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Fatigue causal factor in sudden descent after perceived a collision course
17 April 2012

The Transportation Safety Board of Canada (TSB) released its investigation report on an occurrence in which the first officer of an Air Canada Boeing 767-300 on an overnight flight from Toronto, Ontario, to Zurich, Switzerland, started a sudden descent after perceiving a collision course with an oncoming aircraft over the North Atlantic.

The first officer was napping during a controlled rest period, a common method of combating fatigue where one flight crew member takes short naps at certain times during a flight. 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. 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. As the oncoming aircraft safely passed underneath, the captain regained control and returned to their cruising altitude. Fourteen passengers and 2 flight attendants, who were not wearing seatbelts, were injured during the descent and subsequent recovery. The seatbelt sign had been on for the 40 minutes leading up to the event.

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. The investigation also found that crews did not fully understand the risks associated with fatigue or the procedures for conducting controlled rest. Since the occurrence, both the company and the pilots’ association have taken measures to enhance flight and cabin crew awareness of the controlled rest procedures and to better understand the levels of fatigue experienced by crews flying overnight flights from North America to Europe.

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