What happened
On July 16, 2016, an M16 autogyro, registration HB-WGA, was performing a training flight at Ecuvillens aerodrome (LSGE) under Visual Flight Rules. The flight, which included a pilot and a safety pilot, was intended to follow manufacturer commissioning procedures and practice landing circuits.
After taking off and climbing to approximately 3,300 ft AMSL, the pilot began a descent by reducing engine RPM to 4,100. During this maneuver, the Rotax FlyDat system displayed a "low power" warning, indicating the electrical supply had dropped below 10V. Immediately following this, the engine ceased operation. The crew attempted three in-flight restarts, but all failed. The safety pilot took control and executed an emergency landing in a field located approximately 1 km west of the aerodrome. Both occupants exited the aircraft without injury, and the aircraft sustained no damage.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the engine, fuel system, and electrical components. Investigators found that the spark plugs were blackened, a clear sign of an overly rich fuel mixture during operation. While the fuel system itself showed no major leaks or structural failures, a small piece of foreign matter was discovered inside the right carburetor.
Testing of the fuel pressure regulator, fuel pumps, and ignition systems showed they were all functioning within manufacturer specifications. Although the pilots noted a missing fuel tank cap upon landing, the investigation concluded that the cap's absence did not contribute to the engine failure. The investigation also noted a lack of redundancy in the aircraft's electrical distribution, which could potentially lead to fuel pump power loss.
Findings
- The engine failure was caused by the engine flooding due to an excessively rich fuel-air mixture in the cylinders.
- The rich mixture was likely caused by residue or foreign matter within one of the two carburetors, which disrupted the fuel delivery.
- Exhaust Gas Temperature (EGT) data showed a momentary lean spike followed by the engine stalling, consistent with a fuel delivery anomaly during power reduction.
- The crew's response to the engine failure was appropriate, and the emergency landing was executed successfully.