What happened
On a single day in 2002, two separate Air Canada Airbus A321-211 aircraft experienced significant flight control instabilities during their approaches to Toronto/Lester B. Pearson International Airport. The first aircraft, C-GJVX, was on final approach to Runway 24R with full flaps selected when it encountered roll oscillations at approximately 140 feet above ground level. The flight crew attempted to level the wings, resulting in a firm touchdown.
Roughly three hours later, a second A321-211, registration C-GIUF, experienced similar oscillations at approximately 50 feet above ground level. In this instance, the crew performed a go-around, adjusted flap settings, and successfully landed on a subsequent approach. There were no injuries and no damage to either aircraft. In both cases, the aircraft had accumulated significant mixed ice on the leading edges of the horizontal stabilizer and wing areas not covered by anti-ice systems.
The investigation
The investigation examined flight data recorder information and analyzed the aerodynamic impact of ice accretion. Investigators found that both aircraft had been dispatched with up-to-date weather information and were aware of moderate icing forecasts. Both crews had followed standard procedures by increasing approach speeds to accommodate the icing conditions and using engine and wing anti-ice systems.
Post-landing inspections revealed up to 3/4 inch of ice on the unprotected surfaces of the wings and stabilizers. The investigation also looked into the droplet size of the precipitation, noting that the presence of drizzle suggested the encounter with supercooled large droplets (SLD) may have exceeded the standard certification envelope. Further testing by Airbus confirmed that ice on the flap leading edges and flap tabs altered the aircraft's aerodynamic response in the roll axis.
Findings
- The primary cause of the instability was airplane-pilot coupling, where the combination of pilot inputs, the aircraft's lateral flight control laws, and modified aerodynamics caused an unstable oscillation.
- The A321's normal lateral flight control laws provided higher roll efficiency in the 'CONFIG FULL' flap setting compared to 'CONFIG 3', which significantly reduced the stability margin during icing.
- Ice accumulation on the leading edges of the flaps changed the aerodynamic characteristics of the aircraft in the roll axis.
- The icing conditions encountered likely involved large droplets that were outside the standard certification-defined envelope.
- The flight crews were not specifically trained to recognize or recover from this specific type of airplane-pilot coupling.
Safety action
Following the incidents, Airbus issued technical bulletins and operational recommendations, suggesting that pilots use 'CONFIG 3' for landings when moderate-to-severe icing is anticipated. Airbus also implemented a software redesign for the elevator aileron computer (ELAC) to improve stability margins. Regulatory bodies, including Transport Canada and the FAA, adopted airworthiness directives to mandate these operational limitations and the implementation of the updated software.