Figure 6. QWKNUT Latch In-Process Test Fixture  Discussion and Lessons Learned  The vibration anomaly is an example of how insufficient controls on critical features will, in time, cause a  failure. The first four production lots were not discrepant, and there was no indication of a shortcoming in  the design -- the problem was simply dormant. Design intent must be clearly communicated in  engineering drawings. This requires a thorough understanding of which features are critical to  performance. If adequate engineering controls are not in place, the discrepancy will eventually emerge  with enough repetition (or opportunity). Parts from different lots may both meet the same drawing  requirements but perform differently in a mechanism. Proper identification and inspection of critical  features is essential, and in-process screening at the assembly level provides redundancy in detection of  variation.  Failures are sometimes created as a result of two or more prevailing conditions. Care must be taken to  ensure root cause and all contributing causes are identified before the investigation process is concluded.  Low spring force was initially thought to be the cause because with a certain minimum spring force, even  the discrepant lever geometry could not cause a vibration failure. Increasing spring force did resolve  vibration release, but did not create consistent latch performance. Proper performance under vibration  loads is now ensured by a combination of geometric drawing controls, a rigorous inspection plan,  assembly level screening tests, and finally ATP. These added controls, and a deeper understanding of  the design, were some benefits of the investigation process.  Failure to Release during ATP  Following the resolution of the vibration anomaly, production of the QWKNUT resumed with the vibration  corrective actions fully implemented. The "Gen 1 A QWKNUT configuration now contained dual-springs  for lever retention, and flight units were assembled in parallel with Delta-qualification activity. During  acceptance testing, a flight QWKNUT failed to release preload during an ambient functional test. The unit  operated nominally using the primary SMA circuit, but did not release when the secondary SMA wire  circuit was activated. The testing was stopped and the FRB team was again convened.  Physical observation of the failed unit helped guide us quickly to the physical cause. The lever on the  activated side of the mechanism had disengaged properly from its toggle, but on the opposite side, the  lever was still engaged with its toggle (Figure 7). This prevented the outer race from rotating and  releasing the mechanism. The lever arm appeared to be tilted at an unusual angle, and one end was in  contact with the underside of the cover, which is directly above the lever. The mechanism was in a  partially released condition. In this condition, the mechanism could be released with further travel of the  lever arm. Minimal added force was required, on either side of the lever, to pull the lever free from the  engaged toggle. When the partially released mechanism was activated with the opposite (redundant)  SMA wire circuit, it released full preload. This distinguishes the condition from a "jam" in which increased  force is encountered with mechanism movement. What we observed was a stable lever position at a point  in the travel which should have resulted in complete mechanism release.  80