What happened
On January 10, 2014, an Enström F28A helicopter, registration F-GIZR, was performing a private flight from Millau Larzac to Guéret Saint-Laurent. Approximately 30 minutes into the flight, the pilot noticed a distinct change in engine noise followed by a significant reduction in power. This forced the pilot to execute an emergency landing in a field near Saint-Amans-des-Côts.
To manage the descent, the pilot maintained a speed of approximately 50 knots. As the aircraft approached the ground, the pilot chose to perform a side-slip landing on terrain that was both sloping and cross-sloped. During the touchdown, the pilot lowered the collective pitch and applied aft cyclic input. This maneuver caused a main rotor blade to strike and sever the tail boom, resulting in heavy damage to the airframe.
The investigation
Investigators examined the wreckage and found that while the flight controls, main rotor, and tail rotor were functional, the engine' and fuel systems showed significant degradation. The investigation revealed substantial corrosion within the fuel, control, regulation, and injection circuits. This corrosion was traced back to water infiltration through a leak in the right-hand fuel tank gauge.
Further analysis of the fuel system components—including the metering unit, distributor, and both mechanical and electric pumps—confirmed that the corrosion had been developing over several months. Additionally, the right-hand magneto was found to be malfunctioning due to internal cracks and improper wiring.
Maintenance records showed the aircraft had been out of service for 18 months following a previous accident in 2012, during which time it was stored outdoors. Although a 100-hour inspection and magneto overhaul had been completed shortly before the flight, the pilot noted during pre-flight magneto checks that the RPM drop exceeded the limits specified in the flight manual.
Findings
- The engine's performance was significantly compromised by the combined effect of fuel system corrosion and a faulty magneto.
- The pilot elected to proceed with the flight despite identifying a significant RPM drop during pre-flight magneto tests that exceeded manual limits.
- The decision to perform a side-slip landing on unfavorable, sloping terrain, rather than an autorotation, contributed to the tail boom strike when the combination of low collective and aft cyclic input caused blade-to-tail interference.