What happened
On March 1, 2010, a Cessna 525A departed Stuttgart airport for a ferry flight to Bremen. Shortly after takeoff, as the pilot retracted the landing flaps from 15° to 0°, the aircraft experienced unusual vibrations and a strong leftward yaw. Simultaneously, a "Left Engine Fire" warning activated, and the pilot noted that the Inter-Turbine Temperature (ITT) had risen significantly into the red zone.
In response to the emergency, the pilot moved the left engine throttle to idle and then to the shutoff position. Although the pilot prepared to use the engine fire bottles, the fire warning extinguished on its own after several seconds, and the vibrations decreased. The aircraft returned to Stuttgart and landed safely at 8:22 AM. There were no fatalities and no injuries, though the aircraft sustained light damage.
The investigation
The BFU examined the engine and the aircraft's components. Post-flight inspections revealed that the outer portion of the left engine cowling was destroyed and the engine itself was damaged. Investigators found discharge marks distributed across the engine cowling.
An intensive engine examination, conducted by the manufacturer Williams International under NTSB supervision, identified fatigue cracks at both the front and rear of the diffuser. A portion of the diffuser tube was missing, and the fuel pipe showed signs of wear and fatigue. The investigation also noted that the combustion chamber had holes with soot traces, and parts of the low-pressure turbine housing had been burned away.
Findings
The investigation established that the primary cause of the engine fire was a fatigue failure of the diffuser. Operational vibrations caused cracks to develop in the diffuser, which allowed the component to shift backward and make contact with the fuel pipe. This contact, combined with further vibration-induced fatigue, caused the fuel pipe to break. The resulting uncontrolled flow of fuel into the combustion chamber led to the fire.
Several contributing factors and precursors were identified:
- The presence of fatigue cracks in the diffuser and the fuel pipe.
- Previous indications of engine issues, including the smell of smoke in the cabin during two prior flights and a 20°C to 30°C increase in the left engine's ITT observed over the preceding six months.
- The aircraft's firewall successfully prevented the fire from spreading to the main airframe.
Following the incident, the manufacturer implemented a design change to reinforce the diffuser in the FJ44-3A-24 engine series to prevent similar fatigue failures.