TransAsia Airways Flight 791

Investigation

Taiwan's Aviation Safety Council (now the Taiwan Transportation Safety Board) investigated the accident. The ASC reviewed the flight crew's training records. During recurrent training, the captain had "satisfactory" scores in icing conditions, however the instructor noted that the captain had a "tendency to lose situational awareness". According to his instructor, the captain repeated the training, and his results improved. Despite this, the instructor said that the captain was "still unsteady." There were no issues reported during the first officer's training.

Conclusion

The Aviation Safety Council released its final report on 25 October 2003. The report determined that the flight crew had lacked situational awareness and thus did not respond to the severe icing conditions.

Findings related to probable causes:

The accident flight encountered severe icing conditions. The liquid water content and maximum droplet size were beyond the icing certification envelope of FAR/JAR 25 [located in] Appendix C.

TNA's training and rating of aircraft severe icing for this pilots has not been effective and the pilots have not developed a familiarity with the note, CAUTION and WARNING set forth in Flight Crew Operating Manual and Airplane Flight Manual to adequately perform their duties.

After the flight crew detected icing condition and the airframe de-icing system was activated twice, the flight crew did not read the relative handbook, thereby the procedure was not able to inform the flight crew and to remind them of "be alert to severe icing detection".

The "unexpected decrease in speed" indicated by the airspeed indicator is an indication of severe icing [conditions encountered by the aircraft].

The flight crew did not respond to the severe Icing conditions with pertinent alertness and situation[al] awareness that the aircraft might have encountered conditions which was "outside that for which the aircraft was certificated and might seriously degrade the performance and controllability of the aircraft".

The flight crew was too late in detecting the severe icing conditions. After detection, they did not change altitude immediately, nor take other steps required in the Severe Icing Emergency Procedures.

The aircraft was in an "unusual or uncontrolled rolling and pitching" state, and a stall occurred thereafter.

After the aircraft had developed a stall and an abnormal attitude, the recovery maneuvering did not comply with the operating procedures and techniques for Recovery of Unusual Attitudes. The performance and controllability of the aircraft may have been seriously degraded by then. It cannot be confirmed whether [or not] the unusual attitudes of the aircraft could have been recovered if the crew's operation had complied with the relevant procedures and techniques.

During the first 25 minutes, the extra drag increased about 100 counts, inducing a speed diminishing about 10 knots.

During the [time that the] airframe de-icing system was intermittently switched off, it is highly probable that residual ice [accumulated and] covered on the wings of the aircraft.

Four minutes prior to [the] autopilot [being] disengaged, the extra drag increased about 500 counts, and [the] airspeed decayed to 158 knots, and lift-drag ratio loss about 64% rapidly.

During the 10s [seconds] before the roll upset, the longitudinal and lateral stability [of the aircraft] has been modified by the severe ice accumulated on the wings producing the flow separation. Before [the] autopilot disengaged, the aerodynamic of the aircraft (lift/drag) was degraded of about 40%.