Occurrences
On December 7, 2002, two Air Canada Airbus A321-211 aircraft encountered roll oscillations during final approach to Runway 24R at Toronto/Lester B. Pearson International Airport (LBPIA), Ontario.
Air Canada Flight 457 (ACA457)
At approximately 1607 EST, ACA457 (registration C-GJVX, serial number 1726) was on approach with 123 passengers and 6 crew. At about 140 feet above ground level (agl) with full flaps selected, the aircraft experienced roll oscillations. The flight crew leveled the wings and the aircraft touched down firmly. Post-landing inspection revealed mixed ice on areas of the wing and the leading edge of the horizontal stabilizer not protected by anti-ice systems.
Air Canada Flight 1130 (ACA1130)
At approximately 1859 EST, ACA1130 (registration C-GIUF, serial number 1638) was on approach with 165 passengers and 7 crew. At about 50 feet agl, the aircraft experienced roll oscillations. The flight crew conducted a go-around, changed flap settings, and returned for an uneventful landing. At the gate, ice was noted on unprotected wing and horizontal stabilizer areas. No damage or injuries occurred on either flight.
Background
Both aircraft were dispatched with up-to-date meteorological information indicating moderate icing conditions forecast for the arrival period. En route, pilots received updated weather via datalink. Visual ice indicators showed ¼ to ½ inch of ice accumulation. Both aircraft used engine and wing anti-ice while in icing conditions. Approach speeds were increased to VLS plus five knots as per the Air Canada A321 Aircraft Operating Manual for icing conditions. Autopilot was disengaged between 500 and 1000 feet agl.
Roll Oscillations and Pilot Actions
In both occurrences, roll oscillations began at low altitude (140 ft agl for ACA457, 50 ft agl for ACA1130). The pilot flying attempted to counter the oscillations with stick inputs up to the stops at about 0.5 Hz, but the oscillations increased. In ACA457, the pilot reduced power and landed. In ACA1130, the pilot advanced throttles and executed a go-around, then landed uneventfully with CONFIG 3. No other landing anomalies were reported at LBPIA during that time.
Weather Conditions
Weather reports forecast icing between 2000 and 9000 feet asl, with cloud top temperature about -10°C. Outside air temperature at 6000 feet was -10°C. Pilot reports of moderate icing were received after each occurrence. Drizzle was reported at the surface after ACA1130 landed, with droplet sizes estimated 100-500 microns, suggesting supercooled large droplets (SLD) beyond the FAR 25 Appendix C certification envelope (40-50 microns). No drizzle was observed during ACA457.
Investigation Findings
Flight data recorder data showed aileron and spoiler responses consistent with roll commands. No faults were found in yaw damper or flight control systems. The aircraft had ELAC L81 software.
Airbus testing determined that ice accretion on the leading edge of the flaps modified aerodynamic characteristics, increasing roll spoiler efficiency. This effect was more pronounced with full flaps (CONFIG FULL) than with CONFIG 3. Under normal lateral flight control laws, the reduced stability margin on the A321 in icing conditions led to airplane-pilot coupling (APC), producing an unstable aircraft. Crosswinds were not a factor.
Probable Cause
The probable cause is the A321's normal lateral flight control laws providing higher roll efficiency in CONFIG FULL, reducing the stability margin in icing conditions, combined with ice on the flap leading edges altering aerodynamics, resulting in airplane-pilot coupling that produced an unstable aircraft.
Safety Action
Airbus issued a Flight Operations Telex (FOT) STL 999.138/02 on 20 December 2002 to all A321 operators regarding lateral control events during landing in icing conditions.