Synopsis
A passenger on a Boeing 747-400 saw what appeared to be small orange flames coming from between the No 1 engine exhaust shroud and its pylon. The flight crew was informed. Engine indications were normal, but the commander decided to shut down the engine. The 'flames' continued until the aircraft was slowed for an approach. The aircraft landed uneventfully. Investigation revealed that a rubber seal had torn, and when agitated by the airflow it gave the appearance of flames. There had been eleven previous events reported on similar aircraft.
History of the Flight
The aircraft departed Kuala Lumpur on a scheduled flight to London Heathrow with a flight crew of two captains and two co-pilots. One captain and co-pilot operated the aircraft for takeoff, climb, and early cruise; this captain was designated as the commander for the flight. The other captain and co-pilot flew the cruise portion until about one hour from landing when the original crew resumed control.
The flight was uneventful until shortly before the aircraft commenced its descent towards London. A passenger observed what appeared to be orange flames between the No 1 engine exhaust shroud and its pylon. The passenger pointed this out to a cabin crew member, who immediately informed the flight crew via interphone. The 'cruise' co-pilot was sent to the rear of the cabin to look at the engine through a window. He also saw what appeared to be flames and reported to the flight deck. After discussion, the 'cruise' co-pilot returned to the rear cabin and remained there in interphone communication. The 'cruise' captain also went to the cabin to assess the situation before returning to the flight deck, confirming the co-pilot's report.
The flight crew and the 'cruise' captain evaluated all available information, including engine indications, which were normal. As the aircraft descended, they shut down the No 1 engine and pulled the fire handle to shut off fuel and hydraulic connections and isolate it electrically. Since there were no flight deck indications of an engine fire, they did not discharge the fire extinguisher bottles.
After the engine was shut down, the 'cruise' co-pilot reported that the 'flames' appeared to remain, and when the aircraft decelerated from 290 kt to 250 kt, they appeared to increase slightly.
The flight crew informed ATC of a problem and that there was a "one foot flame" from the No 1 engine. They requested a priority landing with the fire service on standby. ATC explained that an emergency declaration was needed for priority; the flight crew declared a 'PAN' and were radar vectored for a priority landing. When Flap 5 was selected, the 'flames' appeared to extinguish. The commander completed an uneventful automatic approach and landing, and the aircraft was inspected by the airport fire service before taxiing.
Engineering Examination
Initial examination of the rear lower pylon area did not reveal any evidence of burning or fluid leaks. The area showed evidence of a dark 'sooting' type of staining that was dry and of a long-term nature. There was no evidence of hydraulic fluid loss or fuel loss or fuel tank imbalance. The hydraulic and fuel systems were powered up with no fluid leaks observed. The No 1 engine was 'motored' with fuel selected off and on, and no fluid leaks were observed. All pylon panels and engine cowls were opened, and no evidence of any fluid leaks was seen.
The aircraft manufacturer's Safety Services Department informed the AAIB of previous incidents of reported airborne fire in the area of the lower rear pylon and engine tail pipe. In each case, extensive examination found no evidence of fire, but the inboard 'sacrificial' bulb seal between the lower rear pylon and the tail pipe had torn or failed. These bulb seals are circular in cross-section, hollow, approximately 1.25 inches in diameter, 14 inches long, and made from flame-orange-colored, high-temperature silicone rubber. Examination of the aircraft involved showed that the bulb seal on the inboard side of the No 1 lower rear pylon had torn along its fore/aft axis, allowing the outer section to protrude into the airstream.
Following replacement of the engine cowls and pylon panels, low and high engine power runs were carried out. No fluid leaks were noted, and all flight deck indications were normal. An observer seated in the cabin from where the 'cruise' co-pilot had observed the flames noted that during the full power run, the torn bulb seal 'flapped' in the airflow in a way that could easily be mistaken for a flame. This was also seen by ground observers. The torn bulb seal was replaced, and the aircraft returned to service with no further problems.
Previous History
In January 2002, the aircraft manufacturer issued a Fleet Team Digest (FTD) alerting Boeing 747-400 operators with PW 4000 series engines to the possibility of torn or damaged pylon tailpipe bulb seals being mistaken for flames as they fluttered in the airstream. The FTD article advised of Boeing's efforts to find a suitable replacement for the orange/red seal. In March 2006, Boeing advised operators that a suitable 'brown' colored replacement part had been identified, although production and retrofit had not yet been scheduled. Boeing recommended periodic inspection of the affected area and replacement of damaged seals. In light of the manufacturer's continuing review and the provision of an alternative brown seal, the AAIB saw no need for any recommendation.