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SEPTEMBER–OCTOBER 2001

F L I G H T S A F E T Y F O U N D A T I O N

Aviation Mechanics Bulletin

Inadequate Shot PeeningCited in Two Failures of

Left-main Landing Gear onFokker 100

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F L I G H T S A F E T Y F O U N D A T I O N

Aviation Mechanics BulletinDedicated to the aviation mechanic whose knowledge,craftsmanship and integrity form the core of air safety.

Robert A. Feeler, editorial coordinator

Inadequate Shot Peening Cited in Two Failures ofLeft-main Landing Gear on Fokker 100 .......................................................1

Maintenance Alerts ........................................................................................8

News & Tips ............................................................................................... 18

September–October 2001 Vol. 49 No. 5

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On the cover: A close-up of the fractured left-main landing-gear outer wheel on a Fokker 100.Left-main landing-gear components on the airplane failed during two landing incidents inAustralia. (Source: Australian Transport Safety Bureau)

Inadequate Shot PeeningCited in Two Failures ofLeft-main Landing Gear

On Fokker 100

The Australian Transport Safety Bureau said that the twoincidents, which occurred three months apart in the same

airplane, involved cracks in parts of the landing gear that hadbeen repaired and in areas where shot peening was faulty.

FSF Editorial Staff

About 1030 local time July 4, 1999,the crew of a Fokker 100 experienceda severe vibration from the left-mainlanding gear when they applied thewheel brakes during the landing rollat Norfolk Island Airport after a do-mestic flight in Australia. The air-plane received minor damage; noneof the 43 people on board was injured.

Three months later, at 1040 localtime Oct. 9, 1999, the same airplanewas being landed at the same airportwhen the crew experienced a severevibration throughout the airframe.

The airplane received substantialdamage; none of the 84 people onboard was injured.

The incidents were investigated bythe Australian Transport SafetyBureau (ATSB), which issued atechnical report on the analysis ofthe airplane’s left-main landing-gearfailures and air safety occurrencereports on each incident.

ATSB said in its technical report that,in the first incident, the outboardwheel on the left-main landing gear

2 FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • SEPTEMBER–OCTOBER 2001

separated from the wheel hub duringlanding and that the second incidentinvolved the fracture of the left-mainlanding-gear upper-torque-link at-tachment lugs.

A similar landing accident in May2001 involving a Fokker 100 atDallas-Fort Worth (Texas, U.S.) In-ternational Airport resulted inseveral safety recommendations fromthe U.S. National TransportationSafety Board (see “InspectionsRecommended for Some Fokker 100Landing-gear Cylinders,” page 3).

After the first incident, examinationof maintenance documents showedthat the wheel had accumulated 99.8hours in service and 77 landings andtakeoffs since overhaul. The air safe-ty occurrence report said that duringthe overhaul, repairs had been per-formed to remove scoring from thehub that was caused by “rubbingcontact with the brake heat shieldduring service.”

The repairs included the reductionof the hub diameter by 0.02 inch(0.51 millimeter). The work wasconducted in accordance withrequirements of repair no. 15 of theAircraft Braking Systems Corp.maintenance manual of Sept. 27,1998, and the Aircraft BrakingSystems Corp. authorized an increaseof the repair tolerance forthe hub-diameter reduction frombetween 6.41 inches and 6.48 inches

(16.28 centimeters and 16.46 centi-meters) to between 6.39 inches (16.23centimeters) and 6.48 inches. In-structions for the repairs said that af-ter material was removed for thehub-diameter reduction, the repairedarea was to be shot peened. The airsafety occurrence report said thatshot-peening parameters were to be“adjusted to produce a specific sur-face quality.”

The technical report said that a com-parison of the surface of the repairedarea and the original surface of thewheel hub revealed a “markedly dif-ferent” intensity in the shot peeningof each area. The intensity of shotpeening was lower on the repairedsurface, the report said.

“This variation would be expected tolower the resistance of the wheel tofatigue cracking,” the air safety oc-currence report said. “The lower lev-el of compressive residual stressassociated with the less intenseshot-peening process applied to therepaired [area] would also increasethe likelihood of fatigue failure un-der normal loading conditions.”

Shot peening is a method of strength-ening a metal’s resistance to fatigueand other types of stress-induceddamage, typically by using com-pressed air or a rotating wheel to hurlround metallic shot at high speedtoward the surface of the metal.1

Continued on page 4

FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • SEPTEMBER–OCTOBER 2001 3

The U.S. National Transportation SafetyBoard (NTSB), citing a May 23, 2001, ac-cident in which the right-main landinggear of an American Airlines Fokker 100failed during touchdown at Dallas-FortWorth (Texas, U.S.) International Airport,has recommended inspections of Fokker100 series airplanes with the same typeof main landing gear.

The lower portion of the right-mainlanding-gear assembly separated fromthe airplane, and the airplane wassubstantially damaged. None of the 92people in the airplane was injured.

The right-main landing gear was part ofthe airplane’s original equipment and hadaccumulated 21,589 flight hours and15,380 takeoffs and landings duringnearly nine years in service.

Post-accident examination revealed thatthe right-main landing-gear cylinder,manufactured by Messier-Dowty from aforged block of an aluminum alloy, hadbroken into five pieces.

One of the fractures on the forward faceof the main landing-gear cylinder devel-oped from a pre-existing crack that mea-sured 2.5 millimeters (0.1 inch) in depthand 12 millimeters (0.5 inch) in lengthabove a dowel pin bushing and forwardof the up-stop damper abutment. Exami-nation revealed “features typical of aforging fold,” which NTSB defined as “adefect that can be caused by hot metalthat flows back onto itself while it is flow-ing into the die cavity.”

After the accident, U.S. operators ofFokker 100s inspected similar main land-ing gear using eddy current inspections.

“One [main landing gear] identifiedas having flaws similar to those found onthe accident [airplane] was … found tocontain a pre-existing crack … on theforward face of the [main landing-gearcylinder] above a dowel pin bushing andforward of the up-stop damper abutmentand also originated from a forging-folddefect,” NTSB said.

The manufacturer began conductingeddy current inspections of a larger areaof the main landing gear and found sixunits with flaws similar to those on theaccident airplane.

“Several of those cracks were outside thearea covered by the inspections con-ducted by U.S. operators,” NTSB said. “Ifthese flaws and/or cracks had remainedundetected, they could have continued topropagate to their critical crack length andresulted in [main landing-gear] failures.”

In June 2001, Fokker Services issuedservice bulletin (SB) F100-32-128 Revi-sion 1, and Messier-Dowty issued SBF100-32-100 Revision 1. The SBs ad-vised Fokker 100 operators to performone-time, fleetwide eddy current in-spections of the forward face of somemain landing-gear cylinders and, ifnecessary, to repair or to replace them.(SB F100-32-100 Revision 1 said thatthe SB applied to “part/type numbers201072011 through 201072016, whichincludes main-fitting subassemblies201072283, 201072284 and 201251258[main fittings 201072383, 201072384and 201072389]).”

NTSB said, “Because this inspection areagoes beyond that already inspected by

Inspections Recommended forSome Fokker 100 Landing-gear Cylinders

Continued on page 4


U.S. operators and because of the poten-tially catastrophic consequences of [amain landing-gear] failure, [NTSB] is con-cerned that the eddy current inspectionscalled for in the SBs are not mandatory.Therefore, [NTSB] believes that the FAA[U.S. Federal Aviation Administration]should require all operators of Fokker 100series airplanes equipped with the[main landing gears] that are identified inMessier-Dowty SB F100-32-100 Revision1 to immediately conduct an eddy currentinspection of the [main landing-gear] cyl-inders for forging folds and fatigue cracks… and to remove from service all landinggear in which such forging folds or cracksare found until the gear are returned toan airworthy condition.”

NTSB also recommended that FAA:

• Require Fokker Services to “im-mediately determine a repetitive

inspection interval that will preventstructural cracks in main landing gearthat are identified in Messier-DowtyService Bulletin F100-32-100 Revi-sion 1 from propagating to failurebetween inspections”;

• Require operators of Fokker 100 air-planes with the main landing gearidentified in the Messier-Dowty SB toperform periodic eddy current inspec-tions of main landing-gear cylindersfor forging folds or fatigue cracks;

• Require Fokker Services to reviewforging processes and inspection pro-cedures discussed in the Messier-Dowty SB and to modify them, ifnecessary; and,

• Review the processes used byMessier-Dowty to manufacture andinspect landing gear and requiremodification if necessary.♦

The intensity of the shot peeningis determined by shot size and airpressure or the speed of the rotatingwheel.

The result of the process is that re-sidual compressive stress is createdon the metal’s surface; the presenceof the residual compressive stress isdesigned to delay the initiation orextension of fatigue cracks that oth-erwise might develop from featureson the surface.

The U.K. Air Accidents InvestigationBranch said, in an explanation of theshot-peening process that was includ-ed in a report on a 1995 accident, “Es-sentially, applied tensile stresses are

offset by the residual compressivestress from peening.”2

Viewed using low-power magnifica-tion, effective shot peening appearsin the form of small indentations inan even pattern across the surface ofthe treated metal.

The ATSB air safety occurrence re-port on the July 4 landing incidentsaid that the wheel failed because offatigue cracking that began at thesurface of the metal in the repairedarea of the wheel hub.

“No single stress point concentratorhad started the cracking,” the reportsaid. “It had begun at numerous


closely spaced points around the cir-cumference of the hub, known asratchet marks. This was consistentwith sideways flexure of the wheelweb and with crack growth from therepaired surface of the hub. Therewas no indication the growth hadstarted at any crack that had beenpresent prior to the repair.”

The fatigue crack spread in a mannerconsistent with the sideways flex-ing of the wheel web — flexing thatoccurs “when a turning moment(torque) is applied to the main land-ing gear while the wheels [are] rotat-ing, such as during ground turning orcrosswind landings,” the report said.

High crosswind components weretypical during takeoffs and landingsat Norfolk Island, the flight crew said.

The technical report said that the abil-ity of the main landing-gear wheelsto withstand such a turning momentdepends on “the resistance of thecomponent … and the magnitude andfrequency of the alternating stressescreated by the applied loads and anygeometric stress concentrators.”

The final fracture of the wheel occurredjust after touchdown, an indicationthat significant torque was being ap-plied to the left-main landing gear atthe time, the report said.

The incident investigation also re-vealed that, during the last overhaul

before the landing incident, a portionof the left-main landing-gear shim-my damper had been reassembled in-correctly. The error had little effect,if any, on the initiation of the fatiguecrack or the spread of the fatiguecrack, the air safety occurrence reportsaid. (Similarly, the air safety occur-rence report on the second landingincident said that the incorrect assem-bly of the shimmy damper had littleeffect, if any, on the initiation or de-velopment of the fatigue cracks citedin that incident.)

The report on the second landing in-cident said that the upper torque-linkattachment lugs on the left-main land-ing gear had broken during landing.(The upper torque-link attachmentpoint was an integrally forged dou-ble lug with a stiffening web betweenthe two lugs.) Examination of main-tenance documents showed that thelanding gear had accumulated 658cycles since overhaul and 16,579 cy-cles since new.

The technical report said that the up-per torque-link attachment lugs hadfailed because of “the extension ofpre-existing cracking in the lug-stiffening web while torque wastransmitted through the torque links.The cracking in the stiffening webwas caused by stress corrosion.”

The report said that the extension ofthe crack was consistent with load-ing that resulted in sideways flexing


of the wheel rim during such situa-tions as crosswind landings.

“The fracture of each lug section oc-curred as a result of rapid, unstablecrack propagation,” the report said. “Inaddition to the fractures in the lug sec-tions, fracture and crack growth hadextended from the locking-pin hole inthe stiffening web. Initially, crackingfrom both the upper and lowerlocking-pin holes extended on a planeapproximately 45 degrees to thepivot-pin bore. The cracks branchedas they approached the lugs. Onebranch of the cracking extendedaround the circumference of the web,approximately parallel with the planeof the lugs; the other branch extendedon a plane toward the lugs and arrest-ed at the point of change in cross-section between the web and lugs.”

The stiffening web, the pivot-pinbore and the locking-pin hole hadbeen “reworked” during the mostrecent overhaul.

“Material had been removed bylocalized surface grinding in anattempt to remove corrosion,” the re-port said. “The pivot-pin bore surfacehad been peened and repainted witha chromate-based paint primer. How-ever, the paint film exhibited pooradhesion, and the shot-peening cov-erage was haphazard.”

The presence of the stress corrosioncrack in the stiffening web reduced

the fracture-resistance of the lug dur-ing times when tensile stressesexisted in the lug, including duringcrosswind landings, when mainlanding-gear turning moments weretransmitted through the torque links,the report said.

The report said that the inboard lugfractured shortly after touchdown“as a result of the tensile stressescreated by torque transmission [during the crosswind landing] and the owering of the lug fracture resist-ance by the presence of a stress corr-osion crack in the stiffening web. he failure of the inboard lug was ollowed by the bending fracture of he outboard lug.”

oth air safety occurrence reports ncluded a recommendation to the .K. Civil Aviation Authority (U.K. AA) to review the repair process nd the overhaul process for the ailed wheel and for the failed torque inks that were identified during the TSB investigation of the second anding incident to ensure that the rocesses conform to airworthiness equirements.

In response, CAA said that the repairand overhaul processes would bereviewed.

The technical report also included arecommendation for an audit of thecompany responsible for the repair andoverhaul of the left-main landing gear


“to establish why the repaired surfaceof the wheel hub differed from the ‘asmanufactured’ condition.”

The report said, “In particular, itshould be established if the speci-fication of the repair was adequate,or if repair instructions were fol-lowed. The reasons for any inade-quacy or lack of compliance shouldbe established.

“Similarly, the reworking of thetorque link attachment lugs of themain-landing-gear fitting should bereviewed to establish why the surfacetreatment and surface protectionsschemes were inadequate.”♦

[FSF editorial note: This article, ex-cept where specifically noted, isbased on the Australian TransportSafety Bureau (ATSB) Analysis ofleft-main landing-gear failures,Fokker 100, VH-FWI, ATSB AirSafety Occurrence Report 199903327and ATSB Air Safety OccurrenceReport 199904802. The reports in-clude photographs and diagrams.]

References

1. U.K. Air Accidents InvestigationBranch. Report on the accident toDouglas Aircraft Company MD-83, G-DEVR at Manchester Air-port on 27 April 1995, Section1.16 “Tests and research.” Air-craft accident report 1/97 (EW/C95/4/2).

2. Ibid.

Further ReadingFrom FSF Publications

FSF Editorial Staff. “Fatigue CrackLeads to MD-83 Left Main LandingGear Collapse on Rollout.” AviationMechanics Bulletin Volume 45 (May–June 1997).

FSF Editorial Staff. “Corrosion andFatigue-crack Detection RemainsCritical to the Continued Airwor-thiness of Aging Aircraft.” AviationMechanics Bulletin Volume 46(March–April 1998).


MAINTENANCE ALERTS

Safety ActionsRequired for SomePW4000 Engines

The U.S. Federal Aviation Adminis-tration (FAA) has required operatorsof airplanes equipped with somemodels of Pratt & Whitney PW4000turbofan engines to remove the en-gines before they exceed specifiedtime limits.

The requirement is containedin Airworthiness Directive (AD)2001-15-12, which also limits thenumber of PW4000 engines with aparticular configuration of the high-pressure compressor (HPC) sectionthat may be installed on an airplaneand requires specified standards to bemet in rebuilding the affected engines.

“The FAA has noted a growingnumber of takeoff surge events inPratt and Whitney PW4000-seriesturbofan engines,” the AD said.“These surges typically occur within60 seconds after throttle advance to[takeoff] power, a critical phase offlight. These events have resultedin numerous aborted [takeoffs], in-flight engine shutdowns and divertedflights. A surge of this kind on asingle engine of a multi-engine air-plane would not normally result in anunsafe condition.”

Nevertheless, two surge events in-volving surges of two engines on theairplane have been reported, includ-ing one on March 9, 2001, in whichan Air Sudan Airbus A300 lostpower in both engines during takeofffrom Jeddah, Saudi Arabia, FAA said.

Investigations have revealed “no spe-cial causes” of the surges, but FAAbelieves that “a low stall marginresults from open clearances in theaft stages of the HPC” and that “theworst-case open-clearance conditionin the aft stage compressor of theHPC occurs about 60 seconds afterthe throttle is advanced for [takeoff].”

AD 2001-15-12 supersedes twosimilar ADs (AD 2000-22-01 andAD 2001-09-07) whose requirementswere not sufficient, FAA said.

The new AD, which took effect im-mediately upon issuance July 26,2001, requires:

• “Limiting the number of engineswith the HPC CBS [cutback sta-tor] configuration to one on eachairplane within 100 cycles-in-service (CIS) after the effectivedate of this AD;

• “Limiting the number of PW4000engines with potentially reducedstability on each airplane, based


upon airplane and engine con-figuration, within 50 CIS after theeffective date of this AD;

• “Removing certain PW4000 en-gines from service before ex-ceeding cyclic limits on theHPC, based on CSO [cycles sinceoverhaul], within 50 CIS afterthe effective date of this AD;

• “Preventing the buildup ofPW4000 engines that have anHPC with 1,500 or more CSOgreater than the HPT [high-pressure turbine] CSO; and,

• “A minimum rebuild standardfor engines that are returned toservice.”

FAA said that the AD affects 500engines used on U.S.-registeredBoeing 747s and 767s, McDonnellDouglas MD-11s and Airbus A300sand A310s. About 2,100 PW4000engines with the affected configura-tion are in use worldwide, FAA said.

Damaged ElectricalWires Found on A320

The Australian Air Transport SafetyBoard (ATSB) said that damagedbundles of twisted-pair electricalwires found in the leading edge of theright wing of an Airbus A320 result-ed from surface rubbing and contactwith sharp edges.

In a technical report on the wiring,ATSB said that there were two dis-tinct types of damage:

• Surface rubbing damage, whichwas “consistent with the wireexperiencing a shallow angle ofcontact against the corner of asolid object.” In several instanc-es, insulation indentations andrub marks were found near pri-mary areas of damage.

The report said, “Rubbing andscraping of the wires againstthe harpoon tie tongues as theaircraft wing flexes in flightcould be expected to producesuch damage and could producethe characteristic indentationsthat were noted. The end profileof some areas of damage alsomatched the edge radius of theharpoon tie tongues”; and,

• Sharper, “intrusive-type” dam-age sometimes was accompaniedby the partial shaving of the out-er insulation.

The report said, “While notknife-cut in its sharpness, themore intrusive and localizednature of the damaged areas sug-gests a less acute angle of con-tact against a sharper corner oredge along the raceway path.”

The report said that there was noevidence that any of the damageresulted from tooling or “other


influences external to the racewayinstallation.”

FAA Orders Inspectionsof MD-88, MD-90 and

DC-9 Static-port Heaters

The U.S. Federal Aviation Admi-nistration (FAA) has issued Airwor-thiness Directive (AD) 2001-10-10requiring operators of McDonnellDouglas DC-9, MD-88 and MD-90airplanes to inspect the wiring ofstatic-port heaters for signs ofelectrical arcing, chafing and looseconnections and to make necessaryrepairs.

The AD also requires operators to de-termine whether the insulation sur-rounding the heaters is covered withmetalized Mylar (polyethyleneter-aphthalate); if so, the AD says thatthe operators must replace the metal-ized Mylar with Tedlar-coveredinsulation, or take other appropriateaction.

The AD was issued in response to arecommendation by the U.S. NationalTransportation Safety Board, whichsaid that a spark from a static-portheater ignited a fire in the forwardcargo compartment of a Delta AirLines MD-88 on Sept. 17, 1999, af-ter takeoff from Cincinnati/NorthernKentucky International Airport inCovington, Kentucky, U.S. (See“Inspections Recommended for

MD-80, MD-90 and DC-9 Static-portHeaters.” Aviation Mechanics Bulle-tin Volume 49 [March–April 2001]:10–12.) The fire consumed themetalized-Mylar-covered insulationblankets that surrounded the heater.No one was injured; the airplane re-ceived minor damage.

The AD affects 605 U.S.-registeredairplanes.

Stiff Aileron ControlsOn BAE 146 Result inNew Maintenance Test

A British Aerospace (now BAESYSTEMS) 146-300 was parkedfor several hours at a gate in lightrain and in temperatures no lowerthan 7 degrees Celsius (45 degreesFahrenheit), before departure fromAberdeen, Scotland, for a flight toAmsterdam, Netherlands.

The flight crew said that as they flewthe airplane to Flight Level (FL) 250(25,000 feet), the aileron controlsbecame “stiffer to operate and even-tually locked solid.”

The incident report by the U.K.Air Accidents Investigation Branch(AAIB) said, “The aircraft was lev-eled at FL 270, and, because it hadbeen climbing in a wings-level atti-tude, the [captain] decided not to ap-ply excessive force to overcome theaileron jam, as would have been the


case if he had adhered to the ‘aileronjam or uncommanded roll’ drill; theaircraft was therefore maneuveredlaterally using aileron trim.”

The captain said that he believed thatany attempt to overcome the restric-tion on the controls could have led toa sudden, violent rate of roll thatmight have been difficult to control.The flight crew also believed thatrainwater had entered the control runsand had frozen at altitude; after theydeclared an urgency (pan pan) anddescended the airplane to 2,500 feet,the controls gradually became free.The flight crew conducted a long,straight-in approach and a normallanding at London (England) Stanst-ed Airport. None of the 99 people inthe airplane was injured.

A post-incident inspection of the ai-leron controls revealed no signs ofstiffness or jamming.

A similar aileron-restriction incidenthad occurred 10 months earlier andapparently had been corrected by thereplacement of the autopilot aileronservomotor, which drives the aileronlinkage through a gearbox and aclutch. After the second incident,which occurred when the airplanehad accumulated about 20,000 flighthours and 21,000 takeoffs and land-ings, inspections and test flightsrevealed no evidence that the prob-lem was associated with the autopi-lot aileron servomotor.

Nevertheless, the airplane was re-turned to service with a replacementautopilot aileron servomotor. Inves-tigators delayed a planned inspectionof the spoiler camboxes, which sendsignals to their associated roll spoil-ers about roll direction and ailerondeflection, and two more aileron-restriction incidents occurred beforethe inspection was conducted.

The inspection revealed a metallicparticle that “had become trappedbetween the [cam] roller and theslot” and may have been associatedwith at least one aileron-restrictionincident.

The ailerons then were removed anddisassembled.

“The left aileron servo-tab inputmechanism felt ‘notchy’ when oper-ated by hand,” the report said. “Fur-ther disassembly revealed that theback-to-back bearing assembly on theupstream end of the torsion bar wasstiff in operation. The bearings werelocated on a common shaft within afitting forming part of the aileronstructure and were separated by analuminum-alloy spacer. The space be-tween each bearing thus took the formof an annular void, which, it ap-peared, was susceptible to filling upwith water.

“Disassembly of the bearings re-vealed no evidence of corrosion, al-though in the absence of lubrication,


‘galling’ between the stainless steelballs and races hindered smoothoperation. In addition, the solid res-idues from the grease, which proba-bly included dirt, also contributed tothe bearing rolling resistance. … Thedistinguishing feature of the … bear-ings was that they were located atthe edge of a joggle [a step-shapedoffset] in the aileron leading edge,which was thus effectively an endrib. The retaining plate on the exte-rior surface of the end rib was notdesigned as a seal and was thus ableto admit external contaminants, suchas water, to the outer bearing, andthence to the inner bearing via theannular void. The bearings them-selves were ‘on condition’ compo-nents and were pre-packed withgrease at manufacture; there was noprovision for in-service lubrication.”

A revision was made to the bearingtype (part no. SL4421-01) in 1991 —one year after the incident airplanewas manufactured — to allow eitherAeroshell 7 grease or Aeroshell 22grease to be used during manufacture.(Aeroshell 22 grease is less likely toretain moisture, the report said.) Be-fore 1991, only Aeroshell 7 greasewas used.

The report said that examination ofthe operator’s maintenance recordsshowed that “potentially similaraileron problems” had occurred infour of its 10 BAE 146 aircraft andthat after the incident, inspections

revealed aileron bearings on two air-planes that resembled the aileronbearings on the incident airplane. Inother instances, corrosion was foundin the aluminum-alloy spacers thatseparated the back-to-back bearingassemblies. The manufacturer wasunaware of any similar problemsexperienced by other operators ofBAE 146 and BAE Regional Jet (RJ)airplanes, and a low volume ofspare bearing sets had been orderedsince 1983, when the aircraft wereregistered.

“It was not established why otheroperators of this type of aircraft hadapparently not experienced similarproblems,” the report said.

The investigation also did not deter-mine why only one aileron on the in-cident airplane had been affected.

Nevertheless, the report said, “Thesubject bearings have been causingproblems for a number of years, and… the symptoms may have beenmisidentified as autopilot servomo-tor malfunctions. The situation wasfurther confused by the presence ofa contaminant particle within theleft-hand spoiler cambox of [the in-cident airplane]. The bearing prob-lem appeared to be the result ofwater ingress. This would most prob-ably have been rain, as its locationwould not permit direct impinge-ment of the jet from a high-pressurehose during washing operations.


Some of the grease had been washedout, leaving solid residues that im-peded the rolling action of the ballsin the races. Additional rolling re-sistance would have been caused bythe formation of ice crystals as theaircraft climbed above the freezinglevel. It is thus possible that therewould be a higher risk of an aileronrestriction developing where the air-craft climbed on a constant heading,with consequently small aileron de-flections. Large and frequent aileronmovement would be more likelyto crush the ice crystals as theyformed.”

After the incident, the manufacturerrevised the maintenance manual toinclude a “subjective friction test” foraileron servo-tab circuits and eleva-tor servo-tab circuits that involves dis-connecting the linkage near thetorsion-bar input to check for stiffnesswhen the servomechanism is operat-ed by hand. Such stiffness could in-dicate the presence of a worn bearing,which then could be examined andreplaced.

Absence of Oil inTail-rotor Gearbox

Cited inFatal Accident

A Bell 47 flown by a student pilot anda flight instructor was being flownthrough 700 feet after departurefrom Abbotsford (British Columbia,

Canada) Airport when tail-rotor thrustwas lost and the helicopter entered aspin to the right. The helicopter de-scended out of control, stuck theground in a steep, nose-down attitude,broke apart and burned. Both pilotswere killed.

Investigation showed that the tail ro-tor was not turning when the helicop-ter struck the ground and that gearsin the tail-rotor gearbox suffered heatdistortion and had disengaged. No oil— and no remnant of burned oil —was found in the tail-rotor gearbox.No control anomalies were found.

Records showed that maintenancepersonnel had conducted a 100-hourinspection of the helicopter the daybefore the accident. The inspectionrequired that oil be changed in thetail-rotor gearbox, and the task wasassigned to an apprentice aviationmaintenance engineer (AME).

“The apprentice AME drained thetail-rotor gearbox oil, inspected it formetal particles and installed and lock-wired the drain plug,” said the acci-dent report by the TransportationSafety Board of Canada. “In additionto the normal actions of the 100-hourinspection, the forward section of thecables that move the horizontal sta-bilizer were replaced. The [supervis-ing] AME signed the aircraft journeylogbook as having completed the 100-hour inspection. The 100-hour in-spection check-sheet item that called


for draining and refilling of the tail-rotor gearbox was initialed by theapprentice AME.”

The next morning, the student pilotconducted a preflight inspection,which calls for visual inspection,through a sight gauge (small win-dow), of the level of oil in the tail-rotor gearbox.

The flight instructor joined thestudent pilot after the student hadstarted the helicopter’s engine. Theyoperated the helicopter for about 15minutes on the ground and about twominutes in the air before the accidentoccurred.

The report said, “Since there was heatdistortion of the gears and no rem-nant of oil in the tail-rotor gearbox,it is concluded that no oil was in thegearbox when the helicopter startedoperating on the morning of theaccident. It is also concluded that,since the lack of oil was not detectedprior to flight, the apprentice AME,the AME, the student and the instruc-tor did not check the oil level or erredin reading the sight gauge. … It issometimes difficult to tell whetherthere is oil behind the window.”

The report also said that after the heli-copter began to spin, the pilots did notrespond by conducting an immediateautorotative descent — the action pre-scribed by the Bell 47 flight manualin the event of a tail-rotor failure.

Unauthorized Use ofDegreaser Blamed for

Wheel Bearing Failures

After departure, the flight crew of aBeech 1900D was notified that a tirehad been found on the runway, said areport by Transport Canada. (The re-port did not identify the airport or thecountry where it is located.)

“Maintenance [personnel] discov-ered that the wheel had fallen fromthis aircraft as a result of a bearingfailure,” the report said. “The bear-ing failure resulted from a lack of lu-brication caused by the use of adegreasing fluid to clean the land-ing gear.”

The same degreasing fluid had beenused on other aircraft, and in someinstances, their bearings were dry andrequired lubrication.

Investigation revealed that, five daysbefore the incident, a main wheel onthe same airplane had malfunctionedand that its outboard bearing was dryand “self-destructing.”

Investigation also revealed thatthe operator had hired cleaningpersonnel “who had taken it uponthemselves to select a product [thedegreasing fluid] to clean the‘greasy’ wheels” on all of the com-pany’s aircraft, the report said. Af-ter the incident, the company began


inspecting — and, if necessary, re-greasing or replacing — all bearingsand other affected wheel-well com-ponents, and the cleaning personnelwere instructed in correct cleaningmethods.

Inspection of the fleet revealed that75 five percent of wheel bearingswere dry.

Shop Rag JammedSpoiler-control Pulleys

The U.S. National TransportationSafety Board (NTSB) said that a shoprag jammed the spoiler-control pul-ley system of a McDonnell DouglasMD-11, causing the flight crew tohave roll-control difficulty duringtakeoff from Honolulu (Hawaii, U.S.)International Airport.

“Post-landing inspection showed thatthree of the five spoilers on top of theright wing were fully deployed,” thefinal accident report said. “Mechan-ics found a general-purpose shop raglodged in the spoiler-control pulleysystem, jamming it in the deployedposition.”

The rag was found in an open area inthe center-body landing-gear wheelwell. The center-gear doors usuallyare closed when the airplane is on theground, but maintenance personnelcan open the doors to work in thearea.

Employees of a contract fuel-systemsrepair company had worked in thatpart of the airplane two days beforethe incident, when they opened sev-eral lines to check for fuel leaks.

The report said that neither the main-tenance organization nor the mainte-nance technician responsible for therag could be determined.

Stress CorrosionCracking Cited inLanding Accident

Stress corrosion cracking of a landing-gear leg has been identified as thepossible cause of an accident involv-ing a Lockheed L-188C Electra. Theairplane was being flown on a nighttraining flight, which included take-offs and landings at Prestwick Airportin Scotland.

The touchdown for the third landingat first appeared to be normal, butthe report by the U.K. Air AccidentsInvestigation Branch (AAIB) saidthat the captain was “immediatelyaware of a directional controlproblem.”

“The aircraft was veering to theleft to such an extent that he had touse full right rudder, in addition toasymmetric reverse thrust, tomaintain the centerline,” the reportsaid. “Directional control became


progressively more difficult as theaircraft decelerated.”

The captain applied full braking andmaximum reverse thrust on all fourengines and stopped the airplaneabout 10 feet (three meters) from theleft edge of the runway and on a head-ing 70 degrees to the left of the run-way direction.

The airplane received minor damage;neither pilot was injured.

Investigation revealed that thelower (piston) oleo cylinder of theleft-main landing-gear leg hadfractured above the axle and thatthe axle and the two wheel-brake as-semblies had separated from the leg.The upper section of the piston cyl-inder remained in the upper leg andwas further damaged by contactwith the runway as the airplanedecelerated.

Only the lower section of the frac-tured piston cylinder was available formetallurgical examination, whichrevealed that the fracture had begunat the lowest area of the piston’s out-er surface, near the limit of normaltravel by the piston.

“This location was on the inboardside of the piston and was charac-terized by corroded arcs on thefractured surface, which extendedsome 2.5 [millimeters; 0.1 inch] intothe material from the outer surface.

… Visual examination also revealedthe presence of some parallel sec-ondary cracking adjacent to the mainfracture.”

Further examination revealed brittlecleavage cracking in the chromiumplating on the surface and inter-granular cracking in the underlyingsteel in the corroded arc regions.

“Beyond and between these areas, thesteel section had failed in overload,”the report said.

“Several areas of cracking of thesteel substrate were found, and with-in the chromed surface on the frontside of the leg, from 25 [millimeters;one inch] above the normal limit oftravel to below this limit, a patternof fine, vertically orientated cracks[was] observed in the chromiumplating, which was generally about100 micrometers [0.004 inch] thickin this area. Remote from thefracture-initiation zone, the chromi-um thickness was reduced to be-tween 25 micrometers [0.001 inch]and 40 micrometers [0.002 inch],probably due to wear. Some crack-ing in the plating was apparent inboth the vertical [orientations] andhorizontal orientations, but this wasnot generally fully penetrating.Evidence of some remaining grit-blasting debris was present in thechrome/steel interface, but the plat-ing was strongly adherent to the par-ent steel substrate.”


The report said that stress corrosioncracking was the most likely causeof the cracking. Stress corrosion-cracking failures typically result from“a field of cracks produced in a met-al alloy under the combined influenceof tensile stress … and a corrosiveenvironment,” the report said.

Because of the multiple orientation ofcracks on the piston, internal stressesin the steel probably were associatedwith the cracking, the report said.

“The distribution of the cracking de-tected in the chromium plating … wasconsistent with excessive loads hav-ing been generated in the platingby grinding-wheel operations duringmanufacture/refurbishment of thiscomponent,” the report said. “[Al-though] it is not unusual for somecracks to form in chromium ‘as-plated,’ the many cracks observed wereconsidered to have resulted from acombination of the plating processused and excessive grinding action.Those cracks in the chrome surfacehad then allowed the underlyinghigh-strength steel to be exposed tocorrosive conditions, inducing [stresscorrosion cracking].”

The aircraft manufacturer’s recordsshowed that four similar failures had

occurred involving landing-gear-legpiston cylinders. Metallurgical ex-aminations were performed in threeof those instances. The examina-tions showed that one failure result-ed from stress corrosion crackingand that stress corrosion crackingwas a possible cause of the other twofailures.

The manufacturer said that when theaircraft type was certified, there wasno requirement to establish safelives for landing-gear components.Proper maintenance and properplating procedures during overhaulwere considered adequate for inhib-iting stress corrosion cracking. Elec-tra Service Information Letter(SIL) 88/SIL-88A, which discussedstripping procedures and platingprocedures, was issued in October1974; the manufacturer believedthat all aircraft operators possessedthe SIL.

The authorized overhaul life of themain landing gear was 16,000 flighthours. The landing gear on theaccident airplane had accumulated15,400 flight hours since its previousoverhaul. The airplane had accumu-lated 49,500 flight hours and 22,300landings.♦


Portable Drill Press

NEWS & TIPS

Wire-twistingPliers Designed for

Safety Wiring

Diagonal wire-twisting pliers havebeen designed for reliable safetywiring or lock wiring, said the man-ufacturer, Klein Tools.

The pliers are available in a right-handtwist style (model 12213 and model12215) and a reversible twist style(model 12214 and model 12216),which rotates clockwise and counter-clockwise. Both styles are available intwo sizes — six inches (15 centime-ters) and nine inches (23 centimeters).They are designed for wire-diametercapacities of 0.041 inch (one milli-meter) to 0.060 inch (1.5 millimeters).

For more information: KleinTools, P.O. Box 599033, Chicago,IL 60659 U.S. Telephone: (800) 553-4676 (U.S.) or +1 (847) 677-9500.

Vacuum-base DrillUsed for Cutting,

Tapping

The Vac-Force vacuum-base portabledrill press can be used in applicationsthat are difficult for magnetic-basedrills, said the manufacturer, SiouxTools.

Wire-twisting Pliers

The Vac-Force uses an air cylinderthat is controlled by an indirect leverto feed the cutter through the materi-al being cut. The drill is lightweightand can be used in almost any posi-tion, including overhead, and in arange of applications, including cut-ting, drilling and tapping, said themanufacturer.


For more information: Sioux Tools,2901 Floyd Blvd., P.O. Box 507,Sioux City, IA 51102 U.S. Telephone:+1 (712) 252-0525.

Hardness TesterIs Portable

Measuring Tool

The DynaPOCKET compact hard-ness tester is designed to determinethe hardness of large componentsthat cannot be moved easily, said themanufacturer, Krautkramer.

The hardness tester measures 1.5inches (3.8 centimeters) by 6.7 inch-es (17 centimeters) and weighs sev-en ounces (198 grams). The devicecontains standard conversion tablesfor nine material groups, and the mea-sured hardness value is reported on aliquid crystal display.

For more information: KrautkramerBranson, 50 Industrial Park Road,Lewistown, PA 17044 U.S. Tele-phone: +1 (717) 242-0327.

Adhesive ProvidesFluid-resistant Seal in

High Temperatures

The Raychem S1125 Kit 8 is afluid-resistant and diesel-resistantadhesive designed to be used withheat-shrinkable components to seal

connections that are subject tohigh operating temperatures and/orflexural loads, said the manufactur-er, Tyco Electronics.

The adhesive is a two-part epoxy-polyamide adhesive developed forengine-bay requirements in aero-space applications and other appli-cations. The adhesive provides strainrelief and creates a long-term sealthat is resistant to a variety of chem-icals and fluids. Its operating-temperature range, when used withaccepted high-performance com-ponents, is from minus 75 degreesCelsius (C) to 150 degrees C (minus103 degrees Fahrenheit [F] to 302degrees F).

For more information: Tyco Elec-tronics, 300 Constitution Drive,Menlo Park, CA 94025 U.S. Tele-phone: +1 (650) 361-4470.

Grippers ProvideIncreased Grip Forces

The PGN-plus series of parallelgrippers provide increased gripforces for handling heavier loadswith greater efficiency, said themanufacturer, Schunk PrecisionWorkholding Solutions.

The PGN-plus series includes sixversions of grippers in five sizeswith various gripping forces. Thehousing is made from high-tensile


Solid-state Power Controller

Solid-wall Inserts and Studs

strength, hard-anodized aluminumalloy, and all functional parts aremade of hardened steel.

For more information: SchunkInc., 211 Kitty Hawk Drive, Morris-ville, NC 27560 U.S. Telephone(800) 772-4865 (U.S.) or +1 (919)572-2705.

Solid-wall Inserts,Studs Lock Into Place

Solid-wall design inserts and studsdesigned for aerospace applicationshave integral locking stakes that re-sist torque-out and pull-out, said themanufacturer, SPS Technologies.

For more information: SPS Tech-nologies, 301 Highland Ave.,Jenkintown, PA 19046-2692 U.S.Telephone: +1 (215) 572-3718.

Solid-state PowerController Provides

Load Protection

The AMPHION solid-state powercontroller is a plug-in componentthat protects sensors, wiring and28-volt direct-current data busesagainst shorts to ground, shorted sen-sors and overloads, said the manu-facturer, AMETEK Aerospace.

AMPHION detects intermittent arc-ing in the load and wiring and detectsthe fault-current pattern that is typi-cal of an arc. Its circuitry prevents afailure in a short-circuited condition.

For more information: AMETEKAerospace, 50 Fordham Road, Wilm-ington, MA 01887 U.S. Telephone:+1 (978) 988-4639.♦

The solid-wall staked inserts, stakedstuds, swaged inserts and ringlockstuds are available in a range of siz-es, materials and configurations, andtheir integral locking stakes are driv-en into parent materials to lock theinsert in place mechanically. Theycan be installed with simple handtools, said the manufacturer.

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