Originally Posted by
megan
The certification process cannot uncover all the issues that may arise once the aircraft is put into the rigours of commercial use.......Only ever flew the H-34 version, but once again I'm not about to suggest the aircraft had a history of main G/B issues. Stuff happens.
megan,
Your point is excellent. The purpose of qualification testing, including the run-dry test procedure, is to validate all analysis used to certify the design. There is a specific type of certification analysis performed called Failure Mode, Effects and Criticality Analysis (FMECA) that is intended to ensure no credible failure mode will produce a catastrophic event. The FMECA evaluates every component and system from design thru manufacturing and throughout its service life. Of course, the analysis does not consider unforeseen conditions, as you noted above.
The 1982 CH-47 crash in Mannheim is a perfect example of how an unanticipated event involving the lube system of a main rotor gearbox resulted in a catastrophic failure. What makes this case even more unusual is that it was caused by loss of flow at a single oil jet, while the rest of the gearbox lube system continued to function normally. The crew had no idea what was causing the problem, and the final catastrophic structural failure occurred just a couple minutes after the crew became aware of a problem. The picture below shows the rear main gearbox separated from the airframe.
It is quite possible that the aircraft had been flying with the blocked oil jet for a substantial number of hours. Unfortunately, this particular loss-of-lube failure mode was never considered in the FMEA. So no steps were taken to prevent it from happening, or even alert the crew it was occurring so they could take appropriate action.
After it was determined that the root cause of this loss-of-lube failure was the small oil jet orifice becoming obstructed by walnut shell debris trapped inside the gearbox housing oil galleries from grit blast cleaning during an overhaul, the overhaul procedures were modified and the design of oil jets were revised. It is now standard industry gearbox design practice to include a "last chance" filter screen on every lube oil jet to prevent debris from obstructing the small diameter metering orifice holes.
