What happened
On 24 July 2002, a Northwest Airlines Boeing 747-200F, registration N617US, was operating a scheduled cargo flight from Singapore to Bangkok. During the climb phase at 28,000 feet, the aircraft's No. 4 engine experienced a flame-out. This was immediately followed by a fire indication in the engine compartment.
Despite the crew discharging both engine fire bottles, the fire indication persisted. The flight crew successfully returned the aircraft to Singapore Changi Airport, where the landing was completed without further incident. However, upon landing, airport fire and rescue personnel observed flames emerging from the underside of the No. 4 engine. The fire was eventually extinguished using a hand-held extinguisher.
The investigation
The investigation involved a multi-agency effort, including the Singapore Ministry of Transport, the NTSB, the FAA, and representatives from Boeing and Pratt & Whitney. The engine was transported to Minneapolis for a detailed teardown.
Inspectors observed significant thermal damage and sooting extending from the angle gearbox toward the tailpipe. The fire was most intense between the 5:00 and 7:00 o'clock positions. The severity of the heat was such that the bottom of the nacelle was breached, and several engine components were lost overboard, including the N2 tachometer, the main oil pump, and portions of the starter assembly and fuel supply filter.
Maintenance records were also scrutinized. Prior to the incident, the aircraft had undergone maintenance at Singapore Changi Airport, which included repairing a cracked pre-cooler duct and replacing a starter assembly after a sheared coupling shaft was discovered during ground runs.