Summary
On January 10, 2014, an Enstrom F28A helicopter, registered F-GIZR, was involved in an accident during a private flight. The pilot and one passenger were on board. The aircraft sustained heavy damage when the tail boom was severed by the main rotor during a forced landing.
Flight Details
The pilot departed from Millau Larzac airfield (12) bound for Guéret Saint-Laurent (23). Approximately half an hour into the flight, a distinct change in engine noise was perceived, followed by a power reduction. The pilot initiated a forced landing, adopting a descent profile to maintain about 50 knots. Approaching the ground, the pilot opted for a skid landing on an ascending and sloping surface. During touchdown, the main rotor struck and cut through the tail boom.
Examination
Post-accident examination revealed that all airframe damage was consequential to the accident. Flight controls were continuous and functional, and both main and tail rotors were operational. However, further inspection identified:
- Significant corrosion in the fuel system and in the engine control, regulation, and injection circuits.
- A leak from the right fuel tank at the fuel gauge.
- Inoperability of the right magneto in the ignition system.
Water ingress into the fuel tanks led to extensive corrosion of the metering unit, distributor, electric pump, and mechanical pump. Immersion testing of the regulator body and diaphragm indicated that the corrosion had developed over several months. Disassembly of the right magneto revealed improper routing of the coil wire and cracks on the coil body.
Pilot Actions
Before departure, during the magneto check, the pilot noted a drop of 50 rpm on one magneto and 150 rpm on the other. The flight manual specifies a maximum allowable drop of 125 rpm. The pilot proceeded with the flight despite this discrepancy.
During the forced landing, the pilot executed a skid landing. Upon touchdown, the pilot lowered the collective pitch control, reducing main rotor rigidity, while applying aft cyclic input. This combination led to a main rotor blade striking and severing the tail boom.
Maintenance History
The helicopter had been involved in a prior accident in July 2012, which damaged the tail boom. The owner purchased another aircraft of the same type for parts. Before repair, the helicopter was stored in the owner's garden, where the engine was periodically run. The aircraft was not flown for 18 months until its complete repair, which included a 100-hour inspection and magneto overhaul. It was returned to service in October 2013.
Conclusion
The accident was contributed to by the following factors:
- The decision to conduct the flight with a significant magneto rpm drop identified during pre-flight checks.
- The choice to perform a skid landing rather than an autorotation on an unfavorable sloping terrain. In a skid landing, aft cyclic input combined with lowered collective pitch increases the risk of main rotor blade contact with the tail boom.