What happened
On August 18, 1980, a Westland/Agusta Bell AB 47G-3B1 helicopter, registration HB-XHT, was engaged in aerial spraying operations near Villeneuve, Switzerland. Operated by Trans Heli SA, the aircraft was performing agricultural work, specifically treating a tobacco field, as part of a series of spraying missions across the region.
During the mission, the pilot completed an initial pass and then executed a wide left-hand turn to reposition for a subsequent pass. After climbing to an altitude of approximately 10 to 15 meters, the pilot descended to a height of roughly 2 to 3 meters above the ground to stabilize the aircraft for the next pass. During this phase, the pilot attempted to adjust the collective and power to maintain a steady level. However, the aircraft failed to respond as expected. In an attempt to manage the situation and prepare for an emergency landing, the pilot applied cyclic input, which resulted in the tail of the helicopter striking the ground. The impact caused the tail boom to be severed by a rotor blade, and the aircraft subsequently capsized in the soft earth.
The investigation
Investigators examined the wreckage and the flight parameters to determine the cause of the accident. The aircraft was found to be within its maximum takeoff weight of 1340 kg, with an estimated flight weight of 1200 kg. The center of gravity was within prescribed limits. Detailed inspections of the engine, fuel system, and flight controls revealed no evidence of mechanical failure or technical deficiency that could have caused the loss of control.
Physical evidence at the scene showed significant damage to the tail rotor guard and the tail boom, with debris scattered approximately 70 meters from the impact site. The investigation also ruled out the possibility of a vortex ring state, as the flight parameters during the final phase did not meet the necessary conditions for such a phenomenon.
Findings
The investigation concluded that the accident was likely caused by a pilot error in judgment regarding the sequence of maneuvers required to maintain stabilized flight during the start of a spraying cycle. Specifically, the investigators considered two operational possibilities: an inadvertent reduction in rotor RPM during the wide turn, leading to delayed aircraft response, or a late recovery from a flare maneuver intended to reduce airspeed, which caused the tail to strike the ground.