19 June 2009

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_3tail rotor gear


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.”


Ryan said...

Why did Sikorsky fail to inform operators and pilots that the S-92 was fatally flawed.

New allegations about the March 12 crash are emerging in the lawsuit filed by the victim's families that the company actually knew the S-92 was unsafe, but failed to inform operators and pilots. Evidence suggests that the pilots acted on the assumption that the S-92 had the required run dry time of 30 minutes (obviously not the case). If these allegation are true, the company and the regulators that certified them have a lot to answer for.


Edward G. Hollett said...

Likely because it isn't "fatally flawed".

On the same day that suit was filed, the TSB report indicated that the main gearbox was functioning (turning), despite being at zero oil pressure for upwards of 15 minutes at that point.

Your comment demonstrates the dangers of leaping to conclusions before all the evidence is in.

Ryan said...


The evidence is actually coming in (ahhh the internet).

Last time I checked 15 minutes is not 30 minutes. Sikorsky's massive marketing campaign toted the S9A as having the required 30-minute run-dry time (required by the FAA), deliberately misrepresenting the aviation community (afterall this chopper was marketed as one of the safest in the world), reported by the Globe and Mail:


Of course evidence will continue to come in like the FAA's immediate response: This week, the U.S. Federal Aviation Administration ordered changes to the S-92’s flight manual, instructing pilots to land immediately if a gearbox oil leak is detected (Globe and Mail, June 20, 2009).

Of course the courts will ultimately decide who is responsible in this case (should be a good one to keep an eye on, for reasons of justice and prevention), but in the court of public opinion (we are a democracy), something really seems afoul - evidenced by the attention this issue (Sikorsky's alleged failure of a critical safety test, coupled with marketing lies to cover things up) is receiving over the net.

I suspect the foul odour will increase as the evidence mounts.

Edward G. Hollett said...


First - Flying and working at 15 minutes is most emphatically NOT - to paraphrase the speculation to date - failed and caused a crash and therefore the whole thing about run dry was a lie.

The evidence is coming in and it doesn't back up the initial specualtion.

Second - Uniformed opinion and sometimes irresponsible speculation will continue to pursue a line even after evidence to the contrary.

It's not unlike the grassy knoll, Sasquatch and the latest piece of scum-thinking on the Internet - that 9/11 was a government plot and that airliners didn't crash into the building, they were really surplus A-3 Skywarriors owned by Raytheon.

On a less exaggerated but related tack we had completely unfounded, irresponsible comment on the immersionsuits endorsed by a whole host of people including the Premier. He, moreso than most, should have appreciated that there was no reason to make the statements he did.

others get a bit more of a break for the bullshit but in my book they are all on par. At least with the initial MGB issue there was some reason to suspect it.

Third - the changes to the flight manual cleared up what was fairly obviously a misleading or unclear statement, nothing more.

Fourth - Before anyone starts slagging off Sikorsky for what would amount to some pretty serious and potentially criminal offenses, it would be nice to have some solid evidence.

The run dry thing needs to be explored in greater depth and with greater are before anyone suggests the company misled people.

On top of that now that we can see the state of the tail rotor, we need to see what TSB has to say about that.

That's a much more interesting issue.