What happened
On June 3, 2013, China Airlines flight CI781, an Airbus A330-300 with registration B-18317, was operating a scheduled passenger service from Taiwan Taoyuan International Airport to Ho Chi Minh City’s Tan Son Nhat International Airport. The aircraft was carrying 185 passengers and 15 crew members.
Shortly after takeoff, the Electronic Centralized Aircraft Monitoring (ECAM) indicated a bleed air fault in the number 2 engine. The crew followed established procedures and continued the climb to a cruise altitude of 40,000 feet. During cruise, the crew attempted to reset the number 2 engine bleed system, but the fault recurred.
As the aircraft was approximately 110 nautical miles northeast of Ho Chi Minh City, a second fault occurred in the number 1 engine bleed system. This triggered a cabin altitude warning as the cabin altitude rose to 9,536 feet. The crew initiated an emergency descent, utilizing speed brakes to increase the descent rate to approximately 7,696 feet per minute. The crew declared a "MAYDAY" to air traffic control and continued descending until the aircraft reached 10,000 feet, eventually leveling off at 8,000 feet to ensure passenger comfort. The flight landed safely at Tan Son Nhat International Airport without injuries or damage to the aircraft.
The investigation
The Taiwan Transportation Safety Board (TTSB) examined the aircraft's flight data from the Digital Flight Data Recorder (DFDR) and cockpit voice recordings. The investigation focused on the mechanical failure of the engine bleed air systems and the subsequent loss of cockpit voice recorder (CVR) data.
Investigators found that during the post-flight maintenance, technicians failed to pull the correct circuit breaker to stop the CVR from recording, which caused the continuous power supply to overwrite the two hours of critical audio from the incident. The investigation also involved collaboration with the French BEA, Airbus, and the Singaporean AAIB to test faulty components at a facility in Singapore.