The Aviation Safety Council published: A China Airlines Boeing 737-800 aircraft lost cabin pressurization during climb due to the left bleed air system failure and the right air conditioning pack failure. The left bleed air system failure was caused by a broken kiss seal of the pre-cooler control valve; and the right air conditioning pack failure was caused by a duct rupture at the outlet of the air cycle machine. Both failures led to aircraft loss of cabin pressurization during climb.

On July 22, 2010, a China Airlines Boeing 737-800 aircraft, flight number CI-112, registration B-18612, was a regularly scheduled passenger flight from Taoyuan International Airport to Hiroshima Airport, Japan. There were two flight crew, 5 cabin crewmembers and 89 passengers on board. As the aircraft was climbing through 35,300 feet, the Master Warning sounded; the flight crew discovered that the cabin altitude and climbing rate were increasing, and then requested permission of emergency descent and air turn back from ATC. The flight crew made a “Mayday” call and executed emergency descent procedure. ATC vectored the aircraft back to Taoyuan International Airport. The aircraft landed safely without injuries on board.

The ASC launched investigation according to the Aviation Occurrence Act after the occurrence. Several departments were invited for joint investigation, including: Civil Aeronautics Administration, China Airlines, and National Transportation Safety Board of the USA including Boeing Company. Investigation Report was published after approval by the ASC council members on April 29th, 2011, at the 143rd council meeting.

Findings related to probable causes: The occurrence resulted from the left bleed air system failure and the right air conditioning pack failure. The left engine bleed air pressure low occurred at its previous flight, the maintenance determined that it was normal on ground after limited trouble shooting. During climb, the left bleed air system failed due to a broken kiss seal of the left pre-cooler control valve which resulted in the left bleed air system failed to provide compressed air to the left air conditioning pack. Even the right bleed air system automatically switched to high flow mode, the right pack still could not provide enough compressed air for cabin pressurization due to a flexible duct ruptured at the air cycle machine outlet of the right air conditioning pack. The duct rupture was determined possibly by the reinforcement metal rings wearing away the silicone and reinforcing fiberglass at several locations in the hose.

Findings related to Risks: The flight crew possessed situation awareness but did not recognize the unusual cabin pressure at initial cabin pressure abnormal stage and execute the related procedure according by; the aircraft continued its climb with 840 feet/min vertical speed for 90 seconds after the Master Caution light illuminated and the flight crew started the descent when approved by ATC; maintenance were unable to spot the malfunction during the transit check and correct the bleed air malfunction properly; when the oxygen masks fell down automatically, not all cabin crew took seats immediately or used portable oxygen bottles for self protection; area controller did not hear the mayday call from the flight crew and did not pass on the message to the supervisor.

Aviation Security Council issued a total of 15 Safety Recommendations.

Recommendations to Civil Aeronautics Administration include: review “Taipei FIR Northern Control Area Operating Procedure” and personnel work load management, adjust ATC management and manpower distribution; provide general principles of non-routine insufficient on duty staff for reference in the “ATC Operating Manual”; review “Taipei FIR Operating Manual,” clarify the responsibility of assistant Director duty assignment and on-scene supervisor personnel assignment; reinforce the management of area control center over on-duty controller and on-scene personnel allocation; review the ATC training syllabus, emphasizing on human factors and crew resource management and team cooperation.

Recommendations to CAA and China Airlines include: request flight crew to execute related procedure promptly should any abnormality occur; consider add “ATC Notification” into the Memory Item of Emergency Descent Procedure of the aircraft type; refer to Manufacturer service letter 737-SL-21-045 and utilize the built in test equipment of the cabin pressure controllers to set up a cabin pressurization performance monitoring system for the 737-800 fleet; improve the Maintenance personnel knowledge and troubleshooting skill on bleed air system; request cabin crew to follow SOP of depressurization for self protection.

Full investigation report is available for download at http://www.asc.gov.tw

Sherry Liu, Engineer
Tel: 89127388-330   Email:sherry@asc.gov.tw