Thomson B733 near London on Feb 11th 2012, airframe vibration

Last Update: October 11, 2012 / 16:16:08 GMT/Zulu time

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Photo of Thomson Airways G-THOO, Boeing 737-300 — Thomson Airways G-THOO, Boeing 737-300 (Photo credit: Ingy The Wingy / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Feb 11, 2012

Classification
Report

Airline
Thomson Airways

Aircraft Registration
G-THOO

Aircraft Type
Boeing 737-300

ICAO Type Designator
B733

A Thomson Airways Boeing 737-300, registration G-THOO performing some flight from London Gatwick,EN (UK) to an unknown destination with 140 passengers and 6 crew, was climbing through FL200 out of Gatwick when the flight crew noticed a sudden onset of airframe vibrations and a whining noise. All engine indications were normal. Cabin crew reported the noise and vibrations were apparent in the cabin, especially mid cabin. The flight levelled off at FL290, declared initially PAN reporting a problem with an engine, requested to return to Gatwick and 2.5 minutes later declared Mayday when the crew believed they were not given priority. Cabin crew later reported a burning smell of plastics developing in the cabin. While descending through 6000 feet back to Gatwick upon selecting flaps 1 the vibrations and noise suddenly ceased. The aircraft continued for a safe landing at Gatwick Airport, after landing cabin crew reported the smell had further intensified. While the captain instructed cabin crew to be ready for an evacuation, the right hand air conditioning system tripped offline and the smell subsided convincing the crew the problem was related to the right hand pack. The aircraft taxied to the gate, where passengers disembarked normally.

The British AAIB released their Bulletin without identifying callsign or destination of the flight reporting that examination of the aircraft revealed the bearing assembly on the turbofan shaft of the right hand air conditioning system had failed, there was evidence the propeller blade tips had contacted the fan casing. The turbofan detached from its mount and the turbine air duct was split.

The cause of the vibrations was identified as imbalance from the turbofan shaft, the associated burning smell was identified as the fan blades rubbing against the case. The fan was unable to provide adequate cooling for the right hand air conditioning pack which subsequently overheated triggering the trip off light.

It was determined it was likely the bearing had run dry of oil as result of oil leakages at the seals. It was however not possible to verify this.

It turned out that the PAN call by the crew had been overlaid by the transmission presumably from another aircraft, the air traffic controller therefore had not heard the first part of the transmission that included the PAN call. The controller thus planned the aircraft to be handed to London Control at waypoint DIKRO (100nm southsouthwest of Gatwick), but did not communicate this intention to the crew, the crew believing the controller did not facilitate their request for an immediate return to Gatwick thus declared emergency in order to take responsibility for their own routing.