What happened
On May 8, 2001, a Rotorway Exec 90 helicopter, registration OH-XWY, was performing a test flight near Kirkkonummi, Finland. The aircraft was flying at approximately 600 feet above a field when the pilot heard a loud metallic clanking sound followed by unusual vibrations.
Upon hearing the noise, the pilot initiated an autorotative descent. During the landing approach, the pilot realized that the tail rotor had lost power. In an attempt to maintain directional control and keep the aircraft flying straight, the pilot increased engine power. This sudden increase in torque caused the helicopter to yaw sharply to the left by approximately 100 degrees. As the aircraft descended, the right landing gear struck the ground, causing the helicopter to roll over its longitudinal axis and sustain heavy damage.
The investigation
The investigation focused on the mechanical failure of the drive systems and the sequence of events leading to the loss of control. Investigators examined the engine cooling fan belt, the tail rotor drive belt, and the pulleys. A 255 mm long socket wrench extension was also recovered from the crash site, which had been used during recent maintenance work on the aircraft.
Technical inspections revealed that the cooling fan belt had broken and fallen away. The investigation also looked into whether the recovered tool could have interfered with the belt mechanism, though no impact marks were found on the pulleys to support this theory.
Findings
- The primary cause of the accident was the loss of controllability during the final stages of the autorotative landing.
- The loss of control was triggered by the failure of the tail rotor drive belt, which slipped off its pulley.
- The failure of the tail rotor belt was likely caused by the breaking of the engine cooling fan belt, which fell into the space between the tail rotor belt and its pulley.
- The sudden application of engine power during the landing attempt created the torque necessary to yaw the aircraft violently, leading to the ground impact.
- The investigation also noted that parts of the wreckage had been moved and the aircraft was being dismantled before authorities arrived, and the pilot had not initially reported the incident to air traffic control or the police.