G-EZFX was operating a flight from La Rochelle Airport in France to London Gatwick Airport with 105 passengers and six crew on board. During the climb after takeoff the crew noticed some vibration which ceased after a few minutes. The vibration returned intermittently during the flight until, while the aircraft was holding at FL120 before making an approach to Gatwick Airport, it occurred accompanied by a strong burning smell (which went away quickly). The vibration and smell occurred again a few minutes later, so the crew donned their oxygen masks, declared a PAN and asked for an immediate landing. The smell ceased during the approach.
The operator reported that the vibration and smell were caused by a bearing failure in the avionics bay extractor fan. A program to overhaul the fans was already in place for units with more than 20,000 flying hours but the unit in this incident failed after 19,363 hours.
The operator also reported that the manufacturers of the fan and the aircraft were developing a new fan design which would include a vibration monitoring unit to shut down the fan prior to bearing failure.