Finnair A320 near Stockholm on Mar 5th 2011, cabin pressure problems
Last Update: January 24, 2013 / 12:38:07 GMT/Zulu time
Finland's Onnettomuustutkintakeskus (Accident Investigation Board of Finland AIBF) released their final report into the serious incident concluding the probable cause was:
The serious incident on the OH-LXL was caused by rising Cabin Pressure which, in turn, could have been the result of a failure of the Fan Air Valve (FAV) or Temperature Control Thermostate (TCT) grid filter clogging in the right engine's bleed air system (Bleed No2).
With the other system being inoperative for flight, the cooling capacity of only one system proved insufficient. The pre-cooled air was too hot, therefore the temperature sensor of the system worked as per its design and shut off the overheated system.
Maintenance found following the incident flight, that the Fan Air Valve had failed. In addition the temperature limiting thermostat, temperature control thermostat, the over pressure valve and the pressure regulating valve were replaced. All components were found extremely dirty and out of operating limits.
The AIBF analysed that the APU can be started at any altitude and is capable of being used as a backup system for bleed air up to FL200 in case of a dual bleed failure. The AIBF stated however that the APU was never started on the incident flight and concluded, that a second emergency descent became very likely during the remainder of the flight when the bleed air system #2 again reached its maximum operating temperature for about 20 minutes.
With the APU online, the AIBF stated, the aircraft would have been able to maintain a safe altitude of FL200 rather than FL100.
The AIBF analysed further that the crew should have declared Mayday upon initiating the emergency descent, with the investigators prefering the term emergency descent over the term "rapid descent" used in the manuals. The commander used his discretion to not declare Mayday during the incident flight, probably because the aircraft had already been cleared to FL100. The AIBF continued: "It is important to keep the threshold for declaring an emergency as low as possible. An emergency can always be cancelled if it turns out to be disproportionate to the situation. The purpose of the legal and operational status of a declared emergency is to maximise the safety of aviation. The purpose of a MAYDAY call is to inform all possible parties, the air traffic control and any nearby aircraft of an occurrence that jeopardizes the safety of aviation."
Two safety recommendations were released as result of the investigation.
This article is published under license from Avherald.com. © of text by Avherald.com.
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