What happened
On September 26, 2013, a commercial Eurocopter EC 120B helicopter, registration HB-ZIX, was involved in a collision with a structure in Oberes Herrenfeld, Schwyz. The flight began at Buochs airfield, where the pilot initially flew toward Seewen to pick up passengers. During the initial leg of the flight, an engine chip warning light began to flicker, indicating the presence of metal particles in the engine gearbox.
Following a request from the passengers to land at their farm in Oberes Herrenfeld, the pilot diverted to the property. After a brief stop and consultation with a mechanic regarding the warning light, the pilot continued the flight to Trogen to pick up additional passengers. On the return leg, facing approaching thunderstorms, the pilot landed at the farm again at 17:25. To save time, the pilot opted to keep the engine running while the passengers disembarked. As the pilot attempted to lift off and transition into forward flight, the main rotor blades struck a protruding barn overhang. The impact caused the helicopter to lose control and crash from a height of approximately 2 to 3 meters onto a gravel area. One passenger, who was standing near the barn, sustained serious injuries from flying debris, while the other occupants remained uninjured.
The investigation
SUST examined the flight logs, technical records, and the physical wreckage. The investigation focused on the suitability of the landing site and the pilot's decision-making regarding the technical warning light. Investigators analyzed the dimensions of the landing area, noting the 16.3-meter distance between the residential building and the barn, and the 6.4-meter height of the barn overhang. The investigation also reviewed the pilot's prior reconnaissance of the site and the communication with the maintenance provider regarding the intermittent engine chip warning.
Findings
- The primary cause of the accident was the collision of the main rotor with a barn overhang during takeoff from an inadequate landing site.
- The pilot's assessment of the landing area was incomplete; the pilot was unaware of the extent of the protruding roof and failed to account for the narrow clearance between the buildings.
- The pilot was operating under time pressure due to approaching thunderstorms, which led to the decision to allow passengers to disembark while the rotors were still turning.
- Although the pilot was aware of an intermittent engine chip warning, the decision to continue flying was not in accordance with manufacturer guidelines, which recommend an immediate landing and inspection upon such a warning.
- The landing position on the second attempt was shifted closer to the barn, significantly reducing the lateral clearance for the rotor blades.