What happened
On July 31, 2017, an AS350 B1 helicopter, registration I-CSAM, was performing its first flight of the day from a grassy landing pad at Il Picciolo Etna Golf in Castiglione di Sicilia. The aircraft, operated by Butterfly srl, was conducting an aerial work mission for a transfer to Palermo.
During the transition from hover in ground effect to hover out of ground effect, the pilot experienced a sudden drop in engine power. To avoid a more serious uncontrolled flight, the pilot performed an emergency landing on a nearby grassy slope. While the initial landing was not excessively violent, the aircraft experienced intense vibrations immediately afterward. These vibrations caused the main rotor blades to strike the ground and the terrain, leading to the separation of the main transmission and the engine group from the airframe. The impact caused extensive damage to the cockpit, windshield, and fuselage. Two of the three occupants sustained minor injuries and were treated at a hospital.
The investigation
The ANSV investigation focused on the engine performance and the mechanical failure sequence. Technical analysis of the Arriel 1D engine was conducted by Safran, with participation from the BEA. Investigators examined the Fuel Control Unit (FCU) and the engine's compressor and turbine components.
Engine inspections revealed that the compressor inlet showed signs of metallic ingestion, and metallic residues were found on the turbine blades, confirming the engine was operating at the time of the incident. Investigators also identified a leak in the P2/P3 pressure line and found that three of the five fixing screws for the acceleration control unit were missing on the FCU. Furthermore, internal contamination was discovered within the FCU, likely originating from the working piston due to component wear.
Analysis of the wreckage confirmed that the main rotor and tail rotor were still rotating during the impact sequence. While Airbus Helicopters investigated the possibility of ground resonance, they concluded that the damage to the tail boom was a result of the heavy landing and subsequent impact rather than a resonance phenomenon.
Findings
- The primary cause was an incompatible engine power delivery relative to the pilot's power demand during the critical transition from hover in ground effect to hover out of ground effect.
- The engine's performance was degraded by internal contamination within the FCU and a pressure leak in the P2/P3 line.
- Environmental factors, including high ambient temperature and high altitude, reduced the engine's available performance margin.
- The pilot may have performed a sudden collective acceleration, possibly to counteract wind gusts, which exceeded the degraded engine's ability to provide power.
- The heavy landing on uneven terrain contributed to the vibrations that ultimately led to the blade strike and structural separation.