S-92 crash – TSB report – tail rotor, MGB, “run dry”, flotation system

From the Transportation Safety Board report on Thursday:

• Flight data recorder stopped working at approximately 800 feet ASL
• The aircraft was in a controlled, powered-down descent at the time.
• At approximately 500 ft ASL, the tail rotor drive power was lost (Cause unknown).

 

Photo of 491’s tail rotor, showing damaged teeth.

• The tail rotor appears to be directly implicated in the crash, a point previously unknown.  At the time of the crash, there was no apparent loss of power to the main gearbox and it was rotating.  This suggests that it had not seized as would be expected if zero oil state had been attained due to leak and the gearbox had failed entirely.  Without oil, it would seize and stop turning.  Initial reports identified this as the main issue and the whole idea of a run dry capability served as the basis for public comment.  it also appears to be a factor in a lawsuit in the United States, launched Thursday.

Left:  tail rotor take off gear, new

Right:  tail rotor take off gear recovered from  Cougar 491

• The TSB report notes suggest another issue (not the MGB) led to the crash:

“Examination of the MGB indicates that there was no loss of main rotor drive and that the main rotor blades were rotating at the time of the impact. The examination of the MGB also revealed that the tail rotor drive gears had been severely damaged, resulting in a loss of drive, causing it to stop producing thrust. Further examination is being carried out by the TSB Engineering Laboratory to determine the cause and sequence of this loss of tail rotor drive.

The metallurgical examination of the titanium oil filter attachment studs revealed fatigue cracking in the studs as well as evidence of thread damage. A detailed metallurgical examination of the studs, nuts, and filter bowl is under way to identify the origin of the fatigue cracks and to determine the fracture mechanism.” [Emphasis added]

 

• Pilots initiated a main power shut down, immediately  (Time stamps show three seconds), which is standard procedure according to TSB.
• “The helicopter struck the water at approximately 1226 in a slight right-banked, nose-high attitude at an approximate location of 47°26'03" N, 051°56'34.8" W, with moderate speed and a high rate of descent.”
• TSB considers the nose-high attitude consistent with a flared landing approach.  With zero power, the descent would have been spiralling due to loss of the tail rotor.
• The aircraft flotation system did not deploy on impact. “The Sikorsky S-92A flotation system activation switch was found in the armed position after recovery.”
• The investigation revealed an issue with the S-92A flight manual:
• “…there is a perception in some areas of the aviation community that the MGB can be run in a dry state - that is, without lubricating oil - for 30 minutes. FAR 29 does not require run-dry operation of a gearbox to meet the 30-minute”.
• “The Sikorsky S-92A Rotorcraft Flight Manual (RFM) has been reviewed regarding MGB oil pressure loss below 5 pounds per square inch (psi) and the need for pilots to land immediately. An RFM revision has been approved by the FAA and Transport Canada.”
• Other issues are under investigation, as would be expected in an investigation of this type:

“A number of issues regarding survivability such as passenger immersion suit and crew flight suit effectiveness, use of underwater breathing devices, adequacy of survival training, adequacy of general ditching procedures, personal locator beacons, weather/sea state flight limitations, and Sikorsky S-92A flotation system are currently under investigation.”