The Transportation Safety Board is trying to find out what caused a Canadian North jet to roll, shortly after take off last December.

The Boeing 737-300 had just left Fort MacKay, Alta., with 133 passengers and 6 crew members on board.

The TSB says as the airplane had climbed to about 800 feet when it began a 10 degree roll to the left.

The pilots tried to counter to the right, but couldn't. Then there was a drop in air speed.

"The whole manoeuvre last 15 to 20 seconds,” says John Tavernini, a senior technical investigator with the TSB. “It wasn't very long, but it was a period of time when the pilots felt they were losing control of the aircraft. But control was regained before any upset occurred.”

Tavernini says the nose of the airplane also pitched up, when the pilots were trying to point the nose down.

“Once their air speed recovered somewhat, they retracted flaps and the issue resolved itself."

Tavernini says Canadian North grounded the plane when it landed in Edmonton and did a thorough inspection.

It found minor issues that were unrelated, but nothing that would cause the roll. 

The plane was put through a test flight and was later put back in service.

Incident unreported

The TSB didn't find out about the incident until two weeks ago — more than six months after it happened — because the airline failed to report it.

"We got missed in December and through a safety quality assurance audit that the company did, they found the omission and corrected it as soon as they were aware of it," Tavernini says. 

The TSB is now analyzing the flight data recorder files to try to find out what may have caused the plane to roll.