What happened
On August 24, 2012, China Airlines flight CI 947, an Airbus A3/300-300 with registration B-18353, departed Kaohsiung International Airport for Hong Kong International Airport. Due to the presence of Typhoon Libra, the flight crew navigated a northern route to avoid severe weather.
During the cruise phase at 34,000 feet, the flight crew attempted to reset the Engine 2 bleed system after a fault had been noted during takeoff. Following the reset, the crew closed the crossbleed valve. Shortly thereafter, the cabin altitude began to rise steadily. At 18:15 local time, approximately 155 nautical miles northeast of Hong Kong, an 'Excess Cabin Altitude' warning activated. The crew immediately donned oxygen masks and executed an emergency descent. The aircraft descended to 9,000 feet, where cabin pressure stabilized, and the flight landed safely at Hong Kong International Airport without injuries or aircraft damage.
The investigation
The investigation examined the aircraft's maintenance history, flight data recorder (FDR) and cockpit voice recorder (CVR) data, and the mechanical condition of the pneumatic and air-conditioning systems. Investigators looked into why the engine bleed fault was not properly addressed during the previous flight and why the cabin pressure failed to maintain altitude.
Findings
- A leak in the number 1 air-conditioning pack outlet sleeve significantly degraded the aircraft's ability to maintain cabin pressure.
- The engine 2 bleed system was experiencing ongoing issues with the high-pressure valve (HPV) and pressure regulating valve (PRX), which had failed during previous flights.
- Improper maintenance documentation occurred when the flight crew from a previous flight failed to log an 'ENG 2 BLEED FAULT' message in the Technical Log Book, leading to the aircraft being dispatched under an incomplete Minimum Equipment List (MEL) application.
- The flight crew attempted to reset the engine 2 bleed system during cruise; however, because the system was still unable to supply sufficient pressure and the number 1 pack was leaking, closing the crossbleed valve led to the loss of cabin pressurization.
- Heavy workload due to weather avoidance and a tight schedule contributed to the crew not noticing the initial cabin altitude advisory messages before the primary warning occurred.
Safety action
Following the investigation, China Airlines updated its maintenance program to shorten the inspection interval for the pack outlet. The airline also implemented new crew notifications requiring all discovered faults to be logged in the Technical Log Book and briefed to maintenance personnel. Additionally, Airbus implemented changes to the Quick Reference Handbook (QRH) regarding engine bleed resets and issued service bulletins to reduce vibration in the high-pressure valve.