Tail rotor failure
Transportation Cougar pilot attempting controlled landing when tail rotor lost power
MOIRA BAIRD
The Canadian Press
Cougar Helicopters Flight 491 lost power in its tail rotor drive - but not its main rotor - about a minute before it crashed into the ocean off Newfoundland on March 12 killing 17 of the 18 people on board.
The Transportation Safety Board (TSB) says the pilots were making a controlled descent to land in the water. The helicopter was about 500 feet above sea level when its tail rotor failed. Three seconds later, the pilot switched off the engines to attempt an "engines-off landing," a procedure also known as an autorotative landing.
"What it's showing us is that there was control remaining and the guys were doing their best to get the helicopter down on the surface," said Mike Cunningham, TSB investigator-in-charge, in an interview Thursday.
On Wednesday, the TSB updated the families of the crash victims on the progress of its investigation.
Without a working tail rotor, Cunningham says controlling the direction of a helicopter is difficult. The tail rotor also helps the helicopter stay in the air.
"It is one of the more serious emergencies that a helicopter pilot can experience.
"It will rock around and the nose will go up and down, and the pilot is, at that point, just attempting to get the thing down on the ground or the surface of the water, as in this case, as quick as he can."
The flight data recorder stopped working after 800 feet, but investigators were able to piece together information from the aircraft's onboard computer.
They determined the helicopter struck the water at a "moderate" speed.
"Initially, there were some reports that the aircraft had struck the water at a high speed," said Cunningham. "Now, with this data that we have available to us, we know that the forward speed had been slowed down."
He couldn't pinpoint the exact speed, though, saying TSB engineers are continuing their analysis of the data.
The pilot may have also flared the helicopter - a manoeuvre used to slow the rate of descent without the use of power. The pitch of the helicopter is adjusted nose high, which increases rotor revolutions per minute and decreases forward airspeed.
Despite this, the helicopter hit the water with a significant impact - nose upward and banking slightly to the right.
All three of its flotation devices failed as the helicopter hit the three-metre seas.
"The damage that was done to this aircraft during the impact was so significant that ... it might be unreasonable to expect that this (flotation) system could have even operated," said Cunningham. "And would it have made any difference, if it did, is pretty hard to say."
The TSB says the cylinders didn't release compressed gas to inflate the collars that are supposed to keep the helicopter afloat.
"One of the flotation bags was ripped right out of the housing," said Cunningham.
The failure of the flotation collars is in still under investigation.
The TSB also says a metallurgical examination of the titanium mounting studs revealed fatigue cracking and thread damage.
The board is still trying to determine the origin of the fatigue cracks.
During its examination of the helicopter wreckage in March, the board discovered two of the three mounting studs that attach the oil filter assembly to the main gearbox had broken in flight.
A U.S. Federal Aviation Authority directive then ordered the titanium mounting studs be replaced with steel studs. It said "the broken studs resulted in rapid loss of oil."
Without oil in the main gearbox, which powers both the main rotor and the tail rotor, there is no lubrication of rotor gear teeth.
That directive grounded the worldwide fleet of Sikorsky S-92As, including Cougar Helicopter aircraft, until the studs were replaced.
Sheldon Peddle, the president of a union that represents 700 workers on the Hibernia and Terra Nova oil platforms, remains wary of the steel studs.
"I'm not totally convinced that these steel studs are going to be any better in the long run," he said.
mbaird@thetelegram.com