Finnair A320 near Stockholm on Mar 5th 2011, cabin pressure problems

Last Update: January 24, 2013 / 12:38:07 GMT/Zulu time

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Photo of Finnair OH-LXL, Airbus A320 — Finnair OH-LXL, Airbus A320 (Photo credit: Leo-setä / Flickr / License: CC by)

Incident Facts

Date of incident
Mar 5, 2011

Classification
Report

Cause
Cabin pressure problems

Airline
Finnair

Flight number
AY-831

Departure
Helsinki, Finland

Destination
London Heathrow, United Kingdom

Aircraft Registration
OH-LXL

Aircraft Type
Airbus A320

ICAO Type Designator
A320

A Finnair Airbus A320-200, registration OH-LXL performing flight AY-831 from Helsinki (Finland) to London Heathrow,EN (UK) with 140 passengers and 6 crew, had departed Helsinki under minimum equipment list requirements with the bleed air system #1 inoperative. The aircraft was enroute at FL360 about 60nm south of Stockholm (Sweden) when bleed air system #2 became unstable and then shut down due to overheat resulting the cabin altitude climbing. The crew requested a descent and actioned the relevant checklists, ATC cleared the aircraft to FL100, the captain began to descend the aircraft, however, the cabin altitude climbed through 9450 feet resulting in a cabin altitude warning requiring an emergency descent. The crew donned their oxygen masks, the captain deployed speed brakes and increased the sink rate, the aircraft briefly accelerated through its maximum operating airspeed. The first officer in the meantime managed to reset bleed air system #2, the temperatures returned to normal and the cabin pressure returned to normal, too, the passenger oxygen masks did not deploy. The captain therefore levelled the aircraft at FL140 and subsequently climbed the aircraft back to FL250 for the remainder of the flight, during which the bleed air temperature again rose to its maximum operating temperature for a period of about 20 minutes.

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.