthat is manufactured to close tolerances and subject tofrequent inspections during fabrication. It is also a good

4 deal more complex than it might appear to be from theoutside. Here are the main parts of an aircraft tire andsome of their functions.

Tread - The tread is a layer of rubber on the outercircumference of the tire which serves as the wearing sur-face. With the sidewall, it helps protect the cord bodyfrom cuts, snags, bruises, and moisture.

Cord Body - The cord body consists of layers (plies) ofrubbercoated nylon cord. Since a layer of these cords (aply) has all of its strength in only one direction, the cordsof every succeeding ply run diagonally to give balancedstrength. The plies are folded around the wire beads,creating the ply turnups.

Beads - The beads are layers of steel wire imbedded inrubber and then wrapped with fabric. They give a basearound which the plies are anchored and provide a firm fiton the wheel.

Sidewall ~ The sidewall is a cover over the side of thecord body to protect the cords from injury and exposure.

Chafer Strips - Chafer strips protect the plies fromdamage when mounting or demounting the tire, andminimize the effects of chafing contact with the wheel.

Liner ~ The liner in tubeless tires is a layer of rubberspecially compounded to resist diffusion of air. It is

vulcanized to the inside of the tire, extending from beadto bead. In tube-type tires, a thin liner is provided toprevent tube chafing.

Fabric Tread Reinforcement - This consists of pliesadded to reduce tire squirm and increase stability forhigh-speed operation.

Undertread -~ The undertread is a layer of special rubberwhich provides adhesion of the tread to the cord body,and enables the tire to be retreaded.

OPERATING AND HANDLING TIPS

Aircraft tires are amazingly strong. They are built to holdup under heavy loads and tolerate the great variety ofstresses imposed upon them. They will give excellentservice as long as they are handled and maintained pro-perly. Let us take a look at some basic operating tipswhich will help maximize the aircraft tire’s useful life.

Most of the weight of any aircraft is on the main landinggear wheels. Aircraft tires are designed to absorb theshock of landing and will deflect (bulge at the sidewall)about two and one-half times more than a passenger car ortruck tire. The greater capacity for deflection also leadsto some disadvantages; it causes more working of thetread, produces a scuffing action along the outer edge ofthe tread, and results in more rapid wear.

Needless tire damage or excessive wear can be preventedby proper handling of the aircraft during taxiing. If anaircraft tire strikes a chuckhole or some foreign objectlying on the runway, there is a good possibility of itsbeing cut, snagged, or bruised because of the high per-

centage of deflection. Similarly, severe sidewall orshoulder damage may occur if one of the main landingwheels drops off the edge of the paved surface while theaircraft is making a turn. The same type of damage mayalso occur when the wheel rolls back over the edge ontothe paved surface.

To reduce the possibility of damage in taxiing, all per-sonnel should see that ramps, parking areas, taxi strips,runways, and other paved areas are regularly cleaned andcleared of all objects that might cause tire damage.

Flat Spots

Nylon aircraft tires will develop flat spots under staticload. The degree of this flat-spotting will vary accordingto the drop in the internal pressure in the tire and theamount of weight being sustained by the tire. Flat-spotting tends to be more severe and more difficult towork out during cold weather because the nylon cord isless flexible at low temperatures. When a cold tire beginsto warm up from use, the cord becomes more flexible.Usually, a flat spot will disappear by the end of the taxirun. In any case, be sure to allow sufficient time to workflat spots out before attempting high-speed taxi runs ortakeoffs. A flat spot serious enough to be felt by theflight crew during low-speed taxiing can cause real damageto the landing gear or aircraft structure at higher speeds.

Some flat spots are of the permanent variety. Severe useof brakes can wear flat spots on tires and cause them to goout of balance, making premature retreading or replace-ment necessary. Sudden or prolonged application ofbrakes can be avoided by careful control of ground speedand attention to proper taxiing procedures.

Careful pivoting of aircraft also helps prolong tire treadlife and control flat-spotting. If an aircraft turned as anautomobile or truck does - in a rather wide radius - thewear on the tire tread would be materially reduced. How-ever, when an aircraft is turned by locking the wheel (orwheels) on one side, the tire on the locked wheel istwisted with great force against the pavement. Thisscuffing or grinding action takes off tread rubber andplaces a very severe strain on the sidewalls and beads ofthe tire at the same time. Furthermore, a small piece ofrock or stone that would ordinarily cause no damage can,in such a case, be screwed right into the tire.

There is in any case little justification for turning a large,multi-engined aircraft by clamping down the brakes onone side. It is far easier on the tires if the aircraft is first

put into motion forward and then turned, using nosewheel steering and power from the engines on one side orthe other. Only very light brake applications should everbe used to augment steering, and then only when thewheels are in motion. Note too that special care needs tobe exercised in regard to tires when it comes to handlingaircraft with tandem main landing wheels like the Her-cules. A wider radius is required to turn an aircraftequipped with tandem wheels and appropriate allowancesshould be made.

Instances of landing with brakes locked have become lesscommon with the advent of modern antiskid systems, butit still can happen in cases where these devices malfunc-tion. When it does occur, the usual result is a tire sodeeply flat-spotted that it must be removed and retreadedor replaced. Brakes-on landings also cause very severeheat at the point of contact on the tire tread and mayeven melt the tread (skid burn). Heat has a tendency toweaken the cord body and places severe strain on thebeads. In addition, heat buildup during braking mayliterally devulcanize the tire in the bead area. Under thesecircumstances, blowouts are common because air mustexpand when heated. If tires are skidded on the runwayat high speed, the action is similar to tires being groundagainst a fast-turning emery wheel.

More tires fail on takeoffs than on landings because ofheavy fuel loads and higher speeds. Such failures on take-off can be extremely dangerous. For that reason, em-phasis must be placed on proper preflight inspection oftires and wheels and correct tire pressure.

Hydroplaning

In recent years, much emphasis has been put on thephenomenon of hydroplaning. On wet runways, a wave

Deterioration of rubber from high wheel heat

of water can build up in front of spinning tires and whenthe tires ride up on this wave, they may no longer makephysical contact with the runway surface. This results ina complete loss of steering capability and braking action.

Cross-cutting of runways has been completed at some ofthe major airports and reportedly has greatly reduced thedanger of hydroplaning. However, the ridges of concretecreated by this cross-cutting can cause a “chevron” typeof cutting of tread ribs, particularly with the higherpressure tires used on jet aircraft. These cuts are at rightangles to the ribs and rarely penetrate to the fabric treadreinforcing strip. Such damage would not be consideredcause for removal unless fabric was exposed due to apiece of tread rib being torn out.

Storing Tires and Tubes

The ideal location for tire and tube storage is a cool, dry,and reasonably dark location, free from air currents anddirt. While low temperatures (not below 32’F) are notobjectionable, room temperatures above 80°F are detri-mental and should be avoided.

Wet or moist conditions have a rotting effect, and may beeven more damaging when the moisture contains corrosiveagents that are injurious to rubber and cord fabric.

6Example of a tire storage rack.

Avoid storing tires where they are exposed to strong aircurrents. Air currents increase the supply of oxygen andquite often carry ozone, both of which cause rapid agingof rubber. Particular care should be taken to store tiresaway from fluorescent lights, electric motors, batterychargers, electric welding equipment, electric generators,and similar equipment which create ozone.

See that tires do not come in contact with oil, gasoline, jetfuel, hydraulic fluids, or any type of rubber solvent, sinceall of these are natural enemies of rubber and cause it todisintegrate rapidly. Be especially careful not to stand orlay tires on floors that are covered with oil or grease.

The storage room should be cool and dark, preferablywithout windows. If a windowless storeroom cannot befound, paint the windows of the room selected with darkpaint, or cover them with opaque plastic. The less sun-light that reaches the stored tires, the better. The electriclighting provided in the storeroom should be incan-descent; remember that fluorescent lighting is a source ofozone, which is harmful to rubber.

Whenever possible, tires should be stored in regular tireracks which hold them vertically. The surface of the tirerack against which the weight of the tire rests should beflat and, if possible, 3 to 4 inches wide to prevent possibledistortion of the tire.

Avoid stacking tires horizontally, in piles. The weight ofthe upper tires will tend to flatten tires near the bottomof the pile and this may lead to mounting problems.

This is particularly true of tubeless tires; those on thebottom of the stack may have the beads pressed so closelytogether that a bead seating tool will have to be used toforce the tire beads onto the wheel far enough to retainair pressure for inflation. If tires must be stacked, theyshould not be stacked more than four high unless they arein boxes. Stacks of boxed tires must be checked period-ically to determine that the bottom boxes are not crush-ed.

Tubes should always be stored in their original cartons sothat they are protected from light and air currents. Theyshould never be stored in bins or on shelves without beingwrapped, preferably in several layers of heavy paper.

Under no circumstances should tubes be hung over nailsor pegs, or over any other object that might form a creasein the tube. Such a crease will eventually produce a crackin the rubber.

INSPECTIONS

Tire inspections are generally performed while the tire isinstalled on the aircraft and inflated.

Probe foreign object damage with blunt awl:shield work.

A r e sidewall cords exposed?

Carefully examine the tread area for cuts or other injuries,and’ remove any glass, stones, metal, or other foreign ob-jects that might be embedded in the tread or that havepenetrated the cord body.

Use a blunt awl for this purpose (a medium-size screw-driver can be used if a blunt awl is not available.) Whenprobing an injury for foreign material, be careful not toenlarge the injury or drive the point of the awl into thecord body beyond the depth of the injury.

Tires with cuts or other injuries which expose or penetratethe cord body should be removed, deflated, and held forpossible repair or scrapping. Where a cut does not exposethe cord body, taking the tire out of service is notrequired.

Remove any tires that show signs of a bulge in the treador sidewall. This may be the result of an injury to thecord body, or may indicate tread or ply separation.

Sidewalls

Inspect both sidewalls for evidence of weather or ozonecracking, radial cracks, cuts, snags, gouges, etc. (Weather-checking is a normal condition affecting all tires and,unless the cords are exposed and/or damaged, it will in noway affect a tire’s serviceability or safety). Scrap anytire where checking or cracking have extended down tothe cords, or where cuts or snags have damaged the outer

ply.

A light-colored crayon or paint stick should be used tomark all cuts, leaks, or damaged areas while the tire isinflated, and as soon as they are located. It may be diffi-cult or impossible to locate these areas after a tire is de-flated.

Beads

A tubeless tire fits tighter on the wheel than a tube-typetire in order to retain air pressure properly. Therefore,

the flat of the bead (the flat surface between the toe andheel of the bead) must not be damaged as this may causethe tire to leak. This area is the primary sealing surface ofthe tubeless tire, so examine it carefully for evidence ofdamage that would permit air to escape from inside thetire. Bare cords on the face of the bead normally willcause no trouble, and such a tire is suitable for continuedservice if it is otherwise sound.

Check the bead and the area just above the heel of thebead on the outside of the tire for chafing from the wheelflange or damage from tire tools. Any blistering or separa-tion of the chafer strips from the first ply requires repair-ing or replacement of the chafer strip. If cords in the firstply under the chafer strip are damaged, the tire should bescrapped.

If protruding bead wire, bead wire separation, or a badlykinked bead is found, the tire is unsafe and cannot becontinued in service.

Inner Casing

Check any tire which was marked for bulges when the tirewas mounted and inflated. If no break is found on theinside of the tire, determine whether an area of separationexists. If separation is found, the tire should be discardedunless there is only a small localized separation betweenthe tread or sidewall rubber and the cord body. In thiscase, spot repair or retreading may be satisfactory.

Any tire found with loose, frayed, or broken cords insideshould be scrapped.

Important: Do not use an awl or any pointed tool on theinside of a tubeless tire for probing or inspection pur-poses, and do not pierce or cut into the liner. This willdestroy the air-retaining ability of the tire.

Repairability

Tires are repairable which have minor injuries through thechafer strip or slight injuries caused by tire tools in thegeneral bead area, provided the injury does not extendinto the plies of the tire, and if there is no sign of separa-tion in the bead area. If the chafer strips are loose orblistered, they can be repaired or replaced. Shallow cutsin the sidewall or shoulder rubber are repairable only ifthe cords are not damaged.

Liner blisters smaller than 4 inches by 8 inches may berepaired if there are not more than two in any one quartersection of the tire, and no more than five in the completetire. Normally, it would be more economical to postponethis repair work until the time the tire is retreaded.

Due to differences between military and commercialpractices, refer to the applicable documents used by yourorganization for detailed repair procedures.

Tube Inspection

In tube-type tires, failure of an inner tube can easily causeirrepairable damage to the tire in which it is mounted, aswell as to the wheel and the aircraft itself. It is veryimportant that tubes be of the proper size and equippedwith the correct valves.

When inspecting tubes, do not inflate with more air thanis required to simply round out the inner circumference ofthe tube. Too much air places a strain on splices and areasaround valve stems. In addition, excessive air pressure willdamage fabric-base tubes by causing the fabric base to pullaway from the outside of the tube.

Inspect the tube carefully for leaks while under pressure,preferably by submerging it in water. If the tube is toolarge to be submerged in an available water tank, spread

water over the surface as you inspect the tube, and lookfor bubbles.

Examine the tube carefully around the valve stem forleaks, signs of valve pad separation, and bent or damagedvalve stems.

Tubes with severe wrinkles should be removed fromservice and scrapped. These wrinkles are evidence ofimproper fitting of the tube within the tire, and wherevera wrinkle occurs, chafing takes place. A blowout couldresult.

Inspect tubes for evidence of chafing at the toes of thetire beads. If considerable evidence of this chafing ispresent, remove the tube from service and scrap it.

Where the heat is greatest, the tube has a tendency to bestretched over the rounded edge of the bead seat of thewheel. This is one of the reasons why, when mountingtube-type tires, tubes should always be inflated until thetire beads are in position, and then completely deflatedand reinflated to the final pressure. The stretch on thetube is then equalized throughout its inner and outerperiphery.

Also check tubes for possible thinning out due to brakeheat in the area where they contact the wheel and beadtoes. The “set” or shape of the tube can assist in deter-mining when it should be removed from service becauseof thinning in the bead area. In addition, feeling the tubewith the fingers in the area of suspected failure will, aftera little practice, enable you to tell when the life has goneout of the tube and it should be scrapped. On wheelswith only one brake, this heat-set condition will normallyshow up on only one side of the tube. In those caseswhere the brake is a considerable distance from the rim,it is unlikely that this condition will ever be encountered.

In cases where brake heat is a recognized factor, carefulchecking of the tube and tire beads should be made toprevent a failure which might be disastrous. In suchinstances, fabric-base tubes should always be used. Thesehave a layer of nylon cord directly imbedded in the innercircumference of the tube to protect them from thinningout under brake heat. Additional protection is alsoprovided against the chafing action of the tire bead toes,and from damage during mounting and demounting.

A definite schedule should be set up to provide for regularinspections of tires and tubes. However, if an aircraft hasmade an emergency or particularly rough landing, the tireand tube should be demounted and inspected as soon aspossible to determine whether any hidden damage exists.The wheel should also be examined at the same time.

T U B E L E S S A S S E M B L Y : A I R L O S S R E F E R E N C E C H A R T A i I

Items listed in Column A are often over looke d when check ing

a newly installed t ire

The po in t o f l eakage fo r i t ems l i s ted in Co lumn B can be

checked by the app l i ca t ion o f a soapy wate r so lu t ion ( leak

d e t e c t o r ) to the ‘ t i re and whee l assembly o r by immers ing

t h e a s s e m b l y i n water. It is unusual for a t ire to s u d d e n l y

become a leaker after it is partially worn and has not leaked

previously. Pressure loss in used tires is usually caused by

factors other than the tire.

Items i n Co lumn C can be fu r ther checked w i th the t i re

removed.

l I t e m s i n Column D shou ld be checked be fo re a t i re is re -

moved f rom the whee l when a new t i re i s to be ins ta l l ed .

N O T E : Inflation must be at operating pressure or Tire and Rim

maximum pressure when testing for points of air pressure loss.

*Caution: Care should be taken that the bearings and cup area are

not contaminated while testing for leaks.

The chart shown above will help in setting up a uniformcheck list and inspection methods.

Air Pressure Loss

There can be numerous causes for loss of air pressurewithin an aircraft wheel and tire assembly; thus, it iseconomical and wise to follow a systematic check list.Factors which may seem to be only distantly related tothe problem are often the underlying cause of a reportedpressure loss.

Initial Stretch Period (Tire Growth) - Most aircraft tiresare of nylon cord construction, and a certain amount ofstretch occurs after the tire has been inflated to operatingpressure. This in itself will reduce air pressure within thetire. It is absolutely necessary that the tire be inflated toits operating air pressure and let stand at least 12 hours inorder to permit this “growth” of the cord body. It mayresult in as much as 10% drop in air pressure. Compensateby reinflating the tire to the correct pressure.

After reinflating to operating pressure, the pressure dropin any 24-hour period will determine actual pressure lossdue to normal diffusion or any leaks that may be present.This pressure loss check should be made as soon as possi-ble after a tire is mounted and before a tire is placed inservice.

Changes in Air Temperature Was the tire inflated in aheated room and then stored outside? Air pressure willdrop approximately 1 PSI for every 4’F drop in tempera-

ture. Tires should be checked and pressure-adjusted forspecific requirements, otherwise the actual operatingpressure may not be correct. 9

Venting of Tubeless Tires Tubeless aircraft tires arevented in the sidewall area to permit any air that is trap-ped in the cords, or that is diffused through the liner intothe cords, to escape. This venting prevents pressure build-up within the cord body which could cause tread, side-wall, or ply separation.

Vent holes are sometimes inadvertently closed by spilledsolvent or by tire paint. They may also be covered duringthe retreading process. Check for evidence that tubelesstires have been revented after being retreaded.

The vents themselves will not cause undue air loss. Therate of diffusion will vary by manufacturer and individualtire. Discounting tire growth (initial stretch period)during the first 12 hours after inflation to operating pres-sure, the maximum permissible pressure drop due to dif-fusion is 5% in any 24 hour period.

Vent holes penetrate the sidewall rubber to, or into, thecord body and may vary in size, depth, and angle. Theamount of air diffused through these holes also varies.Thus, when water or a soapy solution is brushed over theoutside of an inflated tubeless tires, air bubbles form.Some vent holes may emit a continuous stream of bub-bles. This is normal and does not mean that there is any-thing wrong with the tire. In fact, as long as a tubeless

Tightening wheel bolts with torque wrench

tire is in an inflated condition, air will be coming out ofthese vent holes. When the rate of loss exceeds 5% in 24hours, recheck for possible injuries.

Wheel Inspection

Inspect the entire wheel for damage. Wheels which arecracked or injured should be taken out of service and laidaside for further checking, repair, or replacement.

10

Thermal Fuse Plugs - Some aircraft main wheels, such asthose used by the Hercules and JetStar, have fusible plugsthat are designed to melt at specific elevated temperaturesand relieve air pressure to prevent the tire blowing out orthe wheel breaking. Should air be lost due to the meltingof one of these plugs, it is recommended that the tireinvolved be scrapped. However, an effort should be madeto determine whether air could have been lost around theplug because of its being improperly installed.

A faulty thermal fuse may cause leakage and require re-placement. Usually this is the result of a poor bondbetween the thermal melting material and the bolt body.

The sealing surface for the thermal fuse gasket must beclean and free from scratches and dirt. In some cases, thesurface can bc repaired in accordance with the wheelmanufacturer’s instructions. Be sure that the scalinggasket is the one specified by the wheel manufacturer.The gasket should be free of distortion, cuts, etc.

NOTE: If a tire with inner tube is used with a wheelhaving fuse plugs, the fuse plugs are of course ineffective.However, it is necessary to cover the fuse plug with a high-temperature filler material, such as TYGO-FIL 505, toprevent the sharp edges of the fuse plug and its openingfrom chafing the tube. Procedures for doing this aredetailed in the wheel overhaul handbooks.

Improperly Torqued Wheel Tic Bolts - The torque values Matching tires on dual wheels is necessary so that each tireand torquing procedures specified by the wheel manu- will have the same contact area with the ground and there-facturer should be followed to assure adequate com- by carry an equal share of the load. Only those tires

pression of the sealing O-ring under all temperatureconditions. Low torques, low temperature, and shrinkageof the wheel halves may cause a significant lessening ofcompression on the O-ring seal, leading to air loss.

Wheel Gouges and Scratches - Wheel gouges and scratchesare the result of handling damage or the improper use oftire irons. Leakage may show in the O-ring seal or bead-to-wheel contact area.

Corrosion or Wear in Bead Ledge Area - This usuallyoccurs at the toe area of the tire bead. The leakage willshow at the bead-to-wheel contact area.

Damaged Sealing Surfaces - Look for damage or im-proper machining of sealing surfaces. Care should betaken to see that there is no handling damage. Any ir-regularities should be corrected before remounting thewheel and tire.

Foreign material or paint can impair the sealing surface.Thus, all foreign material should be cleaned from the seal-ing surface before assembly of the wheel. A light. evencoat of primer is permissible. However, the surface mustbe free of dirt.

W h e e l Assembly Holes - Where through bolts are used toattach such items as drive lugs for brake assembly, etc.,the mounting screws or bolts must be properly sealed.The recommendations of the wheel manufacturer shouldbe followed.

Wheel Cracks - A number of methods of crack detectionare available such as penetrant inspection, eddy currentinspection, and ultrasonic inspection. Wheel cracks canusually be traced to fatigue failure. Leakage may occur inthe well area.

Improper Installation of O-rings - O-rings are used be-tween wheel halves to seal the wheel when inner tubes arenot used.

Twisting of the O-ring, or failure to provide lubricantwhen specified may cause loss of air. The wrong size ortype of O-ring, or an O-ring of the wrong compositionmay also cause leakage.

Inspect used O-rings carefully. Be sure they are not thin-ned out, deformed, chipped, damaged, or otherwisedeteriorated. After cleaning, used O-rings should beallowed to shrink at least 3 hours before being reinstalled.

Matching Dual Tire Diameters

having inflated diameters within the following listedtolerances should be paired together on dual wheels.

Tires should not be measured until they have beenmounted and kept fully inflated for at least 12 hours atnormal room temperatures to allow for normal tiregrowth.

Maximum ToleranceO.D. Range of Tires Permissible

up to 24”

25” to 32”

33” to 40”

41” to 48”

49” to 55”

56” to 65”

66” and up

l/4”

5/l 6”

318”

7/16”

1/2”

9/l 6”

5/8”

Courtesy of B. F. Goodrich Co.

Tie Pressures

Undoubtedly, proper inflation is the most importantmaintenance factor in ensuring safe, long service fromaircraft tires. To ensure correct inflation for aircraft grossweight and runway load limits, refer to the proper tech-nical publications.

Tire pressure should only be checked when tires are cool.A wait of at least two hours after a flight is recommendedbefore checking pressures, three hours in hot weather.

Difference of air pressure in tires mounted as duals,whether main or nose, should be cause for concern,because it ordinarily means that one tire is carrying moreof the load than the other. If there is a difference of morethan 5 Ibs., it should be noted in the aircraft records. Therecords should then be referred to on each subsequentinflation check. Impending tire or tube failure can oftenbe detected by this method.

Load Recommendations

There is a limit to the load that any aircraft tire can safelyand efficiently carry. Loading aircraft tires above thelimits can result in these undesirable effects:

Undue strain is put on the cord body and beads of thetire, reducing the factor of safety and service life.

There is greater chance of bruising when striking anobstruction or upon landing (bruise breaks, impactbreaks and flex breaks in the sidewall or shoulder).

The possibility of damaging wheels. Under the severestrain of an extra load, a wheel may fail before the tiredoes.

NOTE: While additional air pressure (inflation) to offsetincreased loads can reduce excessive tire deflection, it putsan added strain on the cord body and increases its sus-ceptibility to cutting, bruises, and impact breaks.

When tires are inflated under load, the recommendedpressure should be increased by 4%. The reason is thatthe deflected portion of the tire causes the volume of theair chamber to be reduced, and increases the inflationpressure reading.

Effects of Underinflation

Underinflation results in harmful and potentially dan-gerous effects. Aircraft tires which are underinflated aremuch more likely to creep or slip on the wheels on land-ing or when brakes are applied. Tube valves can shear off,and the complete tire, tube, and wheel assembly may bedestroyed under such conditions. Too-low pressure canalso cause rapid or uneven wear at or near the edge of thetread.

Underinflation provides more opportunity for the side-walls or the shoulders of the tire to be crushed by thewheel’s rim flanges on landing, or upon striking the edge

Unprepared landing sites are hard on tires and make more frequentinspections necessary.

of the runway while maneuvering the aircraft. Tires mayflex over the wheel flange, with greater possibility ofdamage to the bead and lower sidewall areas. A bruise,break, or rupture of the tire’s cord body can result.Severe underinflation may result in cords being loosenedand the tire destroyed because of extreme heat and strainproduced by the excessive flexing action. This samecondition could cause inner tube chafing and a resultantblowout.

Tie Gauges

Quite often it is found that differences in reported airpressures are due entirely to the variation in accuracy ofdifferent gauges, rather than in any change in air pressure.

It is not unusual to find an inaccurate tire gauge in con-stant use with a tag that states that the gauge reads acertain number of pounds too high, or too low. Un-fortunately, this error may not be consistent as differentpressures are checked. A tire gauge reading 10 lbs. high at80 lbs. pressure may very well read 25 lbs. too high at 150lbs. pressure. Therefore, inaccurate tire gauges shouldeither be repaired or replaced. They should not be con-tinued in service.

12

Cold temperatures may also affect tire gauges and causepressure readings lower than they actually are. Occasion-ally, too, a gauge has been mistakenly treated with oil orsome other lubricant in the expectation of making it workbetter. This, of course, can actually cause incorrect read-ings, and will probably render the gauge unfit for furtherservice.

It is good practice to have gauges recalibrated periodically,and to use the same gauge for performing an inflationcycle - the original 12- or 24-hour stretch period.Good quality, dial-type gauges are highly recommendedfor all tire maintenance installation - regardless of size!

Putting wheel halves into tire.

Vibration and Balance

A major complaint in aircraft operation is that of vibra-tion or shimmy. Although tire balance is usually suspect,there are other items of the tire, wheel, and gear assemblywhich can be the cause of the vibration:

l Wheel assembly installed before full tire growthachieved.

l Axle nut improperly torqued, resulting in a loose wheelbearing.

l A wrinkled or mis-shaped tube, or air trapped betweentire and tube due to improper inflation procedure.

The use of an inaccurate pressure gauge, resulting inimproper inflation pressures.

An improperly mounted tubeless or tube-type tire.

l Poor alignment of the landing gear.

A wheel may be bent or twisted.

l Worn or loose landing gear components may be present.

The tire diameters not matched, or the tire inflation notequalized on dual tire installations.

l A tire may be flat-spotted.

Improperly seated beads and, of course, wheel and/ortire imbalance.

Mounting and Demounting Aircraft Tires

General - Remember that strict attention to detail isrequired when mounting and demounting tires on air-craft. It is not a good practice to try to mount ordemount tires on guesswork. Before beginning, get thespecific data you will need regarding wheel bolt torque,proper inflation pressures, and so forth for the job athand.

Of course, almost every experienced aircraft tire serviceman has developed methods which are more or less hisown, and undoubtedly some of these methods are aspractical as the suggestions that will be given here. In onearea, however, there can be no compromises or shortcuts;

Installing O-ring between wheel halves. Layout of parts required for wheel and tire buildup.

that is in the area of safety. The mounting and demount-ing of aircraft tires should be accomplished with as muchattention to safe working practices as possible. Use onlythe proper tools and suitable protection devices, and makeevery effort to avoid damage to tires, tubes, and wheels.When working on engines or landing gear, tires should becovered so that oil does not drip on them.

The following general items should be helpful: Beforemounting any tire, examine the wheel carefully to makesure there are no cracks or damaged parts. Check thermalfuse plugs and tubeless tire inflation valves for properinstallation and absence of damage.

Nearly all aircraft wheels being made for modern aircraftare of the split-wheel type, including those for theHercules and the JetStar. Tires are much easier to mountand demount on this type of wheel. When inner tubes arenot used, the split wheels are sealed against the loss of airby an O-ring mounted in a groove in the mating surface ofone of the wheel halves. Be sure to inspect the O-ring fordamage and replace it if necessary. When reinstalling-theO-ring, lubricate it as specified by the wheel manufacturerand insert it in the wheel groove. Make sure that theO-ring is free of kinks or twists and is seated properly.

Assemble wheel halves of split-type wheels with the lightsides (an “L” is impressed on the flanges on some wheels)180’ apart from each other to ensure minimum out-of-balance conditions. If the wheel has balance weights,position the wheel halves with balance weights 180’apart.

Be sure that nuts, washers, and bolts used to assemblesplit-type wheels are in proper order and that the bearingsurfaces of these parts are properly lubricated. Tighten torecommended torque values.

When demounting a tire, always remove the valve core anddeflate the tire completely before beginning work. Therehave been many serious accidents caused by failure to

follow this important step. It is also recommended thattires be deflated before wheels are removed from theaircraft. They should then be reinflated if they are to bechecked for leaks.

NOTE: Use caution when unscrewing valve cores; the airpressure within the tube or tire can cause a loosened valvecore to be ejected like a bullet.

With any type of wheel, the tire beads must be loosenedfrom the wheel flange and bead seat before any furthersteps are taken in demounting. Be very careful not toinjure the beads of the tire or the relatively soft metal ofthe wheel. Even with approved tools, extreme care mustbe taken.

Always use a bead breaker to loosen tire beads. Applypressure around the entire circumference of each sidewallto separate the tire from the wheel-half flanges. Do notuse a pry bar, tire irons, or any other sharp tool to loosentire beads because the beads, or the wheel, may bedamaged. Break the beads away from the wheel flangesbefore loosening the tie bolts and pull both parts of thewheel away from the tire.

Tubeless Tires - Tubeless tires fit tighter on the wheelthan do the tube type. It is therefore desirable to lubri-cate the toes of the beads with an approved 10% vegetableoil, soap, or plain water before installation. This willfacilitate mounting, and accomplish proper seating of thetire beads against the wheel flanges. The air pressurecontained in a tubeless tire seals the tire bead ledge toprevent loss of air. When using the lubricant, be carefulnot to get the solution onto this area. The wheel beadledge must be clean and dry to assure proper sealing of thetubeless tire bead.

Be sure to inspect the valve seal when installing a tubelesstire. The valve seal is a packing ring, or O-ring, installed inthe valve hole of each wheel. Its purpose is to prevent theloss of air when an inflation valve is mounted through the

hole. If the valve seal is cracked, or aged in appearance, orin any other way suspect, replace it with a new one.

Care must be taken when demounting tubeless tires toavoid damaging the wheel’s O-ring groove and matingregister surfaces. The ledge area that seals the tire bead,and the inflation valve sealing area are two other criticalpoints. If these places have sustained damage, the wheeland tire unit will fail to maintain the required air pressure.

Do not fasten the valve to the rim until this procedure hasbeen carried out; then install the locking nut (or nuts) andtighten securely. Put on the valve cap and tighten withthe fingers.

NOTE: When tubes are used, cover fuse plugs asdescribed previously.

Inflation

Tube-Type Tires - Dust the inside of the tire and innertube with talc or soapstone before the tube is installed inthe tire. This will prevent the tube from sticking to theinside of the tire during inflation, and lessen the chancesof the tube wrinkling or thinning out. Also rememberthat it is a good practice to always mount the tube in thetire with the valve projecting on the serial-number side ofthe tire.

Install the tube entirely deflated, after the talc has beenapplied. Insert it in the tire (folding may make this easier)and inflate until the tube is just rounded out. The valvecore should be in the valve during this operation.

After the tire and tube are mounted on the wheel, theassembly should be placed in a safety cage for inflation.The cage is best located against an outside wall that isconstructed so as to be able to withstand the effects of anexplosion of tire or wheel, if necessary. The air line fromthe compressor or other air source should be run to apoint at least 20 or 30 feet away from the safety cage, anda valve and pressure gauge installed at that point. The lineshould then be continued and fastened to the safety cagewith a rubber hose extending from that connection. Thisarrangement makes it unnecessary to reach into the cageto check air pressures, or to be anywhere near the safetycage while the tire is being inflated.

Line up the tube and the tire balance marks. Insert thesection of the wheel containing the valve hole into the tireand push the valve through the valve hole in the wheel.

Balance

14Insert the other side of the wheel while holding the valvein position. Be careful during this operation not to pinchthe tube between the wheel sections.

To seat the tube properly on the wheel, first place the tirein a safety cage and inflate the tire to the recommendedpressure. The tire should then be completely deflated,and once more reinflated to the correct pressure. Thisprocedure helps remove any wrinkles in the tube andprevents pinching the tube under the toe of the bead. Italso avoids the possibility that the tube will be forced tostretch unevenly, and thin out more in some areas than inothers. Finally, it assists in the removal of air that mighthave become trapped between the inner tube and the tire.

Balance in an aircraft wheel assembly is very important.From a wear standpoint, when the wheels are in thelanding position, a heavy spot in a wheel assembly willhave a tendency to remain at the bottom and thus willalways strike the ground or runway first. This results insevere wear at one area of the tire tread, and can neces-sitate early replacement. In addition, an unbalanced tiremay cause severe vibration during taxi, which may affectthe operation of the aircraft.

Balance marks appear on certain aircraft tubes to indicatethe heavy portion of the tube. These marks are approxi-mately l/f-inch wide by 2 inches long. When the tube isinserted in the tire, the balance mark on the tube shouldbe located at the balance mark on the tire. If the tube hasno balance mark, place the valve at the balance mark onthe tire.

inserting valve stem into wheel. Guide pin used for alignment of wheel halves.

When mounting tubeless aircraft tires, the “red dot”balance mark on the tire must always be placed at thevalve that is mounted in the wheel unless otherwise speci-fled by the wheel manufacturer.

Tire Valves

Replacements are available for all tubeless tire valves.Valve repair in the case of a tubeless tire is, of course,simply a matter of installing a new valve in place of thedefective one. Repair-type valves are also available formany tube-type tires, but it is often a better practice toreplace the tube if the indicated valve repairs will gobeyond straightening the threads or replacing valve cores.Be sure to consult the appropriate manuals or regulationsapplicable to your organization before attempting valverepairs.

Tube Repair

Most tube repairs involve valve damage of one kind oranother. However, occasionally a tube might be cut,pinched, or punctured by tire tools during mounting ordemounting. Injuries larger than one inch can be repairedby using a reinforcement patch inside the tube. Thisreinforcement should be of the same material that is usedfor repairing the outside of the tube. An injury smallerthan one inch usually will not need a reinforcementpatch. If a tube has an injury that extends for more thantwo inches, it is best to scrap the tube.

Tie Repair and Rebuilding

Many aircraft tires which become injured in service can besuccessfully repaired. When considered economical, spotrepairs can be made to take care of tread injuries such ascuts, snags, etc. which are through no more than 25% ofthe actual body plies of the tire, and not over 2 inches inlength at the surface. Vulcanized spot repairs are alsomade at times to fill in tread gouges that do not go deeperthan the tread rubber and do not penetrate the cordbody.

Aircraft tires which have become worn out, or flat-spottedand removed prematurely, can often be retreaded andrestored to service. The retreading and repairing of air-craft tires has been practiced for many years and has savedaircraft operators considerable sums of money. Tireswhich might otherwise have been discarded have beenrepaired and retreaded for continued use.

Retreading is a general term meaning reconditioning of atire by renewing the tread, or renewing the tread plus oneor both sidewalls. There are actually four different typesof retreading for aircraft tires, depending on the need orprocess used:

Top Capping - Top capping is used for tires worn to thebottom of the tread design, with no more than slight flatspotting and/or shoulder wear. The remaining tread isremoved and a new tread is applied.

Ful l Cupping - This procedure is useful for tires worn allaround, those flat-spotted to the cords, or those withnumerous cuts in the tread area. The new tread materialis wider than that used on a top cap, and comes downover the shoulder of the tire for several inches.

Three-Quarter Retread - A three-quarter retread isapplied to tires needing a new tread, plus renewing of thesidewall rubber on one side, due to damage or weatherchecking. A full cap is applied, and in addition, approxi-mately l/l 6 inch of the thickness of the old sidewallrubber is buffed off one side. New sidewall rubber is thenapplied from the bead to the edge of the new tread, onthe buffed side only.

Bead-to-Bead Retread - A new tread and both new side-walls are applied by this method.

15

Tube balance mark (heavy point) aligned with tire balance mark

(light point).

Worn through tread reinforcement strips only.

0 K for retread. co.

Non-Repairable Aircraft Tires

The following conditions render an aircraft tire unfit forrepair:

. Any injury to the beads, or in the bead area (exceptinjuries limited to the bead cover or finishing strip).

. Tires with weather checking or ozone cracking of treador sidewall that expose body cords.

. Any tire with protruding bead wire or a kinked bead.

Any tire which shows evidence of ply separation

. Any tire with loose, damaged, or broken cords on theinside.

Tires with broken or cut cords in the outside of thesidewall or shoulder area.

16

n Tires that have been run flat, or partially flat, due to themelting or failure of fuse plugs in the wheels shouldnormally be scrapped even though there may be novisible evidence of damage to the inside or outside ofthe tire. The only exceptions would be where it wasknown that the fuse plug leaked air because of its beingdefective,and the tire was not run in an underinflatedcondition.

SUMMARY

Safety First - and Last

Every day, aircraft maintenance personnel have to con-front many potential safety hazards. Some of thesehazards are fairly obvious and have earned a full measureof respect. Other, perhaps less obvious, hazards yielddramatic results if ignored, and thus make an indelibleimpression on the memory. Carelessness with fuel orhydraulic fluid has produced disasters that were im-mediate and spectacular. Accidents involving movingpropellers are also not soon forgotten by those who wit-ness them.

But how is it with tires?Perhaps because tires are so fami-liar, particularly in the relatively harmless low-pressureforms that equip every car and light truck, it is easy toforget that they can be a real source of danger. Evenautomobile tires are far from being immune to suddenfailure. Tines on parked vehicles have been known toexplode without warning, injuring everyone within range.

Aircraft tires, which must operate in an environment ofhigher pressures, temperatures, and stresses, clearly pose a

much greater danger. Don’t be fooled by appearances.Although aircraft tires are tough, and will stand a lot ofhard use, they are also vulnerable, highly-pressurizedvessels which can kill or maim if something goes wrong.Here are a few points to help reduce that possibility to aminimum:

l Don’t attach anything to a tire with staples, tacks, orpins. Anything that makes a hole in a tire can producea weakened spot with potential for later leaks, orworse. Use pressure-sensitive tape or cement to sticktags to tires, but avoid attaching anything to the beadarea where it may damage the sealing surface.

Avoid prying between the flange and the bead withsharp tools. The use of improper tools is credited withcausing more wheel and tire damage than any othersingle factor. Damaged areas on wheels and tires are notjust nuisances that lead to leaks or premature tirereplacement; they are dangerous. Damaged spots areweak spots, and any weak spot can become the startingpoint for the chain of events that can end in sudden,explosive tire failure.

Be sure to use a safety cage and proper inflation equip-ment when inflating a tire after replacement or repair.Do not use air bottles or booster pumps not designedfor tire inflation. Watch pressure carefully! Don‘t usehigh-pressure air sources (over 500 PSI) unless a suitablepressure regulator has been installed in the system.Rapid overinflation of tires is a leading cause of ex-plosive tire failures.

Always approach inflated tires from the front or rear.This is particularly important if the tire is hot fromlanding or a high-speed taxi run, or failure is suspected.If a tire fails suddenly, it is usually the sidewall, not thetread area, that ruptures. Staying out of the potentialline of fire makes good sense.

These hints do not, of course, begin to complete the listof do’s and don’ts in the area of tire safety. The subjectof tire safety, like that of tires itself, is a very big one.But the essence of tire safety, and good tire managementas well, can be summed up as follows: Use good infor-mation, approved procedures, and proper tools. These arethe key steps which will help you take good care of youraircraft tires, and of yourself.

Using the Tach Generator Tester for an Auto-Tat

by W. S. Milford,Service Representative

Have you ever needed an Auto-Tat (precisionRPM test set) for checking RPM and othertroubleshooting on the Hercules, but didn’t haveone readily available? The sketch in Figure 1shows how to fabricate a wire harness to adaptthe Tachometer Generator Tester, Model TTU/27E, (FSN 6625-799-7813) for use as an accurateAuto-Tat.

2.

3.

4.

After making the wire harness, follow thesesimple instructions:

1. Connect the test set power cable to a sourceof 115VAC, single phase power.

Connect the fabricated wire harness to theTEST GEN input connector on the test set;then plug the other end into the test jack onthe synchrophaser test panel.

Turn the test set ON and allow for warmup.

The engine select (ENG SEL) knob on thesynchrophaser test panel is used to select theengine for which you wish to check the RPM.The digital readout on the test set shows theRPM for the selected engine.

MALE JACK PLUG

P J - 0 6 8 72 INCHESFigure I

CANNON PLUG

by W. C. Barre, Service Representative

The crew entrance door of the Hercules is a sturdy anddurable unit. Given normal care and proper attention tolatches, hinges, and gaskets, it will stand up to a lot ofhard use and still last the life of the airplane.

Nevertheless, there’s always a way to make a good thingbetter. Some Hercules operators have found that they canadd to the strength and convenience of the door forground operations by connecting a covered cable betweenthe hand lanyard attachment point and the door closehook.

The installation shown in the picture is one developed bythe men of the Royal Danish Air Force, who report thatthey got the idea from a similar arrangement they sawinstalled on Hercules’ belonging to the Norwegian Air

Force.

The cable gives additional load support on the side of thedoor opposite the standard telescoping tube support, andoffers both a convenient hand hold and a lift rope whenclosing the door from the inside.

In the door-closed position, the cable should be stowed toprevent personnel from tripping and inadvertently open-ing the door in flight by grabbing the door handle.

17

by T. A. Faber, Aerospace Safety Engineer, Senior

There is an old adage in aviation circles that airplanesare made to fly, and the sooner they are airborne, thesafer they are. The air and ground environments are notalways compatible, and compromises must consistently bemade in favor of the air environment where the aircraft isprimarily designed to operate. While on the ground, air-craft are exposed to all kinds of man-made hazards. Some-times they are treated with the same kind of abuse and in-difference normally associated with the operation of carsand trucks, and all too often the results are catastrophic.Airplanes have been taxied into buildings, ditches, andeach other on their way to and from the runway. Othershave been involved in one of the most costly problemsassociated with ground operations: wheel-well fires whichcan be traced back to overheated brakes. In this respect,the Hercules is no exception.

18

The Hercules is a remarkable airfreighter with an out-standing reputation for safety. But on the ground, as wellas in the air, careful attention to correct operating pro-cedures is the key to avoiding hazards that can lead tocostly damage or personal injury. Nowhere is this moretrue than in regard to the proper handling of main landingwheel brakes.

An article entitled “Hercules High Energy Brakes,” pub-lished in the Vol. 2, No. 1, January - March 1975 issueof the Service News, emphasized the need for pilots to usethe Hercules brakes sparingly. The article pointed out thatalthough the trimetallic brakes have more stopping powerand better high-temperature performance than the oldertype, care must still be exercised to prevent the brakesfrom becoming overheated.

Other than an unusual brake system malfunction, themajority of brake fires are caused by repeated brake appli-cations in a short period of time, thus allowing heat tobuild up. As brake stack temperatures increase, the heat istransmitted from the stack to the brake housing; therebysubjecting hydraulic hoses, seals, and brake cylinders toexcessive temperatures. As a result, seals may leak, hy-draulic hoses are subject to failure, or cylinders canrupture. Should any of these conditions develop, the hy-draulic fluid is expelled and atomized under pressure. Ifthe volatile mist comes in contact with the hot brakes, itcan ignite and can cause extensive fire damage unless im-mediately controlled. Brake fires have occurred after re-jected takeoffs (RTOs), during prolonged taxi operations,while executing “windmill taxi starts,” and after takeoffs.However, such problems may be prevented by the exercise

of due care, and by following the recommendations in thisarticle, as well as those in applicable operating or pro-cedural manuals.

Let’s look at some specific points which will help avoidbrake overheating problems.

During taxi, Hercules pilots should make maximum use ofthe reverse pitch propeller for decelerating the aircraft.The brakes should be used primarily for stopping the air-craft when parking, or whenever needed during RTOs orother emergencies. Pilots should be aware that during taxithe heat buildup from successive low-energy stops iscumulative, and eventually can exceed even the amount ofheat that is generated in the brakes during an RTO. Toassure that an RTO stop can be made, Goodyear, thebrake manufacturer, advises that brake temperatures mustbe no higher than 400’F before takeoff.

After some brake fires, it was found that some pilots in-advertently overheated the brakes by taxiing the aircraftwith their feet on the rudder pedals. In this position, thetips of the pilot’s shoes rested against the tops of the rud-der pedals. As they taxied, the slightest foot pressureagainst the top of the pedal resulted in hydraulic pressurebeing applied to the brakes. This caused “dragging brakes”which, on a long taxi run, could set the stage for a brakefire. As a result of this discovery, pilots have been cau-tioned to place their heels on the cockpit floor when taxi-ing the aircraft.

Flight manuals and maintenance manuals contain infor-mation, cautions, and warnings concerning overheatedbrakes. Significant points to remember are:

When taxiing, use low-speed ground idle to reducetaxi speed, and use reverse pitch propeller to de-celerate the aircraft.

Do not allow brakes to “drag.”

The maximum capability of the brakes may easilybe reached during an RTO, and the service lifeshortened to the point where they are no longerserviceable. Have your maintenance crew inspect thebrakes after a heavy aircraft-weight RTO, or any timehot brakes are suspected.

If maximum braking has been used in landing and/ortaxiing, it is recommended that the gear be left ex-tended after takeoff for a minimum of 15 minutesbefore retraction.

19

Remember that peak temperatures occur in the brakeassembly in approximately 1 to 5 minutes, and occurin the wheel and tire assembly in approximately 20 to30 minutes after maximum braking. Two to threehours are required for brakes to cool enough for saferemoval when artificial cooling is not used.

If overheated brakes are indicated, notify the firedepartment and have the brakes inspected. All per-sons other than fire department personnel should staywell clear of wheels and tires. Wheels and tires shouldbe approached from only the front or rear, neverfrom the side.

Do not apply coolant directly on a wheel until afterthe tire has been deflated.

It has often been stated that a well-trained, knowledgeablecrew operating a serviceable aircraft can successfully andsafely complete any assigned mission. It is hoped thatthis discussion has provided that extra bit of informationconcerning the Hercules brakes which will help all air-crews to complete missions successfully and safely.

Remember, take care of your brakes and they will takecare of you when you need them. Abuse them, and theymay not be there when the end of the runway is ap-

Some Dont's for Overheated Brakes

l Do not set parking brakes while an overheated condi-tion exists.

Do not taxi the aircraft into a crowded parking area.After clearing the runway, stop, and chock the nosewheel.

proaching.

detachable panels, etc., below approximately one ohm.Plastic and fiberglass surfaces should be coated with anantistatic paint or other controlled-resistance coatingsufficiently dense to allow electrical charges to leak awayto the surrounding metal structure. Also, an electricalpath between the plastic surface and the airframe must beprovided. Of course, the static dischargers must also be ingood condition and properly bonded to the airframe inorder to eliminate spark discharge from the concentrated-charge and low-pressure points on the airframe to thesurrounding air.

Precipitation Static DissipationA recurring topic for discussion with Hercules operatorsand field representatives has been precipitation static, or“p-static”, noise in the radio receivers.

P-static, as heard in a radio receiver, is caused by anelectrical discharge known as corona discharge from theaircraft. This discharge occurs when the electrical poten-tial of the aircraft is high with respect to the space sur-rounding the aircraft.

20

When flying through precipitation or clouds, a largenumber of particles will strike the aircraft and bounceaway. An electrical charge is transferred at each particleimpact. Under typical “charging” conditions, thispotential can reach values of 100,000 to 200,000 volts ina fraction of a second. These charges can also be inducedwhen flying near or between electrically charged cloudsduring electrical storms, even if no particles are strikingthe aircraft. When the potential reaches a high enoughvalue, it can cause ionization of the air at sharp edgeswhere the charge is concentrated, and short bursts ofcharge will be transferred to the surrounding air or to alow-potential part of the aircraft. It is the energy releasedby these pulses, coupled into the radio antennas, thatcauses noise in the aircraft radio receivers.

P-static will normally be severe on ADF, cause “grass” onLORAN A and C, and be objectionable on HF communi-cations. In general, it will be less severe on VHF or VOR.

P-static interference also can be generated by movablecontrol surfaces, detachable panels, and plastic or fiber-glass surfaces not properly bonded to the airframe whichaccumulate static charges and discharge them to a low-potential part of the airframe. This discharging of sur-faces can couple into the receiving antenna systems like aspark transmitter, producing crackling noises in the head-phones.

To avoid this problem, it is necessary to keep the re-sistance between the airframe and all moving surfaces,

Limited research has been done to determine the cause ofp-static on the Hercules, but it appears that aircraft withdecorative paint on the radomes are the worst offenders.This would indicate that the radome is the most probablesource of p-static troubles, so attention has been focusedin this area.

Lockheed engineering advises that the design philosophyregarding static on the radome is this: The charge buildson the surface paint and bleeds through the antistaticpaint beneath. It is conducted through the antistaticpaint to the silver conductive paint (DuPont No. 4929) onthe edge of the radome, from which it passes through aconductive rubber seal and adhesive, or through theradome hinges or braces to the airframe.

The antistatic paint is checked in production to measure 2to 200 megohms between probes spaced six inches apart,before decorative painting. The resistance should be inthis range when measured between needlepoint probesabout six inches apart which have been pushed through tothe antistatic layer. The antistatic paint-to-fuselageresistance should be less than 100 megohms as measuredwith the needlepoints three or four inches from the edgeof the radome. In each case, several places should bechecked and the needlepoint holes should be touched upafter testing is completed.

If the radome checks do not reveal any suspected break-down in the mechanism for dissipating static charge, thenother possible sources of static noise should be sought.The possibility of an electrically isolated surface should beinvestigated. The grounding of antenna bases is notsuspect because this would cause trouble all the time, notjust in the precipitation conditions. The static dischargersshould also be checked, especially the wire braid type.

One final note regarding coatings: Several types havebeen used to avoid p-static problems, with varying results.Olin Astrocoat antistatic paint has yielded good resultsduring Lockheed usage.

THINNER SERRATED PLATESfor Hercules

by C. R. Bush, Design Engineer Specialist

Main Landing Gear

Occasional contact between the main gear torque strutand the vertical beams, or between the main gear pistonand shelf bracket, has been experienced by some Herculesoperators. This can be caused by the buildup of manu-facturing tolerances, shoe misadjustment, wear, and tireunbalance.

A production change has been made on Hercules aircraftserial number LAC 4710 and up to replace the 0.130-inchthick serrated plates (P/N372602-IL and -1 R) that fitbetween the main gear swivel bracket and the upper shoeswith thinner 0.065/0.070-inch thick (P/N 372602-3 and4) plates.

The use of the thinner serrated plate shifts the position ofthe top of the main gear strut slightly inboard, and thepiston and torque strut area slightly outboard, therebyincreasing the clearance in the contact area. The thinnerplates are interchangeable with the existing plates, andmay be substituted on earlier airplanes if increased clear-ance is desired.

Both plates should be replaced together. If theP/N372602-3 and -4 thin plates are not readily available,the -IL and -1R plates can be modified to the -3 and -4configuration by machining the inside of the web of thechannel section to 0.065/0.070 inch thickness, and re-plating the reworked area with cadmium.

SWBR T E D P L A T E

V I E W L O O K I N G D O W N

Static Ground Assemblies deleted from Landing GearThe trend toward deletion of main landing gear (MLG)static grounding devices got its impetus in 1968 when theU. S. Air Force issued T. 0.4A-I-501 which requiredremoval of those devices from all USAF aircraft. Sub-sequently, various airlines and aircraft manufactures fol-lowed suit and the FAA granted its approval.

Lockheed continued installing the MLG static groundingstraps/wires as a precautionary measure. However, it isnow felt that sufficient data has been accumulated toshow that no adverse effects will occur if these devices arenot installed, as the static charge is sufficiently dissipatedthrough the electrically-conducting MLG tires.

MLG static grounding assemblies have been deleted fromthe JetStar, C-5A, C-141, and L-lOll aircraft with noreported problems to date. They have also been deletedfrom production Hercules aircraft serial No. LAC 4691and up.

NOTE: Only approved tires and retreads should be usedas use of nonconducting tires would not allow adequatedissipation of built-up static charges.

21

TowbarShear Bolts

Recent reports from the field indicate that an explanationof the function of the towbar safety shear bolts would behelpful. The AN6 (3/8-inch diameter) bolts that attachthe towhead to the towtube are designed to protect thenose landing gear assembly during towing operations.Under a condition of extreme load, such as an excessivelysharp turn radius, or a sudden start or stop, the AN6 boltswill shear and thereby prevent damage to the nose landinggear strut. These alloy steel bolts are therefore veryimportant, and should be checked periodically to ensurethat they will function properly.

22

It should be remembered that the AN6 safety shear boltinstalled in the aircraft nose landing gear crossbar and towfitting (see Figure 1) is designed to shear only with a pull-ing force. If the Hercules is being pushed backward, thenose landing gear tow fitting will bottom out against thecrossbar and the crossbar bolt will not shear, leaving onlythe towbar’s AN6 bolts to provide the safety shear func-tion.

It appears that the two AN6 bolts in the towbar shearmore frequently than does the AN6 bolt in the nose geartow fitting. One reason for this is that the bolt in thenose gear tow fitting must shear in two places while thebolts in the tow bar have only one shear point. Also,leverage may be exerted on only one of the two bolts inthe towbar, causing it to overload and shear.

The original Hercules towbar was made by Lockheed(P/N403980), and since the towhead and towtube endconnections were made of high strength steel, the twoAN6 safety shear bolts were installed without bushings.

The newer towbar made by Ventura (P/N 126-000-101)has a towhead and towtube end connector made fromgeneral purpose structural steel. Although the steel usedis of good quality, it does not provide for proper shearingof the AN6 bolts, Therefore, high strength steel bushingsare used in the towtube-to-towhead attachment holes.These bushings must be sharp and flush at the towtubeand towhead mating surfaces. Also, larger washers shouldbe used to prevent the bolt head and nut from pulling intothe bushing holes. (See Figure 2.)

Under no circumstances should the bushings be removedand 5/8-inch bolts installed, since a considerably higher

shear load would be required for separation of the larger-than-standard bolts. Also, the specified bolts must not bereplaced with higher strength bolts of the same size.

If there is a problem with the AN6 bolts shearing in thetowhead attachment, it is likely that the towing instruc-tions and limitations detailed in the Hercules maintenancemanuals are not being followed. The ground handling-towing and taxiing section should be reviewed.

BOLT

W A S H E R

•

w.

HERCULES the airlifter that helps people.

Thi Hercules is the wortchone of the 1if, trt.tnsporting tons of equiprncm 1 ftnd sup1>llos with ease over natural barrietrs 1ha1 defy surface tran.s-portation. Arc1ic tundras, oceans, n1ountoln1, deserts, or junglM - &he l,.e.rculot

knows no boundaries. Whr.tthor it be earth nuwers to carve out funns and roads, drilling mach-­inery for oil wells, or hfeuv1n9

...... _11

·~· --

•

-

supplte$ fO 1 di-satter+wracked

1rea. the depenelob!e Hlf<U._ ti fint on the scene - tnd often the .nJ \va'f 1:he IOIM

""" .... .., be reached.

CUSTOMER SERVICE DIVISION LOCKHEED-GEORGIA COMPANY A OIVISI ON Of l..OCICHEED A I ACFIArT co-.~OAA.T"ION t.'AH.lf.lfA,(ll(>A(UA ~~:)

' • .. '

•

.. • ..

••

•

•• ··~. l