Beta Gimbal Asse mbly (B GA) Hig h Current (P RACA 2685 )    The BGA i s a direct drive mech anism located at the  base of e ach of the ISS solar arrays and i s used to  rotate the array towa rd the sun. Duri ng ea rly ope ration of the  first two arrays on -orbit, highe r tha n  expecte d current spikes were note d. As time and cycles accumulated the se current spi kes eventu ally  reach the ma ximum limit a nd a stall condition o n the joint. The se stall s have been  recove red f rom a nd  manag ed through a combination of op erational procedures to limit the rotation required fro m the joint and   plann ed rotation reversal s. Althoug h the root cause has not been absolutely determi ned an inten sive  investigatio n has narro wed the failure to the bea rings and/or lu bricant. It is beli eved that an anomaly i n  the lea d based lu bricant has cau se some fo rm of debris to be gene rate i n the bearing causi ng errati c  torque  ripple that ove r time can lead to torque  requireme nts beyond  the motor ca pability. Rotation   reversal has contin ued to be an  effecti ve mean s to regain rotation capa bility. A pre planned period of  reversin g rota tions have re duced the fre quen cy of t he stalls and a ppears to date  to be an effective mean s  of controlling  the anomalo us performan ce. Witho ut a defin itive root cau se it is hard to dra w conclusive  lesson s from  this expe rience, however it ca nnot be igno red that the b earing design was chang ed after  developm ent life testing ha d been comp leted, and that the origina l life te sting did not includ e any of the  environ menta l effects th at the be aring would see on orbit. Co st an d sched ule were factors in t he de cision  not re-perfo rming the test s.   

Solar Array Deployment An omaly (PRA CA 2397)     This failu re also occurred d uring the d eployment of t he first solar a rrays on the P6 element. As the arrays  were bein g deployed, dyn amic motion i n the panel s cau sed a failure of the bla nket ten sion mech anism.  This failure was attributed to small stiction forces between the solar array panels where silicon surfaces  were in co ntact. Although the arrays were function ally tested on the ground, the e ffect of these force s was  masked by grou nd supp ort equip ment and the effect of ope rating in a 1-G  environm ent. Failure to  recogni ze th e effect that th ese small force s would have on the system re sulted i n a significan t failure o norbit. Only after the on-o rbit failure occurre d did detailed dynami c analysi s demonst rate th e effect. No  preflight an alysis of this type had be en cond ucted. If this type of analysis h ad been co nducted pri or to  flight, it woul d have identi fied a lack of  force margin  in the blanket tensionin g system that wa s also a   contri butor to the on-o rbit failure.  

Hatch Ha ndle Imprope rly Stowed (P RACA 3348)    The ha ndle o n the ISS common hat ch is used to op erate the latch es on th e hat ch. After use, the handl e is  stowed i n a positio n so th at it doe s not interfe re with the mechanism. T he stowage procedure relie s on  crew training to assure th at this is pe rforme d properly. As was th e case on orbit, an impro perly sto wed  hatch  handle almost caused the lo ss of acce ss to the  airlo ck. A no n-standard work a round luckily allowe d  acce ss to be  regai ned and new procedu res are in place to a ssure th e condition  does not h appen again. In   review of the failure it wa s determi ned t hat the des ign doe s not a dequ ately pro tect again st the miss stow  of the han dle. Adequate design fe ature s to prevent th e miss ope ration of the h ardwa re were not provide d.  Anytime that operator train ing is requi red to prevent what could b e cata strophic consequ ences design  solutio ns sho uld be found  to minimize the cha nce that they coul d occur. In this case gua rds a re bei ng  adde d to the hatch to p reclude imp roper hatch ha ndle stowa ge. 

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