What happened
On March 5, 2011, a Finnair flight, AY831, traveling from Helsinki to London, encountered a serious in-flight emergency while cruising at FL360 over the Baltic Sea, north of Öland, Sweden. The Airbus A320-214, registration OH-LXL, had been cleared for flight under the Minimum Equipment List (MEL) with the number one engine bleed system deactivated due to a previously deferred defect.
Approximately 48 minutes into the flight, the crew observed fluctuations in the number two engine bleed air pressure and cabin altitude. Shortly thereafter, the Electronic Centralized Aircraft Monitoring (ECAM) issued an 'AIR ENG 2 BLEED FAULT' warning. The temperature in the number two bleed system rose above the maximum limit of 257°C, causing the system to shut down. This left the aircraft without any functional bleed air source for pressurization, leading to a rapid rise in cabin altitude.
As the cabin altitude approached the critical threshold of 9,450 feet, the crew received an 'excessive cabin altitude' warning accompanied by a red light and an audible alarm. The crew donned oxygen masks and initiated an emergency descent. After receiving clearance from Malmö ATC to descend to FL100, the pilots managed to restart the number two bleed system during the descent. Once the temperature stabilized, the crew leveled off at FL140 and eventually continued the flight to London at a lower altitude of FL250.
The investigation
The Finnish Safety Investigation Authority (OTKES) examined the technical state of the aircraft and the maintenance history of the bleed air system. The investigation focused on why the remaining functional bleed system failed and why the aircraft was unable to maintain pressure. Investigators analyzed the performance of the Fan Air Valve (FAV) and the temperature control thermostats. The investigation also reviewed the adequacy of the operational instructions provided in the Airbus Operations Engineering Bulletin (OEB) and the Quick Reference Handbook (QRH).
Findings
- The primary cause of the serious incident was the failure of the number two engine bleed system, which left the aircraft without any means of cabin pressurization.
- The failure was likely caused by a malfunction in the number two engine's Fan Air Valve (FAV) or a blockage in the temperature control thermostat (TCT) network filter.
- The cooling capacity of the remaining single bleed system was insufficient to handle the load while the other system was deactivated.
- The crew did not activate the Auxiliary Power Unit (APU) to provide an independent bleed air source during the period of system instability.
- At the time of the incident, the aircraft's Master Minimum Equipment List (MMEL) contained an error regarding the procedures to follow if both bleed systems failed.
Safety action
- Finnair accelerated an internal maintenance campaign to complete required modifications on the bleed air system components for its A320 fleet.
- Following the incident, Finnair's flight operations management issued instructions that A320 flights over the Alps should not be planned if the aircraft is operating with only one functional engine bleed system.
- The Airbus MMEL was corrected in March 2011 to address the erroneous references regarding dual bleed failures.