Theunitwasthensubmittedtoasecondthermalretestbutthistimewithadditionalinstrumentation added toaidwiththetroubleshooting. Withtheaidofavideocamerainstalledinsidethechamber,itwasrevealed thattravelofthelockrackwasinconsistent, jumpy,andwouldoccasionally failtoengageandwouldinstead sendthemechanism reelingbackwardoncepowerwascut.Withthisadditionalinformation, themainarea ofconcernbecamethefitbetweenthelockrackanditsmatingbushings.Theunitwaspartially disassembled toallowinspectionofthelockrackandbushings.Itwasfoundaftermeasuringtheseitems ontheCoordinate MeasuringMachinethatthelockrackwasoversized.Atthispoint,severalotherunits weredisassembled toallowinspectionofthesamepartsandsimilarresultswereobtained.Areviewofthe historyoftheseparticularpartsrevealedadiscrepancy inwhichthepartshadbeeninadvertently completely nitridedinsteadofjustthegearteeth.Thelockrackwasoriginallydesignedwitha15-5basematerialthat hadnitridedteethandwasgoldplated,whilethematingbushingswerealuminumcoatedwitha Molydisulfide lubrication. Thisaddedlayerofnitridewasenoughtothrowthepartoutofitsacceptable tolerancerange.Itwasassumedatthetimethatthepartstillmetprintanditwasgivenause-as-is disposition. Thepartsweresubsequently installedintotheirrespectiveunitsandnothingoutoftheordinarywas detectedastheassemblyisperformedatambientandthepartsslippedthroughtheirmatingbushings withoutincident.Itwasn'tuntiltheunitwasexposedtothecoldenvironment atasystemlevelthata problembecameapparent.Whytheinterference wasn'tdiscoveredduringcomponent-level testingbecame anissue.Areviewofthecomponent testdatarevealedanincreaseinpowerdrawnbythesteppermotor duringcoldtesting.Atfirstglance,thisbulgeinthecurvewasn'tnoticeableandcouldonlybetrulyseenif thetestdatawasblownup.Sincetheunitreacheditsdestination, acloserreviewofthetestdatawas neverperformed. ThelockfuseontheTDLAissignificantly strongeranditisbelievedthatthoughthesameinterference existedonthisunit,itneverexperienced thefailurebecauseitsstifferspringforcedthelockthoughits matingbushings.Inspectionofthelockracksandbushingsonallunitsrevealedasignificantamountof wear.Thisalsorevealswhythefailuredidn'toccurrightaway.Ittooktheadditionaldebriscreatedbythe interference tobindthelock. InsteadofreworkingthelockracktoasmallerOD,newbushings wereinstalled intotheunitwithalarger ID.Theunitwasthentestingagainattemperature andoperated nominally. Allunitswerethenretrofitted withthenewbushings. Lessons: -Don'tjumptoconclusions. Consider allpossible failuremodeswhenreviewing atestanomaly. Everyone wantsaquicksolution. -Consider harness routingandcreatekeepoutzonesaroundoperational mechanisms. -Consider addedthickness duetosurface finishes. -Useasmuchinstrumentation aspossible whentroubleshooting anomalies. 5)F_l_;eT_lCm_try Ithadbeenaknownfactthatoneofthefollower bearings wouldgap(pushawayfromtheracering)duringa toothcrash(Figure 10).Itwasnotwellunderstood, however, theimpacts thisgapping wouldhaveonthe unitduringsystem-level testingandmorespecifically, howitwouldimpactthetelemetry. Duringanambient system leveltestrun,theDLAtelemetry indicated successful lockengagement whileit wasdiscovered thattheunitwasactually inatoothcrash.Thelockisnotengaged wheninatoothcrash soeitherthelimitswitches weremalfunctioning oranother phenomenon wasoccurring. Itwasrevealed that asaresultofthetoothcrash,thefollower bearing gapped significantly enough toallowactuation ofthelimit switches. Sincethistoothcrashcondition wasnottestedforatthecomponent level,itwasn'tdiscovered untilsystem-level testing. Additionally, thecomponent testorientstheunitinamoreupright position 71