Summary
On 1 May 1971 at 0930 local time, an Agusta Bell 204 B helicopter, registration HB-XCQ, was involved in an accident at Domat-Ems, Switzerland. The aircraft was conducting a training flight for autorotation landings. During the fifth exercise, the helicopter struck the ground with its rear, causing the tail rotor to separate. Both crew members were uninjured.
Aircraft and Crew
The helicopter was an Agusta Bell 204 B powered by a Rolls-Royce BS Gnome H 1200-MK 610 turbine engine. It was owned and operated by Heliswiss, Schweizerische Helikopter AG, Belp. The aircraft had a valid certificate of airworthiness issued on 4 February 1971, valid until 31 March 1975. At the time of the accident, the airframe had accumulated 935:50 hours and the turbine 638 hours. The last 300-hour inspection was on 18 February 1971, the last periodic inspection by the Federal Air Office on 24 February 1971, and the last 25-hour inspection on 22 April 1971. Weight and center of gravity were within limits.
The pilot in command, born 1927, held a Swiss professional helicopter pilot license issued on 13 March 1968, valid until 4 March 1971, with qualifications for mountain landings, night flight, and radiotelephony. He had 4040 total helicopter hours, including 915 hours on the 204 B; 43 hours in the last three months (35 on type); and 7:30 hours in the last three days on type. He had two previous helicopter accidents (1966, 1967) investigated by the Canadian Department of Transport.
The trainee pilot, born 1935, Canadian national, held a Commercial Pilot License issued on 7 May 1968 by the Canadian Department of Transport with Swiss validation issued on 23 April 1971, valid until 9 March 1972. He had 7715 total helicopter hours, including 499 hours on the Bell 204 B; 10 hours in the last three months (5:30 on type); and 5:30 hours in the last three days on type. He had one previous helicopter accident (1962) investigated by the Canadian Department of Transport.
Accident Sequence
The accident occurred during a training flight for Heliswiss flight crew. The pilot in command received instructions from the chief pilot to conduct autorotation practice with the trainee pilot to refamiliarize him with the Bell 204 B. At 0900 local, the pilots took off from Domat-Ems with the trainee in the right (pilot) seat and the pilot in command in the left (copilot) seat. After four normal autorotation landings, a fifth exercise was initiated. Climbing to 1000 feet above ground, the trainee positioned the helicopter into wind, reduced power, lowered the collective lever, and stabilized approach speed at 60 knots. Shortly before landing, the trainee flared by pulling back on the cyclic to reduce forward speed. During this maneuver, the helicopter struck the ground hard with its rear, losing its tail rotor. The crew maintained control, and the helicopter came to rest upright on its skids, aligned with the runway, 27 meters from the impact point.
Damage
Both crew members were uninjured. The helicopter sustained the following damage: tail rotor torn off and destroyed, transmission mechanism destroyed, and fuselage damaged. There was no damage to third parties on the ground.
Investigation
The investigation determined that weather conditions (8/8 stratus at 6000 ft, light wind from SW, visibility above 10 km, temperature +12°C, QNH 1006 mb) played no role. The aircraft had valid certificates. The accident was attributed to a misjudgment in the sequence of maneuvers necessary for the correct execution of an autorotation landing. Specifically, the pilot either waited too long before reducing forward speed and performed an excessive flare, or maintained the helicopter in a nose-up attitude too long.