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Release of the GE791 Freighter Occurrence Investigation Report


Publication Date 2005-04-22
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After a two years and four months investigation, ASC released the investigation report of GE791 Freighter occurrence today. This occurrence happened at 0152 Taipei local time on December 21, 2002. It was a TransAsia Airways (TNA) freighter aircraft with type of ATR-72-200, registration number B-22708. The aircraft encountered severe icing condition during flight and crashed into Taiwan Strait at approximately 17 kilometers southwest of Ma-Kung city, Penghu islands. Both pilots on board were missing.
ASC immediately launched a team to conduct the investigation of this occurrence. In accordance with the article 84 of Civil Aviation Law and ICAO Annex 13, the French Bureau D’enquetes et D’analyses pour la Securite de L’aviation Civile was invited as Accredited Representative of state of manufacture. The ATR-72 manufacturer, AVIONS DE TRANSPORT REGIONAL was the adviser of BEA. After a ten months factual information collection, ASC published the factual report on October 25, 2003. The analysis was continued thereafter by ASC.
This final report follows the format of ICAO Annex 13 with a few minor modifications. Firstly, in Conclusions, the Safety Council decided in their 39th Board meeting that to further emphasize the importance that the purpose of the investigation report is to enhance aviation safety, and not to apportion blame and responsibility, the final report does not directly state the “Probable Causes and Contributing Factors”, rather, it will present the findings in three categories: findings related to the probable causes of the accident, findings related to risks, and other findings. Secondly, in addition to the safety recommendations, the Safety Council also includes the safety actions already taken or in progress by the stakeholders. The Safety Council decided that this modification would better serve its purpose for the improvement of aviation safety.
Therefore, based upon the analysis by the Safety Council, the following are the key findings of the GE791 accident investigation.
The findings related to the probable causes identify elements that have been shown to have operated in the accident, or almost certainly operated in the accident. These findings are associated with unsafe acts, unsafe conditions, or safety deficiencies that are associated with safety significant events that played a major role in the circumstances leading to the accident.
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, and 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%.
The Safety Council issued an Interim Flight Safety Bulletin (Issue No:ASC- IFSB- 03- 01- 001) on January 24, 2003. It is recommended that all operators with turboprop aircraft review their training programs to ensure the program contains the necessary training for pilots to recognize and effectively respond to all levels of "Icing Conditions." It is also recommended that operators emphasize additional training in pilot′s situation awareness of icing conditions.
The Safety Council issued safety recommendations to the followings:
To TransAsia Airways
1. Review the managing procedures for the SOC Operations Manual to revise that manual timely when related operation-factor variations existed.
2. Request to the flight crews to check the weather documentation they received from the dispatcher that it is applicable to the flight.
3. Review and improve the implementation and management of ground school courses, flight training and rating to ensure that all pilots are competent in performing their duties.
4. Require pilots to ensure that the adequacy of read and follow the checklist’s procedures in abnormal or emergency conditions.
5. Enhance pilots of the ATR aircraft fleet with their training and rating on areas such as awareness, observing indications of severe icing, briefings and workload sharing, emergency procedures, and unusual attitude recovery.
6. Review the relevant rules and procedures of Flight Crew Reports.
7. Evaluate the retrofit of all company aircraft to use of solid flight data recorders.
To ATR Aircraft Manufacturer
1. Evaluate to include Severe Icing Emergency Procedures as memory items when encountering severe icing condition.
2. Add WARNING remarks to all of the severe-icing-related Chapter/Section in ATR’s relative Manuals to remind flight crew.
3. Proactively develop a more sophisticated icing detection system to enhance the flight crews’ understanding and awareness of icing condition. Evaluate a new system to provide flight crew additional warning when aircraft operates in icing environment with autopilot engaged to reduce the potential risk of pilot’s failure of monitoring and maintaining airspeed. Continuously support and engage a research activity similar to Smart Icing System to reduce the accident s caused by severe icing.
To DGAC, France
1. Proactively develop a more sophisticated icing detection system to enhance the flight crews’ understanding and awareness of icing condition. Evaluate a new system to provide flight crew additional warning when aircraft operates in icing environment with autopilot engaged to reduce the potential risk of pilot’s failure of monitoring and maintaining airspeed. Continuously support and engage a research activity similar to Smart Icing System to reduce the accident s caused by severe icing.
To Civil Aeronautics Administration
1. In addition to ICAO’s regulations, refer to the practices made by HKO and TAWSC. To emphasize the situation awareness of icing en-route to pilots by marking symbols for, at least, moderate icing on the SIGWX charts, where the non-CB clouds above freezing level with supercooled liquid water is possible to be existed.
2. Review the TNA′s pilots training to perform their duties effectively.
3. Evaluate the retrofit of all civil aircraft to use of solid flight data recorders.
4. Continuously review and evaluate the icing detection related Service Bulletin, Advisory Circular and Airworthiness Directive.


※ The final report could be found from:http://www.asc.gov.tw
Aviation Safety Investigator: Tracy Jen
Tel:2547-5200 ext 167

Last updated 2019-08-08
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