Sleep inertia lay behind a bizarre Air Canada in-flight upset after a pilot executed an evasive manoeuvre after misperceiving aircraft lights, and a planet, as a potential collision threat.

Fourteen passengers and two cabin crew were injured when the first officer put the Boeing 767-300 - which had been cruising at 35,000ft - into a descent, losing 400ft of altitude, before the captain took over. As it recovered the aircraft climbed to 400ft above its assigned level.

Flight AC878 had been halfway through a transatlantic Toronto-Zurich service when the first officer had taken a controlled rest. While the rest was supposed to be limited to 40min, he awoke after 75min and, shortly afterwards, was advised by the captain about oncoming traffic.

The oncoming aircraft - a Boeing C-17 military transport - had been below the 767, at 34,000ft.

While scanning for the traffic the first officer initially misperceived the planet Venus as an aircraft, but was corrected by the captain, says the Transportation Board of Canada in its report into the event.

When the first officer then sighted the aircraft's lights, he erroneously thought that they were above the 767 and descending towards it - and reacted by putting the 767 into an evasive dive. He pitched the aircraft 6° nose-down before the captain, who had been monitoring the traffic on his navigation display, responded by disconnecting the autopilot and putting the jet into a climb.

During the incident, which lasted 46s, the collision-avoidance system did not issue either traffic or resolution advisories.

TSB investigators point out that the first officer had suffered interrupted sleep in the 24h before the flight. When combined with the eastbound night flight, coinciding with the biological circadian low point, this increased the risk of fatigue, it says, adding that the first officer "fell completely asleep" during the controlled rest.

The captain allowed the first officer to rest beyond the 40min limit, the inquiry states, but this increased the probability of entering a deeper sleep - arousal from which can worsen the severity of sleep inertia.

It says that the first officer had only been awake for 1min when the captain pointed out the oncoming aircraft and was "not in a state to effectively assimilate the information". Under the effects of sleep inertia, the first officer was "drawn to rely on immediate perceptual information" and was probably "confused and disoriented", perceiving the 767 to be on a collision course.

The attempted evasive manoeuvre subjected the 767 to vertical acceleration ranging from -0.5g to 2g over a 5s period. Its airspeed initially increased by 7kt then fell by 14kt as the captain recovered the twinjet and restored its flightpath.

There had been only 95 passengers on board the lightly-loaded aircraft, and the seat-belt sign had been switched on some 40min earlier owing to possible turbulence. The inquiry says the number of passengers wearing the belts could not be determined. The crew had opted to fly the centreline of the North Atlantic track, rather than offset laterally.

Source: Air Transport Intelligence news