An Air Canada flight that suddenly nosedived, injuring 16 people, was the result of a napping pilot's extreme fatigue and ignorance of the rules, the Transportation Safety Board has concluded.

In January 2011, an Air Canada Boeing 767 on an overnight flight from Toronto to Zurich, Switzerland, made a sudden descent, injuring 14 passengers and two crew members who were not wearing seatbelts.

That's because the first officer, having just awoken from a nap, mistakenly believed the plane was on a collision course with another aircraft, the TSB probe found.

"The captain made a position report, causing the first officer to wake up. At roughly the same time, another aircraft was approaching from the opposite direction a thousand feet below," reads the report.

"The captain, who was the pilot in control of the aircraft, had visual contact with the oncoming aircraft. Under the effects of significant sleep inertia - when performance and situational awareness are degraded immediately after waking up - the first officer perceived the oncoming aircraft as being on a collision course and began a descent to avoid it."

The TSB notes that mid-flight naps are not unusual for pilots. It's called a "controlled rest period" and is a "common method of combating fatigue where one flight crew member takes short naps at certain times during a flight."

But the first officer was too fatigued to fly to begin with, napped at the wrong time and for too long and didn't follow proper controlled rest procedures, the report notes.

"The first officer's level of sleep inertia was magnified by prior fatigue. Also contributing to the significant sleep inertia was napping during a period of the night that made deep sleep more likely, and napping longer than allowed by the company's controlled rest procedure," reads the report.

"The investigation also found that crews did not fully understand the risks associated with fatigue or the procedures for conducting controlled rest."

Air Canada and its pilots' union have since taken measures to ensure staff are aware of the proper protocols, the report said.

"This occurrence underscores the challenge of managing fatigue on the flight deck," said lead investigator Jon Lee. "It also shows that in-flight passenger injuries can be prevented by wearing seatbelts at all times while seated."