Incident Overview
On 24 July 2002, a Northwest Airlines Boeing 747-200F cargo aircraft, registration N617US, was operating as scheduled freighter flight NW908 from Singapore to Bangkok. During the climb at 28,000 feet, the No.4 engine experienced a flameout, followed by a fire indication. The flight crew discharged both engine fire bottles, but the fire indication remained active. The aircraft returned to Singapore Changi Airport for an uneventful landing at 0748 hours local time. After landing, fire was observed coming from the underside of the turbine section of the No.4 engine, and airport fire and rescue personnel extinguished the fire using a hand-held extinguisher.
Aircraft and Engine Details
The aircraft was a Boeing 747-251F (serial number 21121) powered by Pratt & Whitney JT9D-7J engines. The No.4 engine (serial number 662502) had accumulated 83,704 hours and 19,364 cycles total, with 2,456 hours and 520 cycles since its last overhaul. The engine was installed on the aircraft on 13 October 2001. The main gearbox had 10,038 hours and 2,018 cycles since overhaul, and the angle gearbox had 10,028 hours and 2,019 cycles since overhaul.
Fire and Damage
The fire was confined to the No.4 engine but caused extensive damage to the gearbox and nacelle along the bottom of the engine. The severity of the fire resulted in a hole in the bottom of the nacelle, and the following components were lost overboard: N2 tachometer, main oil pump, deoiler drive gear and associated bearings and seals, main scavenge pump, idler gearshaft, starter and starter valve with a section of the starter air duct, and compressor stator control fuel supply filter assembly. There was no damage to the aircraft beyond the No.4 engine and its cowling and nacelle.
Maintenance and Recent History
In the 213 hours before the incident, the average oil consumption of the No.4 engine was 0.22 quarts per hour. A review of the aircraft maintenance log recorded several exhaust gas temperature (EGT) exceedances during take-off on 11, 15, and 19 July 2002. On 21 July 2002, the aircraft arrived at Singapore for an overnight check performed by SIA Engineering Company, which included scheduled tasks on the No.4 engine. On 22 July, an engine ground run revealed issues with the pre-cooler assembly and a chafed pneumatic tube, which were repaired. On 23 July, after installation of a serviceable pre-cooler, a reverse flow leak check was performed. A subsequent engine start attempt was unsuccessful due to a sheared starter drive gearbox coupling shaft, which was replaced.
Investigation Findings
The investigation team, including representatives from Pratt & Whitney, the National Transportation Safety Board, the Singapore Ministry of Transport, the Federal Aviation Administration, Northwest Airlines, and Boeing, conducted a teardown examination of the engine at the Northwest Airlines overhaul facility. The main gearbox and angle gearbox were removed and shipped to Pratt & Whitney for evaluation. The investigation concluded that the fire originated from a failure in the main gearbox. Detailed examinations of the gearbox components, including the main oil pump, scavenge pump, and drive gears, were performed. Analysis of oil samples and filter elements did not reveal significant contamination prior to the incident. The provided probable cause is null.