What happened
On June 16, 2017, at approximately 17:50 local time, an AS 350 B2 helicopter, registration HB-ZPF, was conducting a training flight at Luzern-Beromünster Airfield (LSZO). The flight was part of a training program intended to help the pilot obtain a type rating for this specific model. The aircraft was operating at a high weight, near its maximum takeoff mass of 2250 kg, carrying a flight instructor and four passengers.
After lifting off from a helipad located near the southern end of the hangar, the instructor transitioned the aircraft into a hover-taxi across the grass runway, heading south toward the start of runway 33. During this maneuver, a powered glider was taxiing into position on the same grass strip. To avoid the downwash from the helicopter's main rotor affecting the approaching aircraft, the instructor attempted to rotate the helicopter to the left around its vertical axis.
As the instructor applied right pedal input to realign the aircraft toward a northwest heading, the helicopter experienced a sudden loss of tail rotor effectiveness. This caused the aircraft to spin counter-clockwise for more than one full revolution. The instructor managed to arrest the rotation and performed a controlled landing on the grass. There were no injuries to the six occupants, and the aircraft sustained no damage.
The investigation
The SUST investigation focused on the mechanical and aerodynamic conditions during the hover-taxi. Investigators examined the flight parameters and noted that during the uncontrolled rotation, the tail rotor torque briefly exceeded limits, reaching 113% over-torque for approximately one second. A subsequent visual inspection of the AS 350 B2 confirmed that no structural or mechanical repairs were necessary.
Findings
- The primary cause of the incident was the loss of tail rotor effectiveness during a pedal input maneuver.
- The aircraft was operating at a high weight, near its maximum takeoff mass.
- The pilot's attempt to rotate the aircraft to avoid creating downwash for a nearby glider contributed to the sequence of events.