Drunken passengers aboard a floatplane that crashed near Tofino, B.C., last year likely interfered with the pilot during the short flight, causing him to lose control, according to a Transportation Safety Board report.

"What was happening in the cabin moments before the pilot lost control cannot be accurately determined," according to the TSB report released Wednesday. "However, it can be concluded that this probably involved activity by the unsecured passengers that interfered with the pilot and his control of the aircraft."

The Cessna 185 operated by Atleo River Air Services crashed during a six-minute trip to the community of Ahousat in May 2010, killing all four people on board. 

Katrina English, 22, her brother Edward Sam, 28, and their cousin Samantha Mattersdorfer, 24, all from the small coastal First Nations community of Ahousat on Flores Island, were killed in the crash, along with pilot Damon York, 33, of Tofino.

The aircraft struck the water at an angle and speed consistent with a deliberate dive, or a loss of control, the report said.

"There is no operational reason why the pilot would perform such a manoeuvre," it said.

The TSB reported that passengers were intoxicated at the time they boarded the aircraft, and had previously been argumentative.

The report said it was possible that one of the passengers seated behind the pilot kicked the pilot's seat forward and held it there. This could have pushed the pilot into the instrument panel and the controls forward, inducing a dive.

"It is likely that passenger interference caused the pilot to lose control of the aircraft whereupon it descended in a steep nose-down attitude until it struck the water," the report said.

The report also said the passengers may not have recognized the seriousness of the situation because of their level of intoxication and didn't stop interfering with the pilot in time for him to regain control of the aircraft before impact.

The absence of a locking mechanism on the pilot's seatback, and the fact the pilot was not wearing his shoulder strap, meant "he would have been unable to prevent his upper body from being forced onto the instrument panel," the report said.