What happened
On September 14, 2008, Cathay Pacific Airways flight CX521, an Airbus A330-300 with registration B-HLH, was operating a rescheduled flight from Narita International Airport to Taipei/Taiwan Taoyuan International Airport. The aircraft was carrying 72 occupants, including 59 passengers and 13 crew members.
During the descent from flight level FL400, the aircraft encountered a failure in the bleed air system. Having been dispatched under the Minimum Equipment List (MEL) with the #1 engine bleed air valve closed, the aircraft relied solely on the #2 engine bleed air. While passing through FL380, the crew observed an ECAM message indicating a bleed air fault on the #2 engine. This failure caused the cabin altitude to climb rapidly, eventually triggering a master warning as the cabin altitude reached 9,700 ft.
In response to the loss of pressurization, the flight crew initiated an emergency descent. During the descent, the cabin altitude peaked at 13,424 ft. The crew manually deployed oxygen masks for the cabin. As the aircraft descended, the crew reported a strong burning smell in the cabin, which was later identified as a byproduct of the activated chemical oxygen generators. The aircraft landed safely at Taipei International Airport at approximately 1929 local time with no injuries and no damage to the aircraft.
The investigation
The Taiwan Transportation Safety Board (TTSB) investigated the sequence of events, focusing on the technical failure of the bleed air system and the subsequent cabin environment changes. The investigation examined the maintenance status of the aircraft, the functionality of the engine bleed valves, and the effectiveness of the crew's emergency response and communication with Air Traffic Control (ATC).
Findings
- The primary cause of the failure was that the #2 engine bleed air valve shut down automatically due to an overheat condition.
- Because the #1 engine bleed air system was already inoperative per MEL, the failure of the #2 engine valve left the aircraft without a compressed air source for the air conditioning systems, resulting in the loss of pressurization capability.
- The overheat was caused by a high demand on the remaining active bleed source during descent, combined with a contaminated grid filter in the Temperature Control Unit (TCU) which prevented the fan air valve from providing sufficient cooling.
- The burning smell reported by the crew was attributed to the activation of the chemical oxygen generators used during the descent.
- The investigation also noted several communication challenges, including difficulties in distinguishing call signs on shared frequencies and delays in ATC acknowledging the Mayday calls.