Safety Investigators in Canada have determined that pilots of a Cougar Helicopters Sikorksy S-92 that crashed in the Atlantic Ocean off the coast of Newfoundland on 12 March, resulting in the deaths of 17 of the 18 on board
, were likely trying to autorotate the heavy twin helicopter after a tail rotor drive failure, precipitated (my words, not theirs) by a loss of main gear box (MGB)`lubrication fluid. After the helicopter hit the water, the floatation system, for reasons unknown, failed to inflate. Pictured above is the reconstruction of the crash helicopter by the Transportation Safety Board of Canada (TSB).
Though the details of how successful that autorotation maneouver turned out to be are yet to be determined -- initial data shows the helicopter hit the water with a force of 20Gs -- what is clear is that the problem that initiated the deadly chain of events in the first place might have been flagged up before the helicopter was certified by the US Federal Aviation Administration.
Though the analysis is not yet complete, the evidence strongly suggests that two cracked titanium mounting studs used to hold an external oil filter for the helicopter's main gear box (MGB) cracked and allowed the transmission oil to quickly drain out. Picture below, from TSB, shows a one of the missing oil filter studs on the right side...
An emergency airworthiness directive after the fact required the replacement of those studs with steel equivalents throughout the fleet. Sikorsky earlier had given operators a year to do the changeout, a suggestion spawned by similar but non-lethal failures in the fleet.
Once that oil drained, the rotating machinery had no cooling. In picture below, also from TSB, damage to the tail rotor pick-off gear (connected to the MGB to power the tail rotor) is obvious when compared to a new pinion on the left...
In the aftermath of the crash, the Transportation Safety Board of Canada (TSB) says the FAA and Sikorsky "are working to identify all the modes of failure that might lead to Sikorsky S-92A MGB oil loss, determining their probability of occurrence, and developing appropriate mitigation strategies."
Huh? Shouldn't such a failure modes and effects analysis (FMEA) be done BEFORE certification?
Though under Part 29 certification rules for rotorcraft, failures that result in the loss of lubrication to the drive train, in this case the MGB (the mechanism that takes in power from the S-92's two turboshaft engines to drive the main rotor, tail rotor, power systems, etc), must allow for 30 minutes of flight time after the crew receives an indication of the problem in the cockpit, there's an escape clause of sorts in the regs... If a failure mode is determined to be "extremely remote", the 30 minute rule no longer applies...
It's not clear in the case of the Cougar crash whether pilots, through tribal knowledge, assumed there was 30 minutes to get back to shore despite emergency guidance material for such anomalies. The helicopter ultimately went into the water about 10 minutes after losing its lubrication oil.
And why didn't the "30 minute rule" apply?
"Based on the applicable guidance material at the time of certification, the lubrication failure modes of interest were limited to the failure of external lines, fittings, valves, and coolers," writes the TSB in an 18 June update on the accident.
"This practice was consistent with industry experience, which had found that loss of lubrication tended to be associated with external devices. Therefore, the possibility of a failure at the oil filter was considered to be extremely remote. As a result of the fracture of the filter bowl mounting studs, resulting in the loss of a large quantity of oil, the certification guidance material is being reviewed."