What happened
On July 22, 2010, at 17:09 Taipei time, China Airlines flight CI112, a B737-800 with registration B-18612, departed Taipei Taoyuan International Airport for Hiroshima International Airport. The flight was carrying 89 passengers and 7 crew members.
During the climb, the flight crew noticed irregularities in the cabin pressure. At approximately 31,000 feet, the pilot reported almost no pressure on the left side, with the cabin altitude rising to 8,000 feet. As the aircraft continued to climb through 33,000 feet, the cabin pressure rate of climb was measured at 900 feet per minute. At 35,300 feet, a Master Caution alert was triggered.
Recognizing the deteriorating situation, the crew requested a descent to 32,000 feet. However, as the cabin altitude continued to rise toward 10,000 feet, the crew initiated an emergency descent. At 36,200 feet, an audible warning sounded in the cabin, prompting the crew to execute formal emergency descent procedures. The crew declared a "MAYDAY" at 18,130 feet, set the transponder to 7700, and requested a descent to 8,000 feet. The aircraft eventually returned to Taoyuan International Airport and landed safely at 18:11 with no injuries reported.
The investigation
The Taiwan Transportation Safety Board (TTSB) examined the aircraft's maintenance logs, flight data recorder (FDR), and cockpit voice recorder (CVR). The investigation focused on the failure of the air conditioning and pressurization systems, as well as the performance of the flight and cabin crew and air traffic controllers during the event.
Findings
- The primary cause of the depressurization was the simultaneous failure of both air conditioning systems. The left system failed because a seal on the pre-cooler control valve was damaged, leading to insufficient cooling and an automatic system shutdown due to overheating. Simultaneously, the right system could not maintain pressure because of a ruptured outlet duct in the air circulation unit, caused by friction against a steel ring.
- Maintenance records showed that a low-pressure fault in the left engine air supply system had been noted prior to departure, but the issue was not effectively resolved during the previous inspection.
- The investigation identified several risk factors, including a delay in the flight crew' 90-second window between the Master Caution and the decision to descend, and the fact that some cabin crew members did not immediately take protective measures or use portable oxygen bottles when masks deployed.
- At the Taipei Area Control Center, high traffic volume and staffing shortages contributed to communication gaps, as the controller failed to notify the supervisor of the emergency declaration.