Finnair flight AY831 : on 5 March 2011 at approximately 06:50 a pressurisation failure led to a serious incident on Finnair flight AY831. An Airbus A320-214 airliner, registration OH-LXL with call sign FIN831L, was on a scheduled flight from Helsinki to London.
The aircraft departed with Bleed No1 inoperative according to Minumum Equipment List (MEL 36-11-01).
The aircraft was flying in Swedish airspace, north of the Island of Öland at Flight Level (FL) 360 (ca. 10950 m) with both air conditioning packs supplied by Bleed No2 and cross bleed valve opened. 10 minutes after reaching cruise altitude, the remaining bleed (Bleed No2) failed.
As a result of this, the flight crew had to execute a descent rapidly to a safe altitude.
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).
Finnair flight AY831 was at FL 360 over the Baltic Sea [ … ] the flight crew noticed that the bleed air temperature exceeded its maximum permissible value (257 ºC)
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.