Introduction
On 9 October 2001, at approximately 11:50 local time, an Ecureuil AS350B3 helicopter, registration HB-ZBN, landed in a field near Steckborn, Switzerland. The pilot, born in 1916 and holding a private pilot license, had taken off from Samedan Airport at 11:11 with the intention of landing at the clinic 'Schloss Mammern' in Mammern. Minutes before the planned landing, he experienced a strong urge to urinate and decided to land about 1 km south of Steckborn, close to a forest edge.
Accident Sequence
Immediately after landing, the pilot exited the helicopter, leaving the engine running and the rotor turning. He stood briefly beside the door and then walked several meters in front of the aircraft. There, he was struck on the head by a rotor blade and collapsed. The passenger, a Swiss citizen born in 1944 with no aviation qualifications, went to a nearby road to call for help. She returned to the helicopter and used a mobile phone to ask the helicopter owner how to shut down the engine. She cut off the fuel supply using the fuel shutoff valve, stopping the engine and allowing the rotor to come to a standstill.
Injuries and Damage
The pilot sustained serious head injuries from the rotor strike. Approximately one year after the accident, he died from the consequences of those injuries. The helicopter suffered substantial damage to the rotor mast and rotor head. No third-party property damage occurred.
Helicopter and Autopilot Information
The helicopter was a five-seat turbine-powered aircraft with a three-blade rotor system and fixed skid landing gear, manufactured in 1999. It was equipped with a three-axis SFIM 85 T 31 autopilot, as described in the Flight Manual Supplement No. 16. Examination of the cockpit after the accident revealed that the autopilot was still engaged, with pitch, roll, and yaw channels active, as well as the heading select and airspeed hold modes. The trim system was also engaged. The autopilot control actuators were found in the full nose-down (pitch) and full right turn (roll) positions. A reconstruction flight with a comparable helicopter showed that after landing and releasing the trim release button, approximately 20 seconds elapsed before the cyclic control moved to the full right-forward position, causing the droop stop ring to contact the blade stops.
Metallurgical Analysis
Fragments of broken droop stop ring retaining forks were examined. The fractures were characteristic of rapidly growing fatigue cracks under high loads, occurring within few load cycles with high stress amplitude. No pre-existing damage was found.
Weather and Other Factors
Meteorological conditions at the time included 1-2/8 cloud cover with base at 2500 ft, 7/8 at 6500 ft, visibility around 7 km, wind from 050° at 3 knots, temperature +13°C, dew point +11°C, and QNH 1018 hPa. No fire broke out, and rescue measures were carried out in a timely manner.