... No Matter What You Fly
When considering the Caravan, many industry observers believe many of the type’s problems can be traced to pilots who may be flying an aircraft approved for flight in known icing for the first time. After perhaps years of dealing with icing conditions from the left seat of an airplane armed only with a heated pitot tube, pilots see the big turbine, deicing boots and other equipment installed on the Caravan and consider themselves suddenly immune. But in-flight icing conditions can affect even the most capable aircraft, as Canada’s Transportation Safety Board (TSB) noted earlier this year when it issued its final report involving two similar incidents at the Toronto/Lester B. Pearson International Airport on Dec. 7, 2002. In both incidents, Airbus A321-211 aircraft operated by Air Canada in scheduled passenger operations were involved, disproving the theory that larger aircraft are immune to airframe icing considerations.
According to the TSB, its investigation "revealed that, in both occurrences, the aerodynamic shape of the aircraft's wing flaps was modified by ice, and that, in such icing conditions, the Airbus A321 normal, lateral flight control laws programmed into the elevator aileron computer provided higher roll efficiency, which resulted in a reduced stability, causing the aircraft to roll slightly from side to side." Responding to the rolling moments, flight crews aboard both transports "applied right and left control inputs to try to stop the rolling movement, but the magnitude of the oscillations increased." The pilot-induced oscillations (PIO) were blamed on the ice-modified aerodynamics and pilot input. Both crews used different landing configurations and succeeded in landing without aircraft damage or injuries. In a statement eerily reminiscent of the circumstances surrounding the Oct. 31, 1994, crash of an American Eagle ATR-72 regional turboprop, the TSB noted that "prolonged flight in icing conditions with slats extended should be avoided."