Finnair A333 near Moscow on Dec 22nd 2010, loss of cabin pressure

Last Update: August 1, 2012 / 18:28:23 GMT/Zulu time

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Incident Facts

Date of incident
Dec 22, 2010

Classification
Report

Cause
Loss of cabin pressure

Airline
Finnair

Flight number
AY-1968

Departure
Krabi, Thailand

Destination
Helsinki, Finland

Aircraft Registration
OH-LTS

Aircraft Type
Airbus A330-300

ICAO Type Designator
A333

A Finnair Airbus A330-300, registration OH-LTS performing flight AY-1968 from Krabi (Thailand) to Helsinki (Finland) with 286 passengers and 15 crew, was enroute at 11600 meters (approx FL381) about 130nm south of Moscow (Russia) in exceptionally cold air when the left hand bleed air system malfunctioned. The crew actioned the relevant checklists and started the APU. While the APU was starting the right hand engine's bleed air system malfunctioned as well. The crew requested to descend and was cleared to descend to 5400 meters (FL177). Attempts to restore the left or right bleed air system function proved unsuccessful. The flight crew donned their oxygen masks, the cabin altitude alert (cabin altitude above 9550 feet) came on a short time late prompting the crew to perform an emergency descent. While descending through 6900 meters (FL225) the crew selected the APU as bleed air source. Cabin altitude never reached 14000 feet, the passenger oxygen masks therefore were not released. About 7 minutes after the failure of the right hand bleed air system the aircraft levelled at 5400 meters. Another 7 minutes later the crew decided to climb to 6600 meters (FL217) and continued to Helsinki, the crew managed to restore both bleed air systems prior to landing in Helsinki. The aircraft landed safely in Helsinki.

Finland's Onnettomuustutkintakeskus (Accident Investigation Board AIBF) released their final report into this as well as an earlier incident, see Incident: Finnair A333 enroute on Dec 11th 2010, loss of cabin pressure, concluding the probable cause of the incidents was:

Both serious incidents were caused by malfunctioning of the enginesÂ’ bleed regulated pressure transducers' (Pr). The malfunctioning was caused by freezing of water that had accumulated in the bleed regulated pressure transducers' pressure cell rooms, extremely confined by design.

Due to malfunctioning the transducers provided faulty pressure information to Bleed Monitoring Computers (BMC). Due to the erroneous information the BMCs closed both enginesÂ’ bleed air systems which resulted in loss of pressurisation in cabin, i.e. a increase in cabin air pressure altitude.

The extremely cold air mass enroute during a long time period contributed to the fact that the water froze in the pressure cell rooms.

Furthermore, the relatively rapidly increasing ambient temperatures enroute may have contributed to the enginesÂ’ bleed air system faults.

The AIBF reported that Airbus had identified 58 occurrences of dual bleed air losses between 1997 and end of 2010, 16 of which were caused by overpressure (the failure mode identified by the investigation for both OH-LTS and OH-LTO). That failure mode was first observed in 2008, Airbus identified the cause by end of 2009. A first technical information was transmitted to the operators in October 2009, a second report was distributed in February 2010.

The investigation pointed out a similiar event a week later investigated by Taiwan's ASC, see Incident: Eva Air A332 near Simferopol on Dec 29th 2010, loss of cabin pressure, and remarked that the AIBF did receive materials from the ASC for their own investigation.

The AIBF also pointed out that another Finnair flight done by OH-LTN in the same area on Dec 11th 2012, one hour after OH-LTO, had experience a single bleed air failure, which also turned out to be caused by the same factors.

The AIBF analysed that all 4 flights, OH-LTO, OH-LTS, OH-LTN and B-16312 were equipped with CF6-80E1 engines and had been enroute for more than 5 hours with more than 3 hours in air masses colder than normal and with the temperature rising again just before the bleed air failures occurred. All 4 aircraft had operated above minimum operating temperature of the A330.

The AIBF continued that all Pr transducers on those occurrence flights were of version 2, "which is the most prone to malfunctioning caused by freezing of water due to its confined pressure cell room". The transducers were fitted in a location where they were exposed to freezing and had no heating facility. Thus: "The investigation commission considers that the extremely low ambient temperature contributed to water freezing inside the pressure cell rooms of the transducers, situated in the pylons. The relatively rapid rise in temperature prior to the enginesÂ’ bleed air system fault could have contributed to this."

Airbus said version 3 of the transducers is designed to better tolerate freezing conditions.

The AIBF reported that the crew of OH-LTS had already been aware of the earlier incident of OH-LTO by a bulletin of Finnair's fleet chief and thus in anticipation of a possible failure of the second bleed air started the APU as another alternative of bleed air supply. The AIBF analysed that it was this action that permitted to maintain sufficient cabin pressure to keep the cabin altitude below 10800 feet. The AIBF however stated they would have expected the relief first officer taking his rest would be called back to the cockpit so that the full potential of the crew would be available for handling the emergency.

As immediate safety action Finnair replaced the transducers on their A330 fleet by Dec 25th 2010, however with the same version as previously installed. In September 2011 Airbus recommended all left hand engine transducers to be replaced by the version 3 transducer, Finnair promptly followed the recommendation.

Four additional safety recommendations were issued as result of the investigation.