What happened
On April 10, 2016, a Robinson R4SO was conducting a local flight from the Villefranche Tarare aerodrome with a pilot and three passengers. After approximately 20 minutes of flight, the pilot approached the paved runway 18 from the south. At an altitude of roughly ten meters, the pilot initiated a right turn to vacate the runway and head toward the parking area along the taxiway.
During this low-speed transition, the aircraft experienced a rapid right yaw. In an attempt to maintain control and avoid ground impact, the pilot applied left pedal and increased collective pitch. This maneuver caused the helicopter to climb and enter a second rotation with a high pitch attitude. The yaw eventually ceased, but the aircraft subsequently moved backward and struck the ground violently, rolling onto its right side. The impact resulted in four people slightly injured and the destruction of the helicopter.
The investigation
The investigation focused on the mechanical condition of the aircraft and the environmental factors present during the accident. Technical examinations of the wreckage, specifically the tail rotor, revealed no mechanical failures. Audio analysis from a passenger's video confirmed that the engine, main gearbox, and tail rotor gearbox were operating normally, though the rotor RPM briefly dropped below the threshold, triggering the "LOW NR" alarm.
Investigators also analyzed airport surveillance footage, which revealed that while the pilot believed winds were calm, the actual conditions involved turbulent winds between 10 and 15 knots from a variable direction between 150° and 180°. The footage also confirmed the aircraft was flying at the edge of its ground effect during the maneuver.
Findings
- The pilot's initial right pedal input likely initiated the right yaw.
- Loss of tail rotor effectiveness (LTE) was sustained by a crosswind from the left, which then transitioned into a tailwind, creating a weathercock stability effect that accelerated the yaw.
- The pilot's reaction to the yaw was likely delayed.
- The simultaneous application of increased collective pitch while operating at the edge of ground effect caused a decrease in main rotor RPM, making the aircraft difficult to recover.