What happened
On September 29, 2005, All Nippon Airways flight 198, an Airbus Industrie A321-131 registered JA102A, was operating a scheduled service from Oita Airport to Tokyo International Airport. While cruising at 37,000 feet, the aircraft's air-conditioning system began to malfunction.
At approximately 18:49 JST, the crew received overheat and fault warnings for the first air-conditioning pack. The pilot-in-command attempted to reset the system by switching the pack off and then back on, but the overheat condition recurred, forcing the permanent shutdown of the first pack. Shortly thereafter, at 18:56 JST, the second air-conditioning pack also experienced an overheat and fault condition.
With both packs inoperative, the aircraft could no longer maintain a safe cabin altitude. The crew initiated an emergency descent to 13,000 feet and notified Tokyo Control. As the cabin altitude rose above 8,000 feet, the crew manually deployed the passenger oxygen masks. The aircraft eventually leveled off at 8,000 feet after the crew opened the ram air valve to stabilize the environment. The flight concluded safely with a landing at Tokyo International Airport at 19:33 JST. There were no injuries among the 172 people on board.
The investigation
The investigation focused on the mechanical failure of the air-conditioning components. Investigators examined the air-conditioning packs, the flow control valves (FCVs), and the compressor pneumatic overheat sensors (CPNOHs). Analysis of the digital flight data recorder (DFDR) and quick access recorder (QAR) allowed the team to track the precise temperature fluctuations and air flow changes within the system during the incident.