What happened
On March 23, 2021, an Aérospatial AS350B helicopter, registration JA6050, was en route from Tokyo Heliport to a temporary operation site in Nagano Prefecture when the engine began to lose power. While cruising near Aoki-mura, the pilot observed the engine chip caution light, followed shortly by a noticeable engine sound and a drop in rotor RPM, signaling a flame-out.
In response to the loss of power, the pilot initiated an autorotation maneuver. Aiming for a farm road between rice paddies, the pilot attempted to manage the descent by increasing airspeed and performing a flare maneuver just before touchdown. However, the rate of descent became uncontrollable at a low altitude, leading to a high-impact landing on an asphalt road. The impact was severe, with estimated vertical load factors reaching 35 G. The aircraft was destroyed, and the accident resulted in three serious injuries and two minor injuries among the crew and passengers.
The investigation
The Japan Transport Safety Board (JTSB) conducted a detailed teardown of the Turbomeca Arriel 1B engine. Investigators discovered significant internal damage, including a fractured power turbine shaft and a deformed power turbine bearing support. Metallurgical analysis revealed that the forward part of the power turbine shaft had melted and fractured, with temperatures reaching at least 1,300 °C.
Furthermore, the investigation found a significant quantity of metallic particles, including bearing and shaft materials, within the engine's scavenge pump and oil filter. While the engine's maintenance records showed that all periodic inspections were up to date, the investigation focused on the mechanical failure of the power turbine front bearing, which likely led to the shaft fracture.