Secondary Lockincr Features  For decades, dating back to the development of aircraft engines, the response of fastened joints to  vibration environments has been a critical issue. Failures of critical fastened joints led to the development  of “secondary” locking features that served as a method to guarantee that fasteners would not rotate and  back out under vibration environments. Preload was recognized as a “good method to prevent bolt  rotation, but it was neither highly reliable nor predictable. So, “positive” locking methods were developed  for highly critical joints, such as lockwire, cotter pins, locking tab washers, and safety cable. These locking  methods are very reliable, but are also very labor intensive and thus expensive. To decrease labor,  prevailing torque locking features were developed that rely on friction to help decrease propensity for bolts  to rotate. Prevailing torque locking features include lock nuts, deformed thread keenserts, helicoils, and  various types of locking patches and pellets.  Liquid Locking Compounds (LLCs) including epoxy have also been used as secondary locking features,  but these have proven to be unreliable. LLCs are very sensitive to application process and environmental  factors and cannot be verified after installation without breaking the bond. At JSC, the Engineering  Directorate has been critically evaluating use of LLCs and has implemented policies restricting its use.  Many of the problems with secondary locking features can be traced to a lack of well-defined  requirements. Using explicit language such as this can help:  “Each bolt, screw, nut, pin, or other fastener used in a safety critical application shall incorporate two  separate verifiable locking features. Preload may be used as one of the features combined with a  conventional aerospace secondary locking feature that is positive locking and vibration rated [2].”  Lesson Learned: Clear secondary locking feature requirements need to be specified in program  and project requirements documents.  Over the past two years a significant number of problems have been encountered on many different  pieces of hardware relating to either the lack of a secondary locking feature or the improper use of such  features.  External Stowaae Platform 2  The External Stowage Platform 2 (ESP-2) was a payload mounted in the payload bay of the Space Shuttle  Orbiter on STS-114. ESP-2 had several space station Orbital Replacement Units (ORUs) attached to it.  After docking, the ESP-2 was removed from the payload bay and mounted to the external airlock on the  ISS.  Following a test failure caused by migration of an uncured LLC, NASA Materials and Processes (M&P)  and Structures and Mechanisms personnel advised both the ISS and Shuttle programs to closely restrict  the use of LLCs and to develop application procedures for its use. As a result of the policy, an  investigation of all uses of LLCs on ISS began. It was determined that an LLC had been used in some  locations to assemble the ESP-2. The NASA ISS M&P System Manager advised the ISS Program that the  installation procedures for ESP-2 did not call out the use of primer on the titanium inserts as  recommended and that the design should be corrected by using a standard, verifiable locking feature.  It was decided to attempt qualification of the hardware with the LLC as configured, while following a  parallel path to fix the hardware if the test was not successful. A conservative vibration test used to  generically qualify secondary locking features was adapted to the ESP-2 LLC configurations. The test was  performed at lower-than-flight preloads to help isolate performance of the secondary locking feature from  preload. Five samples each of five different configurations were tested. All samples were assembled using  the same process as on the ESP-2 flight hardware. All 25 samples showed various degrees of rotation  following testing and were therefore determined to be failures. Upon disassembly of several of the  samples, it was observed under microscopic inspection that the LLC had cured, but had failed to adhere  to the titanium properly. The bolts with LLC were removed and replaced with bolts that had a verifiable  locking feature, a Mylar locking patch, to resolve the issue.  116