The wreckage of Cougar Flight 491 was retrieved from the ocean floor and brought to St. John's on a marine supply vessel. (TSB)

Transportation Safety Board investigators say there is reason to believe the pilot of Cougar Flight 491 was trying to make a controlled landing when the helicopter crashed off the coast of Newfoundland, killing 17 people.

"He tried to make a descent using auto rotation, that is to say an emergency descent where you try to keep the speed of the main rotor in order to do a landing that is not catastrophic," said TSB official Charles Laurence, speaking in French.

A TSB update issued Thursday says there was no loss of the main rotor drive on the Sikorsky S92-A and that the main blades were rotating at the time of impact on March 12.

"Engine power was reduced, a descent from 800 feet was initiated and the speed began to decrease," says the TSB in a written statement. At about the same time a flight data recorder commonly known as the black box recorder stopped operating.

Investigators found the tail rotor drive gears had been severely damaged, resulting in loss of drive, causing it to stop producing thrust.

The helicopter dropped 300 feet in 30 seconds "in a controlled manner" but then at 500 feet there was a change.

"A shutdown of both engines was initiated, which is consistent with a tail rotor drive failure emergency," says the update.

Investigators say the helicopter spiralled down from that point, dropping 500 feet in 13 seconds before hitting the water.

The TSB says a flotation device that should have prevented the Cougar helicopter from sinking failed to inflate.

"The Sikorsky S-92A flotation system activation switch was found in the armed position after recovery," says the TSB update. "Examination of the inflation bottles indicates that they had not released their compressed gas to inflate the flotation collars."

The safety board says it is still investigating why the collars failed to inflate when the helicopter hit the water.

Cougar Flight 491 ditched about 65 kilometres off the coast of Newfoundland while carrying workers to offshore oil platforms

One man, Robert Decker, survived the deadly crash.