What happened
On July 2, 2021, an AS350 B3 helicopter, registration I-TNLD, was performing a VFR ferry flight near Caproni Airport (LIDT) in Italy. The flight was a return leg following aerial work in Borgo Valsugana. After departing from Vigolo Vattaro to return to the Trento base, the pilot encountered a red GOV warning light and a 'FLI FAILED' message on the VEMD, indicating an engine governor malfunction.
While attempting to manage the failure and maintain engine parameters, the pilot climbed to approximately 2,000 feet. However, during the approach phase at roughly 300 feet, the pilot experienced a sudden drop in main rotor RPM. This necessitated an immediate autorotation maneuver. The helicopter struck an area near the SS12 road, impacting a small tree with the tail rotor before the fuselage came to rest in a median strip. Both the pilot and the technician on board sustained serious injuries.
The investigation
The ANSV investigation focused on the technical cause of the engine failure and the pilot's response. Investigators examined the engine's Digital Engine Control Unit (DECU) and the specific configuration of the aircraft's fuel control system. It was noted that the aircraft was equipped with a 'pre-MOD' twist-grip mechanism, which differs significantly from other aircraft in the operator's fleet.
In the 'pre-MOD' configuration, the pilot must manually disengage a mechanical stop on the twist-grip to adjust fuel flow during a governor failure. The investigation also reviewed the pilot's recent training and flight experience, noting that while the pilot was highly experienced, they had recently performed a proficiency check on a different AS350 B3 that featured a more modern, automated 'post-MOD' system.
Findings
- The primary cause of the accident was the suboptimal management of the autorotation maneuver following the engine malfunction.
- The engine failure was likely caused by a momentary signal discrepancy (split) within the engine governor's NPT signals.
- The pilot may have experienced confusion or disorientation due to the differing emergency procedures between the 'pre-MOD' and 'post-MOD' twist-grip systems used within the operator's fleet.
- Operational fatigue may have contributed to the incident, as the crew had been on duty for approximately 11.5 hours, including 5.5 hours of flight time.
- The high cognitive load of managing a manual fuel flow adjustment while simultaneously executing an emergency landing likely delayed the pilot's reaction to the rotor RPM decay.