What happened
On December 21, 2002, at 01:52 local time, a TransAsia Airways freighter, an ATR72-200 with registration B-22708, crashed into the sea approximately 17 kilometers southwest of Makung City in the Penghu Islands, Taiwan. The aircraft was operating a flight that encountered severe icing conditions during its cruise. During the encounter, the aircraft experienced an unexpected decrease in airspeed and an increase in drag. The flight crew activated the airframe de-icing system; however, the aircraft eventually entered an unusual state characterized by uncontrolled rolling and pitching, leading to a stall. The aircraft impacted the water at a high diving angle and high speed. Both pilots on board were lost in the accident.
The investigation
The Taiwan Transportation Safety Board (formerly ASC) conducted an extensive investigation involving wreckage recovery, flight recorder readout, and laboratory testing. The investigation team included representatives from the ROC Civil Aeronautical Administration and the manufacturer, Avions de Transport Regional (ATR). Investigators examined the aircraft's maintenance records, the flight crew's training programs, and the meteorological data available to the crew prior to the flight. The investigation also analyzed the performance of the airframe de-icing system and the aerodynamic impact of ice accumulation on the wing surfaces.
Findings
- The aircraft encountered severe icing conditions where the liquid water content and maximum droplet size exceeded the certification envelope defined by FAR/JAR 25 Appendix C.
- The flight crew failed to respond to the severe icing with the necessary alertness and situational awareness, failing to implement emergency procedures such as immediate altitude changes.
- The accumulation of ice on the wings caused significant aerodynamic degradation, including a 40% loss in lift-to-drag ratio and an increase in drag of approximately 500 counts shortly before the autopilot disengaged.
- The aircraft's longitudinal and lateral stability were compromised by ice-induced flow separation, leading to an asymmetric stall and an uncontrolled roll.
- The crew's training and rating regarding severe icing were found to be ineffective, specifically regarding familiarity with the warnings and notes in the Flight Crew Operating Manual.
- There was no onboard system capable of proactively detecting or providing sufficient information regarding the onset of severe icing conditions.