What happened
On September 8, 1996, a Rotorway Exec helicopter, registration OH-XHL, was involved in an accident in Rauma, Finland. The aircraft, which had been homebuilt by the owner, was being used for a ground test intended to check rotor blade vibrations, a process the crew referred to as "tyypitys." Although the crew believed they were not performing a flight, the aircraft was being operated aerodynamically at a low altitude.
During the procedure, the helicopter lifted slightly off the ground. The pilot attempted to maintain control by applying heavy right pedal input, but the aircraft began to rotate unexpectedly to the left. In an attempt to regain control, the pilot increased altitude to approximately 1.5 meters, but the rotation intensified. The aircraft eventually struck the ground, causing the main rotor blades to hit the earth. Immediately following the impact, the engine stopped, but smoke and flames quickly emerged from the engine compartment. The helicopter was almost entirely consumed by fire before emergency services arrived.
The investigation
The investigation examined the mechanical state of the aircraft, the pilot's experience, and the cause of the subsequent fire. Investigators analyzed the transmission system, specifically the drive belts, and the stability of the fuel tank mounting. Photographic evidence from the scene showed that while the engine stopped upon impact, smoke and soot appeared in the engine compartment within seconds, suggesting a flash fire involving fuel vapors or leaking fluids.
Findings
- The pilot lacked a valid helicopter pilot license and possessed very limited flight experience.
- The pilot's perception that low-altitude hovering does not constitute "flying" was incorrect, as the aircraft was being controlled aerodynamically.
- The loss of control was caused by the tail rotor drive belts slipping or the engine running at insufficient RPM, which prevented the pilot from compensating for the torque-induced rotation.
- The fuel tank mounting was found to be inadequate, as the design allowed the rear of the tank to potentially rest against the generator rotor, creating a significant fire risk.
- The engine oil tank vent design contributed to engine compartment contamination, increasing fire hazards.
Safety action
- It is recommended that aviation authorities issue airworthiness directives requiring fuel tanks to be mounted such that the rear ends cannot descend and contact the generator rotor.
- To improve engine compartment cleanliness and fire safety, the oil tank venting should be routed to the exterior of the engine compartment.