What happened
On June 9, 2016, a Bell 206B helicopter, registration HB-XSL, was conducting a proficiency check flight near Heimenegg, Switzerland. The flight, which included a flight instructor acting as an examiner, involved several maneuvers, including slope landings and an intended autorotation exercise over a large meadow.
During the second attempt at the autorotation, as the pilot was performing the flare to reduce vertical and horizontal speed just meters above the ground, a loud bang was heard. Simultaneously, the engine out warning light illuminated and an audible alarm sounded. The examiner immediately took control of the aircraft, stabilized it into a horizontal flight attitude, and performed a controlled landing in the tall grass. Both occupants remained uninjured.
The investigation
SUST examined the wreckage and the aircraft's maintenance history. The investigation revealed significant damage to the engine, including several punctures in the left engine cowling caused by flying turbine components, and damage to a main rotor blade.
Investigators found that the compressor inlet was heavily soiled, suggesting that the compressor cleaning required every 100 hours had not been performed since the last recorded maintenance in November 2015. Furthermore, the investigation identified that the engine had previously experienced a Turbine Outlet Temperature (TOT) excursion. Evidence suggested the warning light had been reset using the ignition switch, allowing the aircraft to continue operating without the manufacturer-mandated inspections required after such an overheat event.
Findings
- The primary cause of the engine failure was the disintegration of the first-stage turbine wheel due to thermal overload.
- The engine had previously undergone a thermal overtemperature event.
- Mandatory manufacturer procedures following a temperature excursion were not implemented.
- The aircraft was not airworthy at the time of the accident because a required airworthiness directive (HB-2005-259) to install an energy-absorbing ring around the turbine wheel had not been completed.
- Discrepancies were noted in the technical logs, where some entries regarding the aircraft's technical state did not match its actual condition, and a maintenance task was recorded as completed before it had actually been performed.