A BAC One-Eleven 510ED, G-AVMS, was operating a scheduled passenger flight from Manchester (England) to Geneva (Switzerland.) on this day in aviation history. After all flight preparations and engine starts are completed the aircraft taxies to Runway 24 for a reduced thrust take-off.
The take-off was uneventful and the aircraft followed the required noise abatement procedure. At 4000 feet the throttles were set to climb power. When the engines reached climb power a "thumb" was heard from the rear of the aircraft. With no cautions or warnings getting the crew's attention and no control issues the only thing noticeable to the crew was a change in the thrust indication from the climb value of 90% to 44-60%. A steward, alerted by the same sound as the flight crew and the abnormal sound of rushing air, checked the rear of the aircraft. His check, including the toilet, did not reveal the reason for the noise.
At the same time, the flight crew was checking the engine and engine controls. The movement of the left-hand engine throttle did not result in a change of thrust. With no control of the engine, the crew shut the left-hand engine down, declared an emergency, and informed ATC that they would return to Manchester due to an engine failure. A short while after the engine shut down the smoke alarm for the aft left toilet alerted the crew. It was found to be filled with a great smoke. A long burst of a Fire extinguisher was discharged into the toilet. This was repeated after some time when, during a subsequent check the smoke was still present, although considerably less than before. When the smoke had almost cleared completely, a cabin crew member removed all possible panels in the toilet, while discharging fire extinguishant behind the remaining panels. There was no sign of fire and no further development of smoke.
The single-engine return to Manchester and landing were completed without further problems. After the aircraft was checked by the airport fire brigade, and no signs of fire were found, the aircraft continued to the terminal where the passengers disembarked.
The failed downstream flange on the HP Shut-Off valve (Source aaib.gov.uk)
An investigation into the incident revealed a failure of the High-Pressure Bleed Air ducting in the stubwing. The downstream attaching flange of the HP shut-off valve had failed around its full circumference.
This caused hot bleed air to escape into the stubwing (between the engine and the fuselage) causing considerable heat damage t0 the aircraft skin and electrical wiring in the stubwing. The aircraft skin transferred the heat to the insulation, which caused the smoke in the toilet. Due to a failure in the Duct Fail Warning system (dirty contacts in a relay), the automatic deactivation of the bleed system did not occur. In the time from the duct failure, to the time the engine was shut down (85 seconds) the hot bleed air caused the damage and smoke in the toilet.
The full investigation report can be read by clicking here.