What happened
On 18 May 2006, a Boeing 747-40 and registration 9M-MPL, operating a scheduled passenger service from Kuala Lumpur to London Heathrow, experienced an incident involving a reported engine fire. During the descent over the Thames Estuary, a passenger noticed what appeared to be orange flames emanating from the area between the No 1 engine exhaust shroud and its pylon. The passenger alerted cabin crew, who then notified the flight deck.
To investigate, a co-pilot was sent to the passenger cabin to visually inspect the engine through a window. He confirmed the presence of the flames and reported back to the flight deck. Although engine instruments showed no signs of fire or overheating, the commander elected to shut down the No 1 engine and activate the fire handles. During the descent, the crew observed that the apparent flames continued and even seemed to increase in size as the aircraft slowed. The aircraft eventually landed at Heathrow without further incident, and the engine was inspected by airport fire services.
The investigation
The AAIB investigation focused on the physical condition of the engine and pylon area. While there was no evidence of actual fire, fluid leaks, or fuel loss, engineers discovered that a high-temperature silicone rubber bulb seal had torn. This failure allowed the inner section of the seal to protrude into the airflow.
Testing confirmed that when the aircraft was subjected to airflow, the torn seal fluttered in a manner that closely resembled flames. This phenomenon had been observed by ground personnel during engine power runs. The investigation also noted that the manufacturer had previously identified a similar issue and was working on a brown-colored replacement seal to reduce the likelihood of such visual misinterpretations.
Findings
- The perceived fire was caused by a torn bulb seal on the inboard side of the No 1 engine lower rear pylon.
- The orange-colored rubber seal fluttered in the airstream, creating a visual illusion of flames.
- There was no actual fire, engine malfunction, or leakage of fuel or hydraulic fluid.
- The aircraft manufacturer had previously issued a fleet alert regarding the potential for similar occurrences with these specific seals.