Occurrence
On 10 January 2008, an Air Canada Airbus A319-114 (registration C-GBHZ, serial number 0813) operating as flight ACA190 was en route from Victoria International Airport, British Columbia, to Toronto Lester B. Pearson International Airport, Ontario. The aircraft carried 83 passengers and 5 crew members. At flight level (FL) 350, it followed a United Airlines Boeing 747-400 (flight UAL896) at FL 370 over Washington State, United States. When separation reached 8.1 nautical miles (nm) with UAL896 ahead, U.S. air traffic control cleared ACA190 to climb from FL 350 to FL 370. At 0648 Pacific standard time, as ACA190 climbed through FL 366, 10.7 nm behind UAL896, sharp jolts were felt followed by a series of rolls. During the oscillations, the aircraft climbed to FL 369 then descended to FL 355, where the crew regained straight and level flight. The crew declared an emergency and diverted to Calgary International Airport, Alberta, landing uneventfully at 0728.
Injuries
Eight passengers and crew received minor injuries and three received serious injuries due to falls and collisions with aircraft furnishings.
Flight Dynamics
Digital flight data recorder (DFDR) data indicated that at 0648, ACA190 was in a stable wings-level climb through FL 366 when three sharp jolts occurred. The aircraft rolled right 5.6°, countered by the autopilot, then rolled left to 27.8°. The captain disengaged the autopilot and autothrottles and attempted correction, followed by four rolls up to 55°. The event lasted 18 seconds; heading varied from 065°M to 086°M. The captain made nine sidestick roll inputs, five to full travel of 20°, and seven successive rudder pedal inputs with six cyclic reversals. Vertical accelerations peaked at +1.57g and −0.77g; lateral accelerations reached +0.49g right and 0.46g left. Analysis showed that after autopilot disconnection, most roll motion resulted from pilot inputs, and lateral accelerations were due mostly to rudder inputs.
Weather
Analysis by the U.S. National Center for Atmospheric Research indicated no remarkable weather systems; likelihood of no more than light turbulence. Both aircraft flew in clear air above cloud, with no significant turbulence before or after the event. Wind at FL 370 was 270°M at 52 knots.
Flight Control Laws
The Airbus A319 uses a fly-by-wire system. Normal law provides envelope protections. The event caused a reversion from normal to alternate law at 0648:14, triggered by an "FCTL ADR DISAGREE" warning due to differences in sideslip-induced angle of attack from air data inertial reference units. The remainder of the flight was under alternate law. The conditions for abnormal attitude law were not met.
Aircraft Inspection
Post-occurrence, a heavy turbulence inspection was conducted. DFDR data showed lateral accelerations exceeded Airbus limits. The vertical stabilizer was removed and inspected; no structural damage was found. The rear vertical stabilizer attachment fitting experienced 129% of limit load, and the rear fuselage fitting 121% of limit load, but certification standards require a minimum 150% factor of safety. No pre-existing faults were identified.
Flight Crew Actions
After stabilizing at FL 355, the crew reported a flight control problem and descended to FL 310. They chose to divert to Calgary due to terrain, medical, and airport facilities. Believing a flight control computer malfunction caused the event, they flew manually with autopilot off.