What happened
On 16 September 2007, a Eurocopter EC135 T2, registration G-IWRC, was conducting a private flight near North Weald Airfield, Essex. The pilot and passenger were returning to the UK from Belgium. While flying at approximately 1,000 ft using the autopilot in a hands-off mode, the pilot heard and felt a dull thud from the rear of the aircraft, accompanied by a change in attitude.
Upon glancing at the triple tachometer gauge, the pilot misinterpreted the position of the rotor RPM needle, leading him to believe the aircraft had suffered a double engine failure. Consequently, the pilot initiated an autorotation to perform an emergency landing in a nearby field. During the landing flare, the tail of the helicopter struck the ground first, causing the tailboom to break and the fenestron drive shaft to fail. The aircraft then slid across the field, with the left skid digging into the soft soil, causing the helicopter to roll onto its right side. There were no injuries to the two occupants, though the aircraft sustained extensive damage to the fuselage, tailboom, and rotors.
The investigation
The AAIB examined the wreckage and analyzed data from the engine's FADEC and the aircraft's warning unit. The investigation focused on the cause of the initial disturbance and the subsequent pilot actions. Investigators reviewed the flight history, noting the pilot's recent experience and the specific flight conditions.
Testing was conducted to determine if the autotrim function could be inadvertently disengaged. While it was possible for external objects or physical contact to trigger the SAS DCPL switch, evidence suggested it was unlikely the pilot or passenger had done so during the flight. The investigation also looked into the source of the 'thud' heard by the pilot, considering possibilities such as a bird strike or a transient disturbance in the hydraulic or flight control systems.
Findings
- The accident sequence began with the uncommanded disengagement of the autotrim function while the pilot was flying hands-off.
- The pilot misread the triple tachometer gauge, which led to the erroneous belief that all engine power had been lost.
- The pilot's decision to enter an autorotation was based on this incorrect assessment of engine status.
- The tail of the helicopter struck the ground before the skids, resulting in the structural failure of the tailboom and fenestron drive shaft.
- The aircraft's roll was caused by the left skid encountering soft, recently cultivated soil during the slide.