Eva Air has reinforced training to enhance its flight crew's awareness and understanding of cabin altitude anomalies after one of its aircraft made an emergency landing last year when the cabin failed to pressurise properly.
The Boeing 747-400, registration B-16411, was on the Taipei Taoyuan-Shanghai Pudong route when the incident happened on 25 March 2012.
The aircraft had taken off at 10:44 local time and during the climb encountered a left outlet valve malfunction and abnormal cabin altitude, Taiwan's Aviation Safety Council (ASC) says in an investigation report.
Findings show that the cabin pressure control system's left outlet valve had failed, resulting in it being partially closed at a nine o'clock position. The position of the valve prevented the aircraft from pressurising normally and resulted in high cabin altitude.
The continuous leaking of cabin pressure led the cabin altitude warning to sound when the aircraft was at 20,800ft (6,340m).
Statements from the flight crew and data from the cockpit voice recorder showed that the crew did not recognise any abnormality until the cabin altitude was at 8,600ft.
When they found out, the captain requested for the aircraft to level to 20,000ft. The pilots then performed the outflow valve left procedure according to the quick reference handbook (QRH), but the cabin altitude continued to rise above 10,000ft, causing the warning to sound.
The captain then declared "Mayday" and called for emergency descent. He also took over as pilot flying, deployed oxygen masks and landed the aircraft safely.
There were 367 passengers and 16 crew onboard the aircraft.
The findings also show that cabin altitude was actually recovering after the first officer manually closed the left outflow valve while performing procedures according to the QRH, but the flight crew failed to recognise that the cabin altitude was controllable.
Simulation flights conducted by the Civil Aeronautics Administration (CAA) thereafter also showed that "emergency descent could be avoided if the flight crew had applied the QRH procedure correctly".
Reviewing the operator's training syllabus, CAA also found that most of its training for emergency descent involves "rapid decompression". It has since asked Eva to review and adjust the relevant training syllabus and scenario to enhance pilots' situation awareness.
The ASC found that the outlet valve problem was likely caused by the motor brake not disengaging properly, possibly from an incorrect voltage at the AC motor. Root cause determinations are ongoing at Boeing and the system supplier.
The ASC also found that the QRH procedure between the 747-400 cargo and passenger aircraft are different. The passenger aircraft version does not include "check the cabin altitude and rate".
ASC has recommended that Eva Air reinforce its flight crew's awareness and understanding of cabin altitude anomalies and cabin pressure, and increase training of relevant operation and procedures.