Investigators have determined that a Cougar Helicopters aircraft crashed belly first on March 12. The crash killed 17 of the 18 people who had been aboard. (Transportation Safety Board of Canada)

Transportation Safety Board investigators said Thursday that Cougar Flight 491 — the helicopter that crashed off Newfoundland, killing 17 people — hit the ocean belly first rather than nose first as originally suggested.

TSB investigator Allan Chaulk told a news conference in St. John's that data suggests the impact of the helicopter was very significant, one "in the plus 20 G loading range."

"The aircraft would have impacted the water somewhat tail low. We don't have a roll figure worked out yet. But certainly it was, basically, for lack of a better term, belly first in an upright position but with a tail-low attitude, and that's what the damage is telling us," he said.

The helicopter had been ferrying offshore oil workers to their platforms on March 12 when the crew reported mechanical problems.

TSB investigator Mike Cunningham added that the impact was quite abrupt with the helicopter opening up "significantly."

"Doors came off, windows came out, a lot of stuff was going on. Exactly how the two [one survivor, one deceased] got out, we're going to have to continue looking at that," he said.

The investigators also said they have a clearer picture of what happened minutes before the crash. A short time into its flight, the gearbox lost oil pressure.

The crew then informed officials in Gander that they would be turning around. Within seconds, the crew called in a mayday. At the time, they were dropping at about 300 metres a minute. And then in the next few minutes, the crew reported that they were preparing to ditch into the water.

While the investigators said there was still a lot of analysis to be done, they could safely say that a gearbox filter bowl assembly stud broke in flight, and the Sikorsky S-92 lost oil pressure as a result.

On Monday, the U.S. Federal Aviation Administration instructed owners of Sikorsky S-92A helicopters to cancel flights until the studs were replaced.

Cunningham said that as of Thursday he understood that 70 per cent of the worldwide fleet of Sikorsky S-92 helicopters have had the studs replaced and were back in service.

The main gearbox has been sent to Sikorsky offices in Connecticut where company, safety board and Cougar officials will continue their investigation.