The Aviation Safety Council (ASC) released the investigation report of an EVA Airways Flight 702, a Boeing B747-400 airplane, encountered cabin pressure control system malfunction and abnormal cabin altitude during climb. The data show that the flight crew might not notice the left outflow valve failure and EICAS fault message until the aircraft reached the altitude 20,000 ft, approximately 9 minutes after the valve failure. When pilots performing the QRH “OUTFLOW VLV L” procedure, the first officer closed the left outflow valve with manual mode, the cabin altitude warning came up almost at the same time.
 
On March 25, 2012, at 1044 Taipei local time, a Boeing B747-400 airplane, registration number B-16411, operated by EVA Airways Corporation performing a scheduled passenger flight BR702 took off from Taoyuan International Airport (elevation 106 ft) for Shanghai Pudong Airport. During climb it encountered a left outflow valve malfunction and abnormal cabin altitude. At 1054, around 47 nautical miles north-east of Taoyuan Airport with altitude of 20,800 ft, the aircraft had the “CABIN ALTITUDE” aural warning. Pilots donned the oxygen masks, performed emergency descent and declared emergency (Mayday) to the Air Traffic Controller. The aircraft returned to Taoyuan International Airport at 1128 without further incidents. The aircraft had 2 pilots, 14 cabin crew members and 367 passengers, total 383 people on board without injuries. The aircraft had no damage.

The ASC launched investigation according to the Aviation Occurrence Act after the occurrence. Parties to the investigation are the Civil Aeronautics Administration (CAA), EVA Airways and The National Transportation Safety Board (NTSB) of the United States. Investigation report was published after approval by the ASC council members on February 26, 2013, at the 8th Council Meeting.

Findings related to probable causes: During initial climb, the Cabin Pressure Control System’s left outflow valve failed such that the valve was stuck in the 64.9% position while under automatic control. The left outflow valve was found to have failures in the AC motor interface between the rotor shaft and the brake shaft, and to have growth of the air gap in the brake at a level that prevented the brake from releasing when commanded. The 64.9% position of the left outflow valve and full closed position of the right outflow valve resulted in cabin air leakage beyond expected climb and cruise levels. The position of the left outflow valve prevented the aircraft from pressurizing normally and resulted in the high cabin altitude conditions that occurred on this aircraft. The data show that the flight crew might not notice the left outflow valve failure and EICAS fault message until the aircraft reached the altitude 20,000 ft, approximately 9 minutes after the valve failure. This resulted that the flight crew were unable to start and complete the OUTFLOW VLV L checklist and manually close the left outflow valve in a timely manner. During climb, the continuously leaking of cabin pressure led to the cabin altitude reaching the point of cabin altitude warning. When performing the QRH ‘OUTFLOW VLV L’ procedure, the first officer closed the left outflow valve with manual mode, the cabin altitude warning came up almost at the same time. While the left outflow valve was gradually closing and the cabin altitude was recovering, the flight crew did not notice the cabin altitude being controllable. The captain decided to perform emergency descent for safety reasons, don the oxygen mask and release passenger emergency oxygen mask. Had the flight crew completed the checklist prior to initiating the emergency descent, they would have been aware that the cabin pressure was controllable.

Findings related to Risks: The defects seen on the outflow valve AC motor are similar to other field returns. The manufacturer can only speculate on the causes as they appear to be from the motor brake not disengaging properly, possibly from an incorrect voltage at the AC motor. Root cause determination activities are on-going at Boeing and the system supplier. Regarding the non-normal procedures for cabin altitude or rapid depressurization, there exists inconsistent QRH procedure between Boeing B747-400 cargo aircraft and passenger aircraft. For the item 3, in addition to the “Verify packs are on and outflow valve are closed”, the QRH of cargo aircraft contains “Check the cabin altitude and rate”, but it is not covered in the QRH of passenger aircraft. The Airplane Flight Manual (AFM) also suggests this non-normal procedure should include this item “Check the cabin altitude and rate”. The QRH of passenger aircraft without this item is not consistent with the AFM suggestion and also not like the cargo QRH such coherent for pilots to perform the next step, to determine “If the cabin altitude is uncontrollable”.

Safety Recommendation to EVA Airways：Reinforce flight crew‘s awareness and understanding of cabin altitude anomaly and cabin pressure control, reinforce the training of relevant operation and procedures.  Recommendation to CAA：Require EVA Airways to reinforce flight crew‘s awareness and understanding of cabin altitude anomaly and cabin pressure control, reinforce the training of relevant operation and procedures.

Further information including the factual data, analysis and safety action taken or being planned; please refer to the Final Report. The Final Report in Chinese and courtesy translation Executive Summary in English of this investigation are available for download at http://www.asc.gov.tw

Sherry Liu, Engineer
Tel: 89127388-330
Email:sherry@asc.gov.tw