What happened
On October 11, 2004, a Kaman K-1200 K-MAX, registration HB-XQA, was engaged in timber logging operations near Grandvillard, Switzerland. After completing approximately 22 to 23 rotations and performing a hot-refueling stop, the pilot attempted to lift a new load of logs. During this process, the pilot heard unusual noises and experienced a significant loss of engine power.
In an effort to protect ground personnel positioned directly beneath the long logs, the pilot intentionally delayed releasing the load. The pilot made several attempts to mechanically release the cargo, but the load remained attached. Eventually, the helicopter entered a nose-down attitude with reduced rotor RPM, and the cargo sling finally detached. The aircraft crashed into a forested area, resulting in one serious injury to the pilot and the total destruction of the helicopter.
The investigation
Investigators examined the wreckage and the mechanical components of the transmission system. The analysis focused on the clutch assembly and the engine-to-rotor connection. Metallurgical examinations of the clutch shaft revealed extreme wear, including a rough, pitted surface and evidence of plastic deformation. The investigation also looked into the behavior of the cargo hook and the timing of the load release. Additionally, the investigators reviewed historical data regarding similar incidents involving the K-1200 clutch assembly reported by operators between 1999 and 2004.
Findings
- The primary cause of the accident was a failure of the clutch assembly.
- This failure was driven by excessive stress and fatigue on the rolling surfaces of the freewheel clutch shaft.
- The delayed release of the cargo from the hook likely contributed to the severity of the accident, as the pilot's attempt to protect ground crew prevented an immediate emergency jettison of the logs.
- The investigation noted that the pilot's decision to prioritize ground safety resulted in the aircraft being in an unrecoverable, nose-down position before the load could be cleared.