The mother of a pilot who died in a plane crash that may have been caused by drunk passengers is calling her son's death sickening and inexcusable.

Damon York, 33, was one of four people who died when his plane went down off the west coast of Vancouver Island, north of Tofino, in March 2010.

The report issued by the Transportation Safety Board on Thursday  concluded  York's passengers were drunk, and likely interfering with his flying, possibly by kicking the back of his seat, pushing him into the controls and causing the plane to nosedive.

York's mother, Lee York, says the family is devastated by the news.

"It has been heart-wrenching. Every day, he dies every day for me," she said. "I'm sick. I am so sick.

"They were just so drunk and to say they didn't know what they were doing, they were so drunk, that just doesn't excuse it," she said.  

Drunk and arguing before flight

TSB regional manager Bill Yearwood said the investigation revealed the three passengers had been drinking heavily and were argumentative before they even got on the plane.

"I can only say there was a high level of alcohol intoxication in the passengers," Yearwood said Thursday.

Investigators believe the passengers may have been so drunk they didn't realize they were causing the plane to crash.

"What was happening in the cabin moments before the pilot lost control cannot be accurately determined. 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," said the report.

Yearwood said the accident wasn't survivable because of the force with which the aircraft struck the water.

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.