What happened
On July 27, 2000, at approximately 17:30 local time, a BK 117 C-1 helicopter, registration D-HBKA, was performing a Helicopter Emergency Medical Service (HEMS) mission near Canto Alto, Italy. The flight had departed from Orio al Serio airport carrying a pilot, a doctor, a nurse, and an alpine rescue technician, originally destined for a medical intervention in Sedrina. After the mission was canceled via radio, the pilot began the return flight to base.
While flying over Canto Alto at an altitude of 1,146 meters, the pilot initiated a descent and a right-hand turn to correct the approach path. During this maneuver, the aircraft entered a descending spiral and impacted a wooded area on the slope. The impact caused significant damage to the airframe, including the main and tail rotors, the transmission, and the tail boom. Despite the severity of the crash, the four occupants escaped with only minor injuries.
The investigation
The ANSV investigation examined the wreckage, the pilot's flight experience, and meteorological conditions. Technical analysis of the wreckage, including the tail rotor drive shaft, ruled out mechanical failure, as the breakage was determined to be the result of extreme torsional stress during rotation. The investigation also considered the aircraft's maintenance records, which showed the helicopter was fully compliant with all airworthiness requirements.
Investigators analyzed the flight dynamics, noting that the aircraft was operating at low speeds with a tailwind during a right-hand turn. The terrain's topography, specifically the recent crossing of a mountain peak, likely introduced downdrafts or turbulence that contributed to the aerodynamic instability.
Findings
- The primary cause of the accident was the occurrence of Low-Torque Engine (LTE) phenomenon, which induced an uncontrolled rightward yaw during the right-hand turn.
- The pilot's delayed perception and inadequate response to the sudden yaw prevented the recovery of the aircraft before impact.
- The proximity to the terrain left insufficient margin for the pilot to counteract the developing spiral.
- The use of left cyclic input during the yaw event, rather than the recommended left pedal application, failed to arrest the rotation.
- An increase in collective pitch shortly before impact may have further exacerbated the yaw rate by temporarily reducing rotor RPM and anti-torque effectiveness.