Narrative:TransAsia Airways flight 791 departed Taipei at 01:05 local time for a cargo flight to Macau. The aircraft was carrying a cargo of fabric, leather, and electronic materials. At 01:51 the co-pilot contacted Taipei Control and requested permission to descend from FL180 to FL160. At 01:52:10, at an altitude of 17,853 feet, a stall warning sounded and the stick shaker activated. The crew disconnected the autopilot and tried maintain control of the airplane. Sixteen seconds later the aircraft descended through 14,465 feet. Within seven seconds the aircraft had descended through 10,246 feet at a speed of 320 knots. At 01:52:47 the plane passed an altitude of 3,027 feet. Three seconds later the cockpit voice recorder stopped working. The aircraft crashed into the sea.
FINDINGS RELATED TO PROBABLE CAUSES:
1. 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 appendix C.
2. 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.
3. 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".
4. The "unexpected decrease in speed" indicated by the airspeed indicator is an indication of severe icing.
5. The flight crew did not respond to the severe Icing conditions with pertinent alertness and situation 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".
6. 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.
7. The aircraft was in an "unusual or uncontrolled rolling and pitching" state, and a stall occurred thereafter.
8. 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 the unusual attitudes of the aircraft could have been recovered if the crew's operation had complied with the relevant procedures and techniques.
9. During the first 25 minutes, the extra drag increased about 100 counts, inducing a speed diminishing about 10 knots.
10. During the airframe de-icing system was intermittently switched off, it is highly probable that residual ice covered on the wings of the aircraft.
11. Four minutes prior to autopilot disengaged, the extra drag increased about 500 counts, and airspeed decayed to 158 knots, and lift-drag ratio loss about 64% rapidly.
12. During the 10s before the roll upset, the longitudinal and lateral stability has been modified by the severe ice accumulated on the wings producing the flow separation. Before autopilot disengaged, the aerodynamic of the aircraft (lift/drag) was degraded of about 40%.
Loss of control
» ASC Taiwan
Official accident investigation report
|investigating agency: ||Aviation Safety Council (ASC) - Taiwan |
|report status: ||Final|
|report number: ||ASC-AOR-05-04-001|
|download report: ||
weather satellite image
This map shows the airport of departure and the intended destination of the flight. The line between the airports does not
display the exact flight path.
Distance from Taipei-Chiang Kai Shek International Airport to Macau Airport as the crow flies is 838 km (524 miles).