The US military has blamed the deadly crash of an Osprey transport aircraft in waters off southwestern Japan on the “catastrophic failure” of a gearbox.
The US Air Force published a report on Thursday of its investigation into the crash of the CV-22 Osprey off Yakushima Island in Kagoshima Prefecture last November. All eight crewmembers onboard died.
The report says the failure of the aircraft’s left-hand gearbox was most likely due to a crack in one of the gears and fatigue cracking of the bearing cage.
The gearbox transmits power to the proprotor, a type of aircraft rotor.
The report says the loss of drive to the left-hand proprotor occurred when at least one piece of the failed gear became wedged in the assembly, grinding against the teeth of another gear until they were all removed.
The report also points out that according to Air Force guidance, the crew are supposed to land as soon as possible when there is a warning of a problem with a proprotor gearbox.
But it says that after the warning was displayed, the Osprey headed to Yakushima Island, not to Iojima or other islands that were closer to the aircraft at the time.
The document adds that the crew had “no further discussion or collaboration” on other landing options or actions to set conditions for an immediate landing or ditching.
Describing the crash as a “mishap,” the report concludes that it resulted from the catastrophic failure of the gearbox and the pilot’s decisions that “unnecessarily extended the flight” after multiple advisories and warnings about the glitch.