Lesso ns lear ned from ea ch of th e major categ ories 

For ea ch of the majo r categorie s the root cau ses, corrective a ctions and recu rrence co ntrols were  reviewed fo r lesson s learned that could be useful  in reducing the risk of simila r failure s on o ther spa ce  flight mech anism p rograms. An attempt wa s made t o provid e lesson s lea rned that affected  the highe st  numbe r of PRACA’ s in ea ch catego ry. 

Tolerancing   This catego ry wa s used to captu re failures attribute d to insufficie nt design tol erancing for eith er thermal or  mech anical deflectio ns. For exa mple, several mo ving me chan ical a ssembl y binding fai lures, whi ch  occurre d during envi ronmental testin g, we re caused by a la ck of suffici ent clea rances. Root cause  investigatio ns determin ed that adequ ate tolera nces had not b een prov ided to  account fo r the therma l  expan sion coefficient differen ces of t he va rious compon ents i n the systems. Also, ot her ha rdware  anomali es were fou nd to be caused b y poor me chanical toleran cing of m oving com ponents. In seve ral  cases, limit switch riggin g toleran ces did not acco unt for the mechani cal tolera nces and run  out of the  interfaci ng m echanical co mpone nts, re sulting in the  failure to achieve co nsistent and repe atable limit   switch activation.   

Most of the problems resulting from tole rancing errors could have best been prevented with good de sign  reviews and more rigorous design analysi s. It was found in a majo rity of  these p roblems that the   compl exity of either the th ermal or load environ ment was not considered in the origin al de sign. Whe neve r  compl ex or e ven sim ple mech anical systems are b eing revie wed careful  atte ntion shoul d be ma de to   toleran ces that could b e affected by tem peratu re or loads. In seve ral probl em re ports, the roo t cause wa s  not accou nting for sufficie nt clea rance in moving m echanical int erfaces fo r the gro wth or contra ction of  comp onents unde r the environme nt they were bei ng designed to o perate. In  other cases, the  orientation   of the sy stem  in a g ravity e nvironm ent o r unloa ded condition affe cted the operation of the mechani sm a nd  resulted in anomal ous perfo rman ce. In seve ral cases, failure to provi de ad equate margi n for  manufa cturing toleran ce build up resulted in  failures of the units unde r test. A detaile d design tole rance  analysi s accounting for ma nufactu ring and wo rst ca se thermal a nd mechani cal conditio ns can be u sed to  mitigate the ri sk of havin g hard ware failure in test o r in operation. 

Design  Failures attri buted to desi gn we re characteri zed by probl ems in the detailed p arts of the h ardware. For  example several cases of  rotating com pone nts with  out prope r use of wa shers o r bushing resulted i n  failure of the  mechani sm durin g testing. Also, inte rferences with fillets o r other compone nts, or having   wiring route d too clo se to moving com ponent s resulted in test failure s and the need to re design hardware.  Includ ed in this catego ry are failu res attributed to insufficie nt stre ngth of a fastener o r othe r retentio n  device s that sho uld have been caug ht durin g de sign reviews or analysi s. Failure to provide ade quate  design featu res to protect again st gallin g, contami nation, or to pro vide pro per force or to rque margi ns are  also in cluded in this cate gory. 

Although under initial  revi ew thi s category a ppeared to be a case of 20 -20 hindsight, further revie w  resulted in some inte resting le ssons l earned. Th e most striking of these wa s that the f ailures were  prima rily cau ght du ring qualificatio n or acce ptance tests. T he IS S pro gram was fo rced by bud getary  constraints to  waive th e require ment s for many test s and thu s relied on  design revie ws and analy sis as  substitutes. I n revie w of th e resultin g hardware pe rformance, many  years late r, it is clear that often time s  this is a ri sk that may resul t in unsu spected hard ware failure s. Man y design e rrors or defi ciencies can only  be foun d in t est, and whenever a nalysi s or inspe ction is used in place of tho se tests, a higher d egree of  rigor is required to a ssure that desi gn errors are ca ught. Another l esson th at sh ould b e drawn from thi s  categ ory is that de sign reviews n eed to  include a thorough peer review. In several cases simple d esign  errors, that senior d esigners would h ave cau ght, were  missed an d only caught i n subsequent  testing.  It is  felt that many of these co uld have bee n prevente d by having ad equate revie w by senio r desi gners and  analyst. Detai led de sign checklist s could  be u sed to help systemati cally review new design fo r errors that  had be en fo und in p revious designs. Design to mi nimum ri sk criteria when applie d correctly can h elp  identify area s in a desig n that need close review. 

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