What happened
On July 31, 2011, a Guimbal Cabri G2, registration PH-WTW, was conducting flight training at Lelystad Airport. The flight involved an instructor and a student pilot performing autorotation exercises. During the final phase of a third autorotation maneuver, the instructor realized the aircraft was overshooting the intended landing area. To avoid the risk of the helicopter tipping while crossing a raised taxiway, the instructor suggested performing a power recovery instead of completing the autorotation.
As the helicopter descended to approximately 10 feet above taxiway D, the engine throttle was opened. During the impact with the ground, the aircraft experienced a hard landing with the nose slightly downward. The force of the impact caused the collective lever to be abruptly pushed downward by the inertia of a crew member's arm. This sudden movement, combined with low rotor RPM, caused the main rotor blades to strike the tail boom, resulting in the disintegration of the tail rotor and significant structural damage to the tail boom.
The investigation
Technical examinations conducted with manufacturer experts revealed that the engine, fuel systems, and carburetor heating systems were all functioning correctly. There were no indications of mechanical failure in the rotor system prior to the event. The investigation focused on the physical interaction between the blades and the airframe.
Investigators found that two of the three rotor blades had sliced into the tail boom, with the first blade penetrating 20 mm into the skin and exposing the tail rotor drive shaft. The second blade penetrated 4 and 40 mm respectively, striking the drive shaft and causing it to bend. This impact forced the flexible coupling to move axially, eventually pulling it from the pulley and causing it to break at the weld seam. The tail rotor itself disintegrated due to the high inertial forces and the sudden seizure of the drive shaft.
Findings
- The primary cause of the accident was suboptimal coordination between the instructor and the pilot during the transition from autorotation to power recovery.
- The helicopter experienced a hard landing characterized by low rotor RPM.
- The structural failure was triggered by a combination of low rotor speed, the inertia of the crew member's arm pushing the collective down, and the specific cyclic position of the aircraft.
- There were no pre-existing technical malfunctions in the engine or rotor systems.