The Air Force, Navy and Marine Corps on Wednesday took the extraordinary step of grounding hundreds of aircraft after a preliminary investigation of last week's crash indicated that a materiel failure - that something went wrong with the aircraft - and not a mistake by the crew led to the deaths.
The crash raised new questions about the safety of the Osprey, which has been involved in multiple fatal accidents over its relatively short time in service. Japan grounded its fleet of 14 Ospreys after the crash.
Lieutentant-General Tony Bauernfeind, head of Air Force Special Operations Command, directed the standdown "to mitigate risk while the investigation continues," the command said in a statement.
"Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time."
In a separate notice, Naval Air Systems Command said it was grounding all Ospreys. The command is responsible for the Marine Corps and Navy variants of the aircraft.
The Air Force said it was unknown how long the aircraft would be grounded. It said the standdown was expected to remain in place until the investigation determined the cause of the Japan crash and made recommendations to allow the fleet to return to operations.
The US-made Osprey is a hybrid aircraft that takes off and lands like a helicopter but can rotate its propellers forward and cruise much faster, like an aeroplane, during flight.
Its unique design has been a factor in multiple incidents. While the investigation into last week's crash has only just begun, it renewed attention on the aircraft's safety record, particularly on a mechanical problem with the clutch that has troubled the program for more than a decade. There also have been questions as to whether all parts of the Osprey have been manufactured according to safety specifications.
In August, the Marines found that a fatal 2022 Osprey crash was caused by a clutch failure, but the root cause was still unknown. In its report on the crash, the Marines forewarned that future incidents "are impossible to prevent" without improvements to flight control system software, drivetrain component material strength, and robust inspection requirements."
Air Force Special Operations Command has 51 Ospreys, the US Marine Corps flies as many as 400 and US Navy operates 27.
The Osprey is still a relatively young aircraft in the military's fleet. The first Ospreys only became operational in 2007 after decades of testing.
But more than 50 troops have died either flight testing the Osprey or conducting training flights in the aircraft, including 20 deaths in four crashes over the past 20 months.
An Osprey accident in August in Australia killed three Marines. That accident also is still under investigation.
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