What happened
On June 10, 2020, a Bell 505 Jet Ranger X, registration HB-ZWC, was conducting a training flight near Nods, Switzerland. The flight, part of a refresher course, focused on practicing autorotations. During the third of seven planned exercises, the flight instructor transitioned the engine from idle to flight power to initiate a power recovery.
As the instructor attempted to recover power, the torque increased much more rapidly than anticipated. To prevent exceeding torque limits, the instructor limited the upward movement of the collective lever and maintained a horizontal attitude. However, this prevented the aircraft from fully arresting its descent, resulting in a hard landing. The helicopter slid approximately 0.5 meters before coming to a stop. The crew, consisting of a flight instructor and a student, sustained one light injury, while the other crew member was uninjured.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) examined the aircraft's flight data from the Garmin 1000 unit. The data revealed that immediately after switching the engine to flight power, the collective lever was raised while the rotor RPM was still low. This action increased aerodynamic drag on the rotor blades, causing a sharp spike in torque.
Physical inspections of the aircraft revealed a 3 mm deep dent in the tail boom skin and damage to the paint on the main rotor mast. The investigation also confirmed that the aircraft's weight and center of gravity were within manufacturer limits and that weather conditions were favorable at the time of the incident.
Findings
- The primary cause of the incident was the inability to sufficiently arrest the descent during the autorotation recovery due to a rapid torque increase.
- The rapid torque spike was triggered by raising the collective lever immediately after switching the engine to flight power while the rotor RPM was still low.
- The pilot's decision to limit collective movement was an appropriate response to prevent exceeding engine torque limits, though it prevented a full stop of the descent.
- The forces generated during the hard landing and subsequent control inputs led to a phenomenon known as mast bumping, where the rotor head contacted the rotor mast and a rotor blade tip struck the tail boom.