Investigation Summary
The accident originated from the fracture of two titanium studs on the main gearbox oil filter assembly. These studs failed due to galling and fatigue, a known vulnerability that had prompted Sikorsky to issue an Alert Service Bulletin (ASB) in January 2009 recommending their replacement with steel studs. The investigation revealed that the S-92A had been certified under an 'extremely remote' failure probability clause, exempting it from the standard 30-minute 'run dry' requirement. This certification was based on the assumption that all major oil leaks would occur in the oil cooler system, which could be bypassed. However, the Flight 91 leak occurred at the filter bowl, which could not be bypassed. When the oil pressure dropped to zero, the crew activated the bypass as per the checklist, but because the leak was at the filter, the bypass was ineffective. The crew's decision to continue flight for 11 minutes instead of ditching immediately was influenced by the lack of secondary indications (like rising oil temperature) and a misunderstanding of the gearbox's 'run dry' capabilities. The eventual failure of the MGB gears led to the loss of the tail rotor drive, making the aircraft uncontrollable just before impact.
Final Conclusions
The TSB identified 16 contributing factors, primarily focusing on the mechanical failure of the titanium studs and the regulatory certification that allowed the S-92A to operate without a proven 30-minute run-dry capability. The crew's decision-making was hampered by a checklist that did not emphasize the absolute necessity of an immediate landing regardless of secondary indications. The investigation also highlighted deficiencies in the flotation system, which failed to deploy properly, and the lack of standardized maintenance for pilot immersion suits.
Photographic Evidence (1)
Video Analysis
Was This Helicopter Doomed By Design? Air Disasters — A detailed look at the investigation into Cougar Flight 91, focusing on the Sikorsky S-92A's main gearbox design and the certification process that bypassed the 30-minute run-dry test.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Pilot Flying (PF) during the emergency. Identified as Matthew Davis, age 34.
First Officer
Pilot Not Flying (PNF) during the emergency. Identified as Tim Lanouette, age 47.
Systems & Failure Modes
Main Gearbox (MGB)
The central transmission system that drives the main and tail rotors. It failed due to total oil starvation after the filter bowl studs fractured.
MGB Oil Bypass
A system designed to isolate the oil cooler in the event of a leak; it was ineffective because the leak occurred at the filter assembly, not the cooler.
Multi-Purpose Flight Recorder
A combined CVR and FDR unit manufactured by Penny and Giles that provided the data for the investigation.
Interesting Facts
- 01The S-92A main gearbox failed only 11 minutes after total oil loss, despite a certification assumption that such a failure was 'extremely remote'.
- 02Sikorsky had issued an Alert Service Bulletin (ASB) to replace titanium studs with steel ones after a similar non-fatal incident in Australia in 2008.
- 03The aircraft struck the water with an acceleration of approximately 20g.
- 04The aircraft's integrated flotation system was activated but failed to operate correctly; however, no deficiencies were found in the Mustang flotation vests worn by the crew.
- 05A post-accident inspection of available Cougar Helicopters pilot immersion suits found 16 out of 25 were unserviceable.
Safety Actions & Advisories
FAA Emergency Airworthiness Directive 2009-07-53
Required the immediate replacement of titanium MGB filter bowl mounting studs with steel studs on all S-92A aircraft.
RFM Procedure Revision
The S-92A Rotorcraft Flight Manual was modified to mandate an immediate landing if MGB oil pressure drops below 5 psi, regardless of other symptoms.
Known Controversies & Unanswered Questions
Certification 'Run Dry' Exemption
Reference ↗The S-92A was certified despite failing the standard 30-minute 'run dry' test (it lasted only 10 minutes). Certification was granted based on the 'extremely remote' possibility of a total oil loss, an assumption that proved incorrect as it only accounted for leaks in the oil cooler system and not the filter bowl.
Fraudulent Misrepresentation Lawsuit
Reference ↗In 2010, Cougar Helicopters sued Sikorsky for over $25 million, alleging the manufacturer 'fraudulently misrepresented' the S-92's run-dry capabilities and failed to adequately warn operators about the severity of the 2008 Australian incident. The case was settled out of court in 2011.