What happened
On 11 February 2012, a Boeing 737-33V, registration G-THOO, was climbing through FL200 after departing London Gatwick Airport when the flight crew experienced sudden airframe vibrations and a high-pitched whining noise. While engine and system indicators remained stable, cabin crew reported a burning smell in the mid-cabin area, which later intensified toward the rear of the aircraft.
Initially, the co-pilot declared a pan to Brest ATC, requesting a level-off at FL290 due to a technical issue. Shortly after, the crew upgraded the declaration to a mayday and requested a direct route back to Gatwick. During the descent, the vibration and noise ceased at approximately 6,000 ft, but the illumination of the right pack trip-off light prompted the crew to shut off the right air conditioning pack. The aircraft landed at Gatwick without further incident, and all 140 passengers and 6 crew members disembarked normally.
The investigation
An inspection by airport fire services found no external damage to the aircraft. However, subsequent maintenance examinations by the operator's engineers revealed that the bearing assembly within the right air conditioning pack turbofan had failed. This failure caused the turbofan to detach from its mounts and resulted in a split turbine air duct. Evidence showed that the impeller blade tips had rubbed against the casing.
Investigators also looked into a pre-existing defect regarding the right ram air inlet door, which had been locked open following maintenance actions. While the manufacturer suggested this could potentially cause the turbofan to operate during high-speed flight, investigators could not confirm if this specific condition contributed to the failure.
Findings
- The primary cause of the airframe vibration was an imbalance in the turbofan shaft caused by the failure of the bearing assembly.
- The burning smell was produced by the impeller blades rubbing against the casing due to the shaft imbalance.
- The failure of the turbofan led to inadequate cooling for the right air conditioning pack, triggering the overheat indication.
- It is suspected the bearing assembly may have run dry due to oil leakage at the seals, though this could not be definitively proven.