History of the Flight
After departing London Gatwick Airport, while climbing through FL200, the flight crew noticed a sudden onset of airframe vibration and an accompanying whining noise. There were no abnormal engine or system indications, but a smell of burning was reported in the cabin. The commander declared a mayday and initiated a diversion back to London Gatwick Airport. The aircraft landed uneventfully.
Air Traffic Control Communication
On first contact with Brest ATC, the aircraft was cleared to climb to FL330, but the co-pilot requested a level-off at FL290 due to a "slight technical problem." The controller approved and cleared the aircraft to proceed on its planned route. After levelling off, the flight crew could not identify the vibration source, and the commander decided to return to Gatwick. The co-pilot declared a pan with Brest ATC, advising of a technical problem with one engine. After further exchanges, the co-pilot upgraded to a mayday. The flight crew declined routing via waypoint DIKRO and initiated a direct route back to Gatwick.
Cabin and Crew Actions
Cabin crew reported the vibration and noise were noticeable throughout the aircraft, particularly in the mid-cabin area, and passengers became alarmed. Later, the rear cabin crew reported a smell similar to burning rubber. The flight crew briefed the senior cabin crew to prepare for a precautionary landing. Upon selection of flap 1 at approximately 6,000 ft, the vibration and noise ceased. After landing, Airport Fire Services inspected the aircraft externally and found no problems. While the inspection was ongoing, the burning smell intensified. The commander made a PA call to place cabin crew on standby, and the right pack trip off light illuminated. The flight crew turned the right air conditioning pack switch off and consulted the QRH. The burning smell reduced. The aircraft taxied to the terminal, and passengers disembarked normally.
Aircraft Examination
Inspection by the operator's engineers determined that the bearing assembly on the turbofan shaft within the right air conditioning pack had failed. There was evidence that impeller blade tips had rubbed against the turbofan casing. The turbofan had detached from its mounts, and the turbine air duct was split. The turbofan and turbofan valve were sent to a repair facility for strip examination.
Relevant Defects
On the day prior to the incident, a defect was raised because the right ram air inlet door was indicating fully open throughout the flight. The B737 Dispatch Deviations Guide permits continued operation if the door is locked open. Maintenance actions required disconnecting the electrical connector to the ram air actuator and isolating the electrical supply. This was done on 11 February, and the aircraft operated two further sectors before the incident flight. The defect recurred after the incident and was resolved in late April 2012.
Discussion
The cause of the airframe vibration was identified as the imbalance in the turbofan shaft resulting from failure of the bearing assembly. The burning smell was attributed to the turbofan impeller blades rubbing on the casing due to the imbalance. In this condition, the turbofan could not provide adequate cooling for the right air conditioning pack, leading to an overheat and illumination of the right pack trip off light. The turbofan operates normally only during ground operation or slow flight with flaps extended; sustained vibration suggested it was operating during high-speed flight. The aircraft manufacturer considered possible electrical anomalies, but due to maintenance interventions, the precise reason could not be determined. Workshop examination concluded that the bearing likely ran dry of oil due to oil leakage at seals, but limited detail prevented verification. The strip examination report could not confirm whether high-speed flight contributed to the failure.
The radio telephony review revealed that the first part of the pan declaration transmission was blocked. The controller planned to route the aircraft via DIKRO but did not communicate this to the flight crew. Believing the controller was not facilitating an immediate return, the flight crew upgraded to a mayday to take responsibility for routing. As a result, Brest ATC Safety Department conducted a review to improve emergency flight handling.