ASC Releases The Investigation Final Report On China Airlines Flight 947, An Aviation Occurrence Involving Excess Cabin Altitude Leading To Emergency Descent
Aviation Safety Council (ASC) releases a final investigation report on a China Airlines Airbus A330-300, B-18353, a scheduled passenger revenue flight as CI 947 departing from Kaohsiung International Airport for Hong Kong International Airport. Due to the engine 2 bleed fault, latent leaking at No.1 air-conditioning pack outlet, combined with flight operations and fault indicating issues, during cruise the flight encountered excess cabin altitude which led to emergency descent.
On August 24, 2012, China Airlines flight 947 with 2 flight crews, 12 cabin crews, and 248 passengers in total of 262 onboard, departed at 1735h from Kaohsiung for Hong Kong. At 1815:15h, around 155 nautical miles north-east of Hong Kong International Airport with cruise altitude of 34,000 ft., a “CAB PR EXCESS CAB ALT” warning occurred. Flight crews donned their oxygen masks and declared emergency (Mayday) to ATC. The flight landed in eventless at Hong Kong International Airport at 1846h. Neither injury to the passengers nor damage to the airplane was reported.
The Aviation Safety Council conducted the investigation according to the Article 6 of the ROC Aviation Occurrence Investigation Act and the content of the International Civil Aviation Organization (ICAO) Annex 13. The investigation team included members from China Airlines, Civil Aeronautics Administration Taiwan (CAA) and France BEA (Bureau d'Enquêtes et d'Analyses pour la sécurité de l'aviation civile) and technical advisor from Airbus Industrie. The final investigation report received approval for publication at the 14th council meeting on August 27, 2013.
The conclusion of the investigation are as followings:
Findings related to probable causes:
- Before the occurrence flight, there was an unknown leaking at the no.1 pack outlet which degraded the performance of air-conditioning system and cabin pressurization capability.
- Both no.2 engine bleed system high pressure valve, HPV, and pressure regulating valve, PRV, failed at the previous two flights, however both items were not appropriate rectified which adversely affected following operations of engine bleed air system.
- The HPV and PRV already failed at the previous flights with “AIR ENG 2 HPV NOT OPEN” and “AIR ENG 2 BLEED FAULT” alert messages. The flight crew of previous flights reported that they thought that the “AIR ENG 2 BLEED FAULT” alert message was triggered by his procedure and system checking; therefore he did not write down that bleed fault message on the Technical Log Book post-flight.
- The maintenance personnel who performed maintenance actions for the previous flight and dispatched the occurrence flight might access same message from AIRMAN system, but he did not get further understanding of both “AIR ENG 2 HPV NOT OPEN” and “AIR ENG 2 BLEED FAULT” alert messages. He reported that he could not recall the bleed fault message; however he believed that ENG BLEED fault message will show if HPV failed. Due that on the Technical Log Book showed the ECAM message “AIR ENG 2 HPV NOT OPEN”, therefore the airplane was dispatched with Minimum Equipment List, MEL, 36-11-07B which corresponded to the high pressure valve failed only. The MEL item for the failure of “AIR ENG 2 BLEED FAULT” was not applied which resulted in the proper MEL operation procedures of engine 2 bleed failure were not taken.
- The Bleed Monitor Computer of present A330 airplane with GE engine configuration might not correctly indicate bleed system failure while the PRV up-stream pressure sensor failed to provide right signal to the computer.
- During cruise, pilot reset the engine 2 bleed system and the fault message disappeared accordingly. Pilots thought that the engine 2 bleed system returned to normal and closed the cross bleed valve. Due to the engine 2 bleed system still failed which could not supply pressure air for no.2 pack operation. Due to the no.1 pack leaking at no.1 pack outlet could not provide enough compressed air for cabin pressurization. At 34,000 ft of cruise altitude, the cabin altitude increased continuously after the closure of the cross bleed valve, the cabin altitude warning occurred followed by the crew’s emergency descent.
Findings related to risks:
- Five defects of multi-zone cabin temperature too high or unable to control recorded on the Technical Log Book on month prior to the occurrence flight. During climb of the previous flight, the cabin altitude increased obviously while the cross bleed valve closed when flight crew closed the cross bleed valve. Both symptoms indicated that before occurrence the pre-existed broken sleeve at no.1 pack outlet led to the leaking of conditioned air and the difficulty to control cabin temperature.
- According to the accumulated flight hours of the occurrence airplane and the broken condition of no.1 pack outlet sleeve, the inspection interval of pack outlet on the original airplane Manufacturer MPD and original Operator AMP could not find out the sleeve defect in a timely manner.
- The accumulated Time Since Check in flight hours (FH) of failed high pressure valve and pressure regulating valve removed from the occurrence airplane were 170 FH and 245 FH respectively, which were less than the mean time between failure 2,703 FH and 3,799 FH respectively.
- The air-conditioning system of A330 airplane may not provide pack operation status to pilot in a timely manner while pack did not receive enough upstream air pressure/ air flow for normal operation.
- If pilots could check the ECAM BLEED system page, on the occurrence flight at the time of before/ after engine start or after resetting the engine 2 bleed they might find the failure of engine 2 bleed. The proper MEL or ECAM actions could be taken then the cabin pressure anomaly may be avoided.
- On A330 QRH, there is no requirement to check related ECAM system page of the computer reset procedure to verify the system reset result back to normal.
- The flight time from KHH to HKG was short and the weather was bad on the occurrence day. Besides the normal workload, flight crew had to avoid thunderstorm which was very time constraint. Flight crew did not notice the “CABIN ALTITUDE” advisory message and ECAM BLEED system page display before airplane reaching the altitude of ’CAB PR EXCESS CAB ALT’ warning under the heavy workload.
- Flight crew did not follow the Chapter 4, Abnormal Procedure, prescribed on the A330 Airplane Operation Manual to verify the abnormal condition shown on ECAM display.
The Aviation Safety Council issued a total of 4 safety recommendations.
Recommendations to China Airlines include:
- Reinforce flight crew‘s discipline of logging the ECAM alert message on Technical Log Book, reinforce the system training and operation procedures related to the engine bleed air system and cabin pressurization system.
- Reinforce maintenance personnel’s discipline of using ECAM alert message and Minimum Equipment List, and the system training related to the engine bleed air system and air-conditioning system. Reinforce maintenance personnel motoring of repeated defect to reduce the time of trouble shooting.
Recommendations to CAA include:
- Require China Airlines, to reinforce flight crew‘s discipline of logging the ECAM alert message on Technical Log Book, reinforce flight crew‘s system training and operation procedures related to the engine bleed air system and cabin pressurization system.
- Require China Airlines, to reinforce maintenance personnel’s discipline of using ECAM alert message and Minimum Equipment List, and the system training related to the engine bleed air system and air-conditioning system, reinforce maintenance personnel’s motoring of repeated defect to reduce the time of trouble shooting.
The full investigation report is available for download at www.asc.gov.tw (Chinese version only)
Contact：Sherry Liu, Engineer
Tel: +886-2-89127388 ext. 330