Introduction
On 9 June 2015 at approximately 09:50 local time, a Cabri G2 helicopter registered HB-ZOA was involved in an accident at Confrin des Praz, Semsales, in the canton of Fribourg, Switzerland. The helicopter was operated by Swiss Helicopter AG and was conducting a visual flight rules (VFR) training flight. The crew comprised an instructor and a student pilot. The student, who held no pilot license, had accumulated 61 hours 48 minutes of flight time, with 56 hours 23 minutes on type. The instructor held a commercial pilot license (helicopter) and had a total of 3107 flight hours, including 176 hours on the Cabri G2 and 1754 hours as an instructor.
Flight Sequence
The lesson, scheduled from 08:00 to 10:00, was the first flight together for the instructor and student. The briefing defined the program, aiming to improve the student’s precision in autorotation landing points. After a weather briefing and pre-flight checks, the helicopter departed normally. Several autorotation exercises were conducted at two different fields before the instructor selected a large meadow west of Semsales. The first autorotation on this field necessitated a power recovery due to late corrections by the student. The second was executed correctly, prompting the instructor to keep the throttle closed and continue to the ground. A third autorotation was initiated by the instructor at an altitude of 3700 ft QNH (approximately 1000 ft above ground) by setting the throttle to idle. The student responded correctly, lowering collective and restoring rotor RPM. The student performed a flare to reduce speed and rate of descent, then leveled the helicopter near the ground and pulled collective to cushion the touchdown. The instructor had his hands on the controls and accompanied the maneuver. The left skid touched the ground, followed immediately by the right skid. The helicopter bounced forward and became unstable. The instructor took over the controls. Upon a second impact with the ground, a suspicious noise was heard. The helicopter came to rest on both skids. The instructor noted damage to the tail boom and the tail rotor driveshaft. He shut down the engine, braked the rotor, and cut the battery. Both crew members were uninjured.
Meteorological Conditions
At the time of the accident, Switzerland was on the edge of a high-pressure area. A layer of stratus covered the northern Alps, with a base at 3300 ft AMSL. Visibility was approximately 6 km. The wind was from 360 degrees at 5 knots. Temperature was 14 °C with a dew point of 11 °C, and QNH was 1019 hPa. Hazards included low cloud base and bise turbulence.
Aircraft Information
The Cabri G2 (HB-ZOA) is a two-seat helicopter with skid landing gear, a three-blade main rotor, and a Fenestron tail rotor. It was manufactured in 2013 by Hélicoptères Guimbal. The engine was a Lycoming O-360-J2A rated at 108 kW (145 hp) at 2650 RPM. The maximum takeoff mass is 700 kg. At the time of the accident, mass and center of gravity were within limits. The last 50-hour inspection was performed on 29 May 2015. Fuel quantity at departure was 58 liters; at the accident, it was 34 liters.
Impact and Damage
The accident site was a flat, obstacle-free field with short grass. The first skid impact marks were approximately 5 cm deep; subsequent impressions exposed loose soil. The helicopter sustained serious damage: the right horizontal stabilizer and the tail boom were struck by the main rotor blades. The upper part of the tail boom was severed, and the Fenestron tail rotor driveshaft was cut and torn off. The lower fuselage showed an impact from contact with the skid support tube. The tips of the main rotor blades bore impact marks from contact with the tail boom.
Probable Cause
The accident was caused by the main rotor blades colliding with the tail boom because the flight instructor intervened too late at the end of an autorotation exercise that had not been previously discussed and was continued to the ground with the engine at idle.