Quote:
The fastening system for the oil filter bowl of the S-92A used anodized titanium alloy studs, silver-plated steel nuts and cadmium-plated steel washers. Anodizing and plating increases resistance to corrosion, wear and galling.
Really?!
Wasn't galling the failure mode here?! This just begs more questions than it answers.
Your not reading the report are you NF.
I say again, read the following paragraph as to why the S-92 was
unique in having problems with the titanium studs (historically not an issue on the 53 and 60 Series with millions of flight hours between them) and note that the
design scheduled number of times that the oil filter bowl should have been removed during the life of the MGB was just five, but Cougar was averaging around 220 hours instead of the anticipated 500 to 1000 hours.
1.6.3.3 S-92A Main Gearbox Oil and Filter Replacement
When digested, then take a butchers at points 1. to 5. of the conclusions.
3.0 Conclusions
3.1 Findings as to Causes and Contributing Factors
1. Galling on a titanium attachment stud holding the filter bowl assembly to the main gearbox (MGB) prevented the correct preload from being applied during installation. This condition was exacerbated by the number of oil filter replacements and the re-use of the original nuts.
2. Titanium alloy oil filter bowl mounting studs had been used successfully in previous Sikorsky helicopter designs; in the S-92A, however, the number of unexpected oil filter changes resulted in excessive galling.
3. Reduced preload led to an increase of the cyclic load experienced by one of the titanium MGB oil filter bowl assembly attachment studs during operation of CHI91, and to fatigue cracking of the stud, which then developed in a second stud due to increased loading resulting from the initial stud failure. The two studs broke in cruise flight resulting in a sudden loss of oil in the MGB.
4. Following the Australian occurrence, Sikorsky and the Federal Aviation Administration (FAA) relied on new maintenance procedures to mitigate the risk of failure of damaged mounting studs on the MGB filter bowl assembly and did not require their immediate replacement.
5. Cougar Helicopters did not effectively implement the mandatory maintenance procedures in Aircraft Maintenance Manual (AMM) Revision 13 and, therefore, damaged studs on the filter bowl assembly were not detected or replaced.