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An Avoidable Accident

On March 12, 2009, a Sikorsky S-92A helicopter operating as Cougar Helicopters Flight 491 crashed into the ocean 30 nautical miles east of St. John’s, Nfld., killing 17 of the 18 on board. While the TSB investigation report is to be released shortly, much is already known about the causal factors contributing to the outcome.


January 25, 2011
By Ken Armstrong

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On March 12, 2009, a Sikorsky S-92A helicopter operating as Cougar Helicopters Flight 491 crashed into the ocean 30 nautical miles east of St. John’s, Nfld., killing 17 of the 18 on board. While the TSB investigation report is to be released shortly, much is already known about the causal factors contributing to the outcome.

While the final report on the accident has not been released, the TSB has provided the following preliminary data and Sikorsky has already settled by paying an undisclosed amount for its liability in the accident. Further litigation is ongoing.

The primary cause initiating the emergency was the loss of oil pressure to the main transmission. The metallurgical examination of the titanium oil filter attachment studs revealed fatigue cracking in the studs as well as evidence of thread damage. This resulted in the rapid loss of oil supply and a drop in oil pressure below the five-psi range, which requires an emergency descent and immediate landing according to the Rotorcraft Flight Manual (RFM).

TSB examination of the main gear box (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. However, this information is not yet conclusive that the helicopter had full and unimpeded auto-rotational capability after the engines were shut down by the crew.

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The helicopter was en route at 9,000 feet to an offshore rig at 110 knots when MGB oil pressure dropped rapidly. The crew turned towards the coastline of Newfoundland and descended to approximately 800 feet above the water and cruised at 133 knots for a period of time.

Just before the flight recorder stopped working for an unknown reason at 1225:17, engine power was reduced and a descent from 800 feet was initiated until about 1225:44, at which time driving power to the tail rotor was lost due to the overstressing and complete destruction of the gears in the tail rotor drive shaft at the main transmission. While indicating 85 knots at 1225:47 and descending through approximately 500 feet, both engines were shut down. Subsequently, the aircraft experienced a number of large and rapid attitude changes. At 1225:54, Flight 491’s pitch attitude increased from approximately 10 degrees nose down to about 16 degrees nose up, which is consistent with a flare for an engines-off landing. The helicopter struck the water at approximately 1226 in a slight right-banked, nose-high attitude at moderate speed and a high rate of descent.

The Sikorsky S-92A flotation system activation switch was found in the armed position after recovery; however, the inflation bottles had not fired to inflate the flotation system. This is undergoing further investigation to determine if there was a mechanical failure of the system or whether the crew failed to inflate the bladders since, by all indications, they had chosen to attempt to reach the shore.

So, what could the crew have done differently to avert the tragedy? Whatever kind of helicopter you fly, from a Bell 47 to a Boeing CH-47, a loss of MGB transmission pressure dictates an immediate landing. Moreover, a stressed transmission does not suggest pulling in heaps of power to speed up its destruction. Levelling off high above the terrain/water placed all occupants in an un-survivable situation. Attempting to extend the flight by approximately another 20 minutes to a more desirable landing area against the RFM procedure simply provided more stress to the transmission and reduced the likelihood of survival.

Some aviators believe FAA part 29 requires the transmission to be able to run dry for 30 minutes. It does not. While I realize some will observe hindsight is 20/20, the only prudent action by the ill-fated crew was essentially “written in stone.” Had the pilots initiated an immediate and accelerated descent to landing on the water with a deployed emergency flotation system and prepared to abandon the helicopter if the sea state dictated evacuation – everyone could have lived.

So, what’s the lesson here? Not only should a pilot abide by operational limitations of engines and transmissions but also, when the rare emergency occurs, one must follow the procedures of the appropriate flight manual unless some unusual and unanticipated circumstances dictate modification of an emergency procedure.

The Transportation Safety Board of Canada (TSB) is scheduled to hold a news conference on Wednesday, Feb. 9, to make public the final TSB investigation report (A09A0016) into the crash of Cougar Helicopters Sikorsky S-92A.


Ken Armstrong has flown 89 helicopter types and provides accident reconstruction and expert opinions to law firms and the courts.


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