Eva B744 near Taipei on Mar 25th 2012, loss of cabin pressure
Last Update: April 1, 2013 / 13:36:08 GMT/Zulu time
Incident Facts
Date of incident
Mar 25, 2012
Classification
Incident
Cause
Loss of cabin pressure
Aircraft Registration
B-16411
Aircraft Type
Boeing 747-400
ICAO Type Designator
B744
Findings Related to Probable Causes
- During initial climb, the Cabin Pressure Control SystemÂ’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 was controllable.
Findings Related to Risk
- 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”.
The first officer was pilot flying for the sector. Both outflow valves were near open (0.9% and 0.8% closed) in automatic mode when the aircraft departured Taipei and began to close as the aircraft departed. The left outflow valve reached 64.9% closed position at about 698 feet MSL, but did not close any further, while the right outflow valve reached its nominal closed position at 102.1% closed at 4603 feet MSL. During the climb, about 40 seconds prior to the cabin altitude warning, the crew detected the left hand outflow valve wasn't closed and called for the left outflow valve checklist. When the aircraft climbed through 20,800 feet MSL, cleared to climb to FL370, the crew got an aural alert "Cabin Altitude", donned their oxygen masks, released the passenger oxygen masks manually, and upon captain's decision initiated an emergency descent to FL100 with the captain assuming the role as pilot flying. Prior to the initiation of the emergency descent the first officer had already started the left hand outflow valve checklist and began to manually close the valve.
The flight data recorder revealed, that one second prior to the aural alert the left outflow valve moved out of the 64.9% closed position and reached 102.1% fully closed position 14 seconds later, then remained in that position until after landing, while the right hand outflow valve opened to 82.5%.
The flight crew reported that upon climbing through FL200 the EICAS page cabin altitude popped up showing the cabin altitude between 6,000 and 7,000 feet in white with a rate of climb of about 1,200 fpm, the left hand outflow valve was in the 9 o'clock position while the right hand outflow valve was closed. Soon after the cabin altitude turned amber while climbing through 8600 feet followed by an "OUTFLOW VLV L" EICAS message without any aural tone. When the aircraft descended through 8000 feet, the cabin altitude had returned to normal, the crew removed the oxygen masks and continued for a safe landing back to Taipei.
A postflight examination revealed the left outflow's AC motor had failed. The motor was replaced and the aircraft returned to service.
The removed AC motor was sent to the manufacturer for further analysis that revealed the rotor shaft/brake assembly had worn, the ASC continued: "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." The root cause determination is still going on.
Incident Facts
Date of incident
Mar 25, 2012
Classification
Incident
Cause
Loss of cabin pressure
Aircraft Registration
B-16411
Aircraft Type
Boeing 747-400
ICAO Type Designator
B744
This article is published under license from Avherald.com. © of text by Avherald.com.
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