What happened
On April 27, 2013, during a "Heli-Weekend" event at the Grenchen Regional Airport, a collision occurred between the rotor components of two helicopters. The MD Helicopters Inc. Model 369E, registered as D-HMGD, had recently landed on a grass area and was in the process of shutting down its engine. At approximately 15:01, an Eurocopter EC 135/P1, registered as HB-ZJE, took off from a position very close to the stationary aircraft.
As the HB-ZJE climbed to an altitude of roughly 17 meters, the powerful rotor downwash generated by its departure hit the decelerating main rotor blades of the D-HMGD. This aerodynamic force induced significant vertical oscillations in the D-HMGD's blades, with amplitudes estimated at over one meter. These vibrations caused several of the five rotor blades to strike the tail boom of the D-HMGD, resulting in heavy damage to the aircraft's tail structure, rotor blades, and damper elements. No injuries were reported among the five occupants involved in the two aircraft.
The investigation
SUST examined radio communications, witness statements from ground personnel and pilots, and airport webcam footage. The investigation focused on the coordination between the event organizers and the flight operations. Investigators looked into the positioning of the aircraft, the timing of the takeoff, and the adequacy of the event's parking and stationing concept. The investigation also reviewed the technical condition of both the MD 369E and the EC 135/P1, finding no pre-existing technical defects in either aircraft.
Findings
- The primary cause of the accident was the rotor downwash from the departing HB-ZJE inducing oscillations in the decelerating blades of the D-HMGD, leading to a collision with the tail boom.
- The uncoordinated timing of the takeoff of the departing helicopter was the direct cause.
- A deficiency in the organizer's parking and stationing concept acted as a systemic factor, as the assigned parking spots did not sufficiently account for the need for rapid departure of emergency-ready aircraft.
- The pilot of the HB-ZJE performed a safety takeoff at a low altitude, which failed to sufficiently mitigate the downwash impact on the nearby aircraft.