Spinwing, the BAH Chinook crash was caused by a transmission failure.
Briefly a big ring-gear (about 2 feet across) is fitted to the vertical mainshaft with a lot of bolts to take power from the horizontal high-speed shaft. Some, that is some, of these bolts were found to be losing torque slightly over the x000hr inspection period. A hugely complex and costly fitting of oversized bolts with higher torque capability was instigated.
Primarily due to a change in the washers used under these new bigger bolts (a lubrication channel was deleted) salt particles got into the thread area and started a corrosion pit which propagated a circumfrential stress fracture in the ring gear which finally failed. The 'box was fitted with a comprehensive chip detector system. Chip detectors cannot detect a stress fracture until metal starts spalling off, by which time it's probably far, far too late.
Post modified in deference to the remarks in the post below, though very little of the original was apparently incorrect.
Addendum...
At the time it was widely & popularly believed within the BAH community that the modifications were unnecessary, that the BA and Boeing engineers had developed a perfectly acceptable solution to the torque loss that did not involve oversizing bolts (and thus the incorporation of the non-scalloped shim. This, iirc, did involve Loctite) and that this mod had been forced against their better judgement by the CAA despite strident objections. Further, it was stated categorically within my hearing by people who should have known that one feature of the scalloped (grooved) shim was that it allowed lube oil to flush salt contaminants out of the joint, thus removing a potential cause of corrosion. The new-mod solid shims could not do this, and the view was that this lack of flushing almost certainly promoted, if not caused, tha corrosion pit that started the process.
Cetainly not a forseeable error chain, but a good example of the complexity of such events.
Last edited by Agaricus bisporus; 12th Apr 2009 at 13:25.