History of Flight
Following takeoff from Aberdeen, failure messages related to the Instrument Landing System (ILS) and Ground Proximity Warning System (GPWS) illuminated. Shortly after, the cabin crew became aware of a strange smell in the forward cabin area. The flight crew were informed and asked the cabin crew to monitor the situation. The strange smell then became evident on the flight deck, and there was an increase in cabin temperature. Initially, it was thought that the smell emanated from the galley ovens, but as it was early in the flight, these had not yet been switched on.
The flight crew donned their oxygen masks, declared an emergency, and carried out an uneventful return to Aberdeen. Following a thorough inspection by the attending fire service, the aircraft was taxied onto the stand for a normal disembarkation of the passengers. After landing, the smell had dissipated.
Previous Flight
On the previous flight, ILS system 1 and the pressurisation system were not working correctly. After landing, the 'AVIONICS VENT SYS FAULT' message appeared on the engine and warning display.
Engineering Investigation
After the event, a thorough inspection of the engines, auxiliary power unit, and air conditioning system did not reveal any problem that could have caused the strange smell. However, due to the ILS fault, GPWS fault, and the earlier 'AVIONICS VENT SYS FAULT' message, it was suspected that the source of the problem lay with the avionics cooling system. Consequently, the avionics vent fan and filter were replaced. There was no repeat of the strange smell on subsequent flight sectors.
The avionics vent fan was sent for repair, where examination revealed noisy and worn bearings.
Background
There have been previous failures of the avionics vent fan on A319 and A320 aircraft, generally related to the bearings. The symptoms of a bearing starting to fail include a low rumbling noise followed by the smell of burning in the cabin. The bearing failure can then reduce the amount of avionics cooling and produce warning messages such as 'AVIONICS VENT SYS FAULT'.
The avionics vent fan manufacturer has issued a Vendor Service Bulletin 3454-21-108, which replaces the current bearings, containing steel ball bearings, with an improved type utilising ceramic ball bearings. Consequently, the aircraft manufacturer has issued a service information letter (SIL 21-141) which notifies aircraft operators of the improvement.