What happened
On June 21, 2011, at approximately 0007 EDT, a Boeing B-7-57-232, registration N6714Q, experienced a fire in its number one engine during takeoff from Atlanta’s Hartsfield International Airport (ATL). The aircraft, operated by Delta Air Lines as flight 1323, was performing a domestic passenger flight to Los Angeles International Airport (LAX).
During the climb through 3,000 feet, the captain observed a small yaw followed by a left engine fire indication during gear retraction. The crew declared an emergency, shut down the number one engine, and initiated a turnback to ATL. The aircraft completed a successful overweight single-engine landing on runway 26L. Following the landing, an emergency evacuation was performed on the right side of the aircraft. Of the 172 passengers and 6 crewmembers on board, 3 passengers sustained minor injuries during the slide evacuation.
The investigation
Post-incident examination of the number one engine revealed extensive fire damage and thermal distress within the fan and core compartments. The most significant damage was located in the fan compartment, specifically between the intermediate case and the turbine exhaust case.
Investigators found that the end housing of the fuel flow transmitter (FFT) had backed off from the main housing, creating a gap of approximately 0.3 inches. This separation resulted in the plastic deformation of the FFT-to-fuel distribution valve (FFT-to-FDV) fuel line. While metallurgical analysis of the FFT hardware showed no material defects, torque tension tests and finite element modeling indicated that the load capability of the bolted joint was marginal under certain assembly variances and operational temperature changes. Specifically, the testing showed that the main housing threads might fail to maintain preload, leading to the loosening of the joint.
Findings
- The fire was caused by fuel leaking from the fuel flow transmitter, which ignited upon contact with hot engine components.
- The fuel leak originated because the end housing of the fuel flow transmitter separated from the main housing, creating a gap for high-pressure fuel to escape.
- The separation occurred because the bolted joint's ability to maintain preload was compromised by specific assembly variances and fluctuations in operational load and temperature.