What happened
On July 5, 1997, a student pilot was conducting solo hovering exercises at Buttwil Airfield in an Enstrom 280FX helicopter, registration HB-XEQ. The flight was part of a private pilot training session. During a diagonal maneuver within a designated training square, the pilot noticed an unusual odor inside the cockpit. Upon landing, engine instruments appeared normal; however, before the next takeoff attempt, thin wisps of smoke began entering the cabin through the passenger seat crevices.
The pilot immediately shut down the engine, closed the fuel valve, and evacuated the aircraft. Outside, a plume of white smoke was observed emanating from the engine's intake and ventilation ports, which quickly darkened into visible flames erupting from the engine compartment. Due to the aircraft's full fuel tanks, personnel on the airfield refrained from using portable fire extinguishers to avoid the risk of explosion. The Buttwil fire department arrived shortly thereafter and extinguished the fire using foam.
The investigation
Investigators examined the wreckage and consulted forensic experts from the Zurich City Police Scientific Service. The investigation focused on the engine compartment, specifically the area around the turbocharger. Forensic analysis identified the fire's origin in the front left section of the engine compartment.
Investigators found that the oil check valve, a component made of light alloy, had completely melted away. There was no evidence of electrical failure, fuel system leaks, or any signs of intentional or negligent ignition. The investigation also reviewed the aircraft's maintenance history, noting that while a 25-hour inspection had been performed recently, the check valve's specific integrity was not a primary focus of that interval, and no previous issues with the oil system had been recorded in the technical logs.
Findings
- The primary cause of the fire was a leak in the oil check valve.
- Dripping oil from the leaking valve landed on the extremely hot turbocharger housing, which operates at temperatures between 700°C and 800°C.
- The ignition of the oil caused the leak to expand, leading to the rapid spread of fire within the engine compartment.
- The pilot's actions—shutting down the engine, closing the fuel valve, and evacuating—were correct and prevented further escalation.
- The specific reason for the valve's failure (whether due to a material defect or improper installation) could not be determined because the component was destroyed by the fire.