What happened
On April 30, 2002, at approximately 16:00, an Eurocopter AS350B2, registration HB-XJC, was undergoing start-up procedures at the Zermatt Heliport. The pilot, who had recently taken over the aircraft for a ferry flight, initiated the engine start sequence. Following the completion of the start, the aircraft experienced intense vibrations and ground resonance, accompanied by a loud, rising engine noise heard by witnesses. During the acceleration phase, the helicopter experienced an uncontrolled lift. While the pilot attempted to reduce power at approximately one meter above the ground, the aircraft fell back to the surface, rotating roughly 9/0 degrees on its vertical axis. The hard landing caused visible damage to the transition area between the fuselage and the tail boom. The pilot sustained no injuries.
The investigation
Investigators examined the engine and the fuel control unit (FCU) to determine the cause of the abnormal engine behavior and the subsequent lift. The investigation revealed that the engine's FCU exhibited several functional discrepancies, including an internal leak in the non-return valve and an acceleration curve where fuel flow was too low. Furthermore, the investigation found that the FCU was two years past its calendar service limit and lacked a required warranty seal on the NG maximum stop. The engine inspection also showed evidence of operating beyond normal temperature limits, with compressor blades having rubbed against the cover.
Findings
- The primary cause of the accident was the uncontrolled lifting of the collective during the engine acceleration process.
- The investigation could not definitively determine why the collective failed to remain locked or why it was not secured.
- Technical defects in the turbine created an unusual start-up behavior, specifically a significant delay in acceleration after reaching 50% NG, which required heightened pilot attention.
- The pilot's focus was likely diverted by managing the fuel control lever and monitoring instruments during the abnormal engine response, potentially preventing the detection of the rising collective.
- The engine's fuel control unit was out of service limits and possessed functional flaws that could have been identified through mandatory periodic checks, which were not recorded in the engine logbook.