The Air Canada Boeing 767-333 (registration C-GHLQ, serial number 30846) was operating as flight ACA878 from Toronto, Ontario, to Zurich, Switzerland. Approximately halfway across the Atlantic, during the hours of darkness, the aircraft experienced a 46-second pitch excursion. This resulted in an altitude deviation of minus 400 feet to plus 400 feet from the assigned altitude of 35 000 feet above sea level. Fourteen passengers and 2 flight attendants were injured. The seatbelt sign had been selected "on" approximately 40 minutes prior to the pitch excursion. The flight continued to destination whereupon 7 passengers were sent to hospital and were later released.
At 0155, the captain made a mandatory position report with the Shanwick Oceanic control centre. This aroused the FO. The FO had rested for 75 minutes but reported not feeling altogether well. Coincidentally, an opposite-direction United States Air Force Boeing C-17 at 34 000 feet appeared as a traffic alert and collision avoidance system (TCAS) target on the navigational display (ND). The captain apprised the FO of this traffic. Over the next minute or so, the captain adjusted the map scale on the ND in order to view the TCAS target 5 and occasionally looked out the forward windscreen to acquire the aircraft visually. The FO initially mistook the planet Venus for an aircraft but the captain advised again that the target was at the 12 o'clock position and 1000 feet below. The captain of ACA878 and the oncoming aircraft crew flashed their landing lights. The FO continued to scan visually for the aircraft. When the FO saw the oncoming aircraft, the FO interpreted its position as being above and descending towards them. The FO reacted to the perceived imminent collision by pushing forward on the control column. The captain, who was monitoring TCAS target on the ND, observed the control column moving forward and the altimeter beginning to show a decrease in altitude. The captain immediately disconnected the autopilot and pulled back on the control column to regain altitude. It was at this time the oncoming aircraft passed beneath ACA878. The TCAS did not produce a traffic or resolution advisory.
