What happened
On April 25, 2011, a mountain biker sustained serious chest and lung injuries following a crash on a downhill trail near Dättlikon, Switzerland. Due to the steep and slippery terrain, ground rescue teams determined that a helicopter winch extraction was necessary to transport the patient to a hospital.
An Agusta AW109SP, registration HB-ZRX, arrived at the scene to perform the rescue. The crew, consisting of a pilot, a doctor, and a paramedic, identified a clearing for the operation. The pilot hovered approximately 15 meters above the treetops while the winch operator lowered the doctor via a 50-meter cable directly to the area where the patient was waiting.
Immediately after the doctor was lowered, the pilot began to climb and maneuver the aircraft. During this phase, the downward airflow from the helicopter's rotors caused a previously damaged tree trunk to break. The approximately seven-meter-long trunk fell onto the patient, who was lying in the danger zone prepared for evacuation, resulting in a complex pelvic injury.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the sequence of events using witness statements and helmet camera footage. The investigation looked into the condition of the aircraft, the weather, and the environmental factors at the landing site.
Investigators found that the tree, an oak, appeared healthy to the local forester, but a closer inspection of the break point revealed advanced rot and a woodpecker hole located just below the fracture site. The investigation also noted that the ground rescue team was unable to establish radio contact with the helicopter crew prior to the operation, and several bystanders remained within the danger zone during the winch maneuver.
Findings
- The primary cause of the accident was the breaking of a damaged tree trunk caused by rotor wash during the doctor's deployment.
- The tree possessed hidden structural weaknesses due to internal rot and a woodpecker cavity.
- The doctor was positioned in the immediate vicinity of the patient rather than at a decentralized location.
- Ground personnel were not monitoring the overhead area for falling debris.
- Bystanders were present within the operational danger zone during the winch procedure.