What happened
On 10 January 2008, an Air Canada Airbus A319-114, registration C-GBHZ, was operating a scheduled flight from Victoria, British Columbia, to Toronto, Ontario. While cruising at flight level 366 over Washington State, the aircraft encountered significant disturbances while following a United Airlines Boeing 747-400.
The aircraft experienced three sharp jolts, followed by a series of intense roll oscillations. During the event, the aircraft's heading shifted significantly, and the pilot disengaged the autopilot to manually correct the rolls. The turbulence caused the aircraft to descend to flight level 355 before the crew regained stable flight. Due to the perceived flight control issues, the crew diverted to Calgary International Airport. The incident resulted in three serious injuries and eight minor injuries among the passengers and crew, caused by collisions with the cabin interior during the turbulence.
The investigation
Investigators analyzed data from the digital flight data recorder to reconstruct the flight dynamics. The analysis revealed that the aircraft experienced peak vertical accelerations of +1.57g and -0/77g, along with significant lateral accelerations.
Technical examination of the flight control systems showed that the aircraft transitioned from normal to alternate flight control law due to a disagreement between the air data inertial reference units. The investigation also looked into the aircraft's structural integrity. While the vertical stabilizer and its attachment fittings were subjected to loads exceeding their limit loads, no structural damage was found during subsequent non-destructive testing.
Findings
- The primary cause of the upset was the encounter with wake turbulence from the preceding Boeing 747.
- The pilots were not notified by air traffic control of the potential for wake turbulence encounter.
- The crew's manual recovery efforts included significant rudder pedal inputs, which contributed to the lateral accelerations experienced by the aircraft.
- At the time of the event, maintenance manuals did not provide specific instructions for inspecting the vertical stabilizer following high lateral g-force events.