The Air Force, Navy and Marine Corps 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.
Lt. Gen. 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.
In Japan, where U.S. military Ospreys had a non-fatal crash once and a number of incidents, the latest accident has rekindled safety concerns just as the Japanese government builds a new base for its fleet of Ospreys.