What happened
On March 25, 2012, an EVA Airways Boeing 747-400, registration B-16411, was operating scheduled passenger flight BR702 from Taoyuan International Airport to Shanghai Pudong Airport. During the initial climb, a malfunction occurred within the cabin pressure control system. Specifically, the left outflow valve became stuck in a partially open position, while the right outflow valve moved to a fully closed position.
As the aircraft climbed, the cabin altitude began to rise steadily. At approximately 20,800 feet, the flight crew received an aural "CABIN ALTITUDE" warning. The crew immediately donned oxygen masks, initiated an emergency descent, and declared a "Mayday" to Air Traffic Control. The aircraft descended to 8,000 feet, stabilized the cabin pressure, and performed an air turn-back to Taoyuan International Airport. The flight landed safely without injuries to the 367 passengers or the 16 crew members on board, and the aircraft sustained no damage.
The investigation
The investigation examined Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) data to reconstruct the sequence of events. The FDR revealed that the left outflow valve failure began during the early climb phase, at an altitude of approximately 698 feet, where the valve became stuck at 64.9% open. The right valve subsequently closed completely at 4,603 feet.
Investigators also analyzed the EICAS (Engine Indication and Crew Alerting System) messages and the crew's response. While the crew reported not noticing the advisory messages until the cabin altitude reached 20,000 feet, the investigation noted that the failure had been active since the initial climb. The investigation also looked into the mechanical failure of the left outflow valve's AC motor, which involved a breakdown at the rotor shaft and brake interface.
Findings
- The primary cause of the incident was a failure of the left outflow valve AC motor, which prevented the brake from releasing and caused the valve to remain stuck in a partially open position.
- This malfunction, combined with the fully closed right valve, led to continuous cabin air leakage, making it impossible to maintain normal pressurization during climb and cruise.
- The flight crew did not recognize the abnormal cabin pressure status until the cabin altitude had already reached approximately 8,600 feet, which delayed the execution of the necessary checklists.
- During the attempt to perform the "OUTFLOW VLV L" checklist, the cabin altitude continued to rise, eventually triggering the aural warning and necessitating the emergency descent.
- A discrepancy was identified between the Quick Reference Handbook (QRH) for passenger and cargo aircraft; the passenger version lacked a specific instruction to check cabin altitude and rate, which is suggested in the Airplane Flight Manual (AFM).