What happened
On a scheduled flight from Québec International Airport/Jean Lesage to Montréal International Airport/Pierre Elliott Trudeau, an Airbus A310-308, registration C-GPAT, experienced a significant loss of altitude control during its initial climb. After departing Runway 06, the aircraft climbed rapidly, reaching a rate of 6300 feet per minute. As the crew attempted to level off at the assigned 3000 feet altitude, the aircraft's trimmable horizontal stabilizer (THS) shifted to a nose-down position.
This change in trim caused the aircraft to descend through its target altitude, dropping as low as 1393 feet altitude. During this period, the aircraft accelerated significantly, reaching speeds of up to 370 knots, triggering multiple overspeed and terrain warnings, including 'Pull Up' and 'Too Low Terrain' alerts. The pilot flying inadvertently engaged go-around mode while attempting to manage the descent, further complicating the flight profile. The co-pilot eventually took control of the aircraft to stabilize the pitch. The crew declared a PAN PAN urgency message and later an emergency due to perceived airspeed indication issues, before successfully landing in Montréal without further incident. There were no injuries.
The investigation
The investigation focused on the mechanical and procedural factors that led to the uncontrolled descent. Investigators examined the aircraft's flight data, which showed the THS moving from a nose-up to a nose-down attitude during the level-off phase. The investigation also looked into the cockpit environment, noting that the co-pilot's headphones were partially unplugged during takeoff, which delayed a rotation speed warning.
Technical analysis of the flight simulator used for training revealed a modeling error in the artificial feel unit, where the control column force did not increase realistically at high speeds. Additionally, the investigation reviewed the crew's handling of the situation, noting that several inadvertent radio transmissions were made on the tower frequency during the high-workload period of the descent.