Accident Description
On September 9, 1998, a Hiller UH-12E helicopter was operating from a parking area near Aschau, Germany, in support of a forest restoration project by transporting tree seedlings as an external load attached to a 40-meter-long cable. The day before the accident, numerous similar transport flights had been conducted.
On the accident day, the forester and the helicopter crew met at the parking area around 07:00. After a preflight inspection, flight operations began at approximately 07:30. A short-notice decision was made to conduct a reconnaissance flight without transporting any plants.
During takeoff, the helicopter initially climbed vertically and then transitioned to forward flight. When the helicopter reached a speed of 30–40 knots and an altitude of about 40 meters above ground, a member of the ground crew warned the pilot via radio that the external-load cable was still suspended beneath the helicopter. The warning came too late; moments later, the cable caught on a pasture fence running perpendicular to the direction of flight. When the fence wire broke, the tensioned cable whipped upward into the main rotor blades. The main rotor blades severed the cable, and a portion of it was thrown into the tail rotor, which was extensively damaged.
As a result, the helicopter entered an uncontrolled flight condition and impacted a meadow out of control, sustaining catastrophic damage.
Investigation
The investigation was conducted by two flight accident investigators at the scene. Since the sequence of events was clearly reconstructed and technical causes were ruled out, the investigation focused on operational aspects.
Conclusions
According to the investigation report, the accident occurred when the pilot initiated a reconnaissance flight with the external-load cable still attached from the previous day's transport flights. The short-notice change in flight planning—from a transport flight to a reconnaissance flight without external load—influenced the accident sequence. The pilot apparently forgot to release the cable and chose a takeoff profile that would have been correct for a flight without the cable but was unsuitable for a flight with the cable. The ground crew's radio warning came too late.
By unfortunate circumstances, the cable hook caught the wire of a pasture fence. After the wire broke, the tensioned cable whipped into the main rotor, causing severe damage to both the main and tail rotors, resulting in loss of control and an uncontrollable flight condition. The accident could not be prevented.