Introduction
On 11 November 2020, a Guimbal Cabri G2 helicopter, registered HB-ZVN, was involved in an accident during takeoff from Sion Airport (LSGS). The helicopter was operated by Héli-Alpes SA and was being used for a training flight. The aircraft sustained substantial damage, and a nearby stationary helicopter was also damaged. Both occupants, a student pilot and an instructor, sustained minor injuries.
Flight Crew
The instructor, a 60-year-old Swiss national, held a valid Commercial Pilot License (CPL(H)) issued by the European Union Aviation Safety Agency (EASA). He had accumulated 6,396 total flight hours, including 184:10 hours on the Cabri G2, of which 174:10 were as an instructor. In the 90 days preceding the accident, he had flown 85:17 hours total and 48:39 hours on type.
The student pilot, a 48-year-old Swiss national, was undergoing initial helicopter training and did not hold a license. He had a total of 21:09 flight hours, all on the Cabri G2, with 7:37 hours in the last 90 days. He had flown with four different instructors prior to the accident.
Pre-flight and Takeoff
The student arrived at 08:00 and performed a pre-flight inspection of the helicopter, which was in the hangar. He added 1 liter of oil and moved the helicopter to parking spot H4. He ordered 35 liters of AVGAS, bringing the total fuel on board to 60 liters. The student and instructor discussed the lesson plan, agreeing to practice autorotations in the Bas-Valais region.
The student occupied the right seat, the instructor the left seat. Both fastened their harnesses and donned radio headsets. The student started the engine following the checklist, and the instructor obtained takeoff clearance from the control tower.
Accident Sequence
During takeoff, the student pulled up on the collective control. The helicopter immediately began a left yaw rotation. The instructor shouted, "Put right pedal" and simultaneously pressed the right pedal. He felt strong resistance in the anti-torque pedals as the student continued to raise the collective. The helicopter ascended rapidly while spinning left. The instructor pressed the right pedal with full force, and a sudden loud cracking sound was heard in the cockpit. The helicopter continued to turn left, and the instructor ordered the student to release all controls, stating, "I think it's done for, let go of everything." The instructor had no clear recollection of subsequent actions, did not hear the low rotor rpm warning, and did not shut off the fuel.
The helicopter descended from about 10 meters, still rotating left, and struck the tarmac hard. Upon impact, the skids detached, and the wreckage rotated on the ground. The main rotor blades struck a stationary helicopter on parking spot H3. Both occupants released their restraints and exited the aircraft unassisted. No fire occurred. The emergency locator transmitter (ELT) activated. Airport fire services subsequently applied foam to the wreckage as a precaution.
Injuries
Both crew members sustained minor injuries. An on-site medical examination and breathalyzer test were negative.
Aircraft Wreckage and Examination
The Cabri G2 (serial number 1212) had a maximum takeoff mass of 700 kg. At the time of the accident, the mass and center of gravity were within prescribed limits. The helicopter had undergone a 50-hour inspection on 4 November 2020, with 948 hours in service. The last state inspection by the Federal Office of Civil Aviation (FOCA) was on 16 October 2020.
The helicopter was destroyed. The empennage was rotated 90 degrees to the left due to a break in the tail boom forward of the horizontal stabilizer. The right pedal of the instructor's anti-torque pedals was found in an advanced position relative to the left pedal, indicating an asymmetric position. Examination of the pedal positioning mechanism revealed that the cam on the instructor's right pedal was severely worn, consistent with extreme force applied to it.
The continuity of the tail rotor control system was confirmed, and no defects were found in the cyclic and collective control systems. The investigation did not identify any technical element that could have caused the accident or contributed to its initiation.
Weather
The weather was sunny with scattered clouds at 6000 ft AAL, visibility more than 10 km, wind 050° at 5 kt, temperature 4°C, dew point 2°C, QNH 1027 hPa.