Introduction
On 30 September 2021, at approximately 14:02 local time, a Guimbal Cabri G2 helicopter, registered as HB-ZVK, was damaged during a training flight at Ägerten, municipality of Neuendorf, Canton of Solothurn, Switzerland. The helicopter was operated by Swiss Helicopter AG and was being used for autorotation training with a flight instructor and a student pilot. Neither occupant was injured.
Flight Crew Information
The flight instructor, a Swiss national born in 1966, held a Commercial Pilot Licence Helicopter (CPL(H)) issued by the European Union Aviation Safety Agency (EASA) through the Swiss Federal Office of Civil Aviation (FOCA), with a Flight Instructor (FI(H)) rating. He had accumulated 2691 hours and 38 minutes total flight time, including 155 hours and 2 minutes on the accident type, with 118 hours and 54 minutes in the last 90 days.
The student pilot, also Swiss and born in 1980, was undergoing initial training for a Private Pilot Licence Helicopter (PPL(H)). He had logged 29 hours and 38 minutes total flight time, all on the Guimbal Cabri G2, with 14 hours and 49 minutes in the last 90 days.
Aircraft Information
The Guimbal Cabri G2 is a two-seat, single-engine light helicopter with a skid landing gear, a three-blade main rotor, and a fenestron tail rotor. The helicopter was manufactured in 2021 (serial number 1279) and was powered by a Lycoming Engines O-360-J2A four-cylinder air-cooled engine. At the time of the accident, the airframe and engine had accumulated 223 hours and 48 minutes since new. The last scheduled maintenance, a 100-hour inspection, was certified on 10 September 2021 at 199 hours and 5 minutes. The takeoff weight was approximately 650 kg, within the maximum allowable takeoff mass of 700 kg, and the center of gravity remained within limits throughout the flight.
Flight Sequence
The flight instructor and student met on the morning of 30 September at Heliport Pfaffnau (LSXP). After briefing and preflight inspection, they conducted an initial training flight of about one hour in the Pfaffnau area. Following a debriefing and lunch, they returned to the heliport at 13:00, performed another preflight check, and prepared for a second flight focused on autorotation training.
The helicopter, registered HB-ZVK, departed Pfaffnau at 13:37 and flew northwest to Neuendorf, where they performed the first autorotation exercise. Subsequently, they moved to a nearby clearing surrounded by woodland, where the student executed six more autorotations over approximately 15 minutes. During each exercise, the instructor reduced the throttle twist grip without prior warning, deactivating the engine governor. At about 200 meters above ground, the instructor reopened the throttle, reactivating the governor, and confirmed that the engine and main rotor tachometer needles were synchronized. The student then continued each autorotation to a hover condition. No anomalies were noted in the helicopter's behavior or engine power during these exercises.
Accident Sequence
During the eighth autorotation, the student approached a landing area on the right side of a road. The final approach became slightly high, so it was extended to the left of the road, requiring a left turn of about 45 degrees. Near the ground, the student began to flare, reducing rate of descent and forward speed. He then pulled the collective pitch control to reduce sink rate and stabilize the helicopter in a horizontal hover. Simultaneously, he applied right pedal to counter the expected left yaw due to power application.
According to the crew, during this phase, the engine and main rotor rpm dropped significantly, triggering an acoustic warning. The electronic display showed the main rotor and engine rpm needles synchronized. The instructor attempted to turn the helicopter nose back into the direction of flight by applying left pedal, but was unsuccessful. The helicopter struck the ground hard, causing a main rotor blade to contact the tail boom and sever it. The instructor then likely closed the throttle twist grip and shut down the engine by closing the fuel valve and switching off the electrical system.
Damage
The helicopter sustained substantial damage. No third-party damage was reported.
Manufacturer Information
The helicopter manufacturer states that the main rotor can produce lift at rotor speeds well below the nominal minimum of 450 rpm in hover. However, under such conditions, the fenestron tail rotor does not produce sufficient thrust to maintain effective yaw control. Even with full right pedal input, the helicopter will yaw uncontrollably to the left. If such a loss of control occurs, pulling the collective further reduces main rotor rpm and exacerbates the yaw motion.
Probable Cause
The official report does not state a probable cause.