“Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time,” U.S. Air Force Special Operations Command (AFSOC) said in a statement.
The tilt-rotor aircraft developed by Boeing and Bell Helicopters crashed during a routine training mission last week off Yakushima Island, about 1,040 km (650 miles) southwest of the capital, Tokyo.
At least 400 of the transport aircraft have been delivered and are mainly used by the U.S. Air Force, Marines and Navy, according to Boeing. Japan’s Self-Defence Forces also has a small fleet of Ospreys.
Japan grounded its V-22s immediately after the crash. The U.S. initially said it was suspending Osprey flights from the unit that the doomed aircraft belonged to, but said other aircraft would continue to fly after safety checks.
It is not unusual for the U.S. military to ground entire fleets, especially after fatal accidents. A spokesperson for Japan’s defence ministry said on Thursday its aircraft remained grounded. The deployment of the aircraft in Japan has been controversial, with critics of the U.S. military presence in the country’s southwest islands saying it is prone to accidents. “The standdown will provide time and space for a thorough investigation to determine causal factors and recommendations to ensure the Air Force CV-22 fleet returns to flight operations,” AFSOC said.
According to the Flight Safety Foundation, at least 50 personnel have died in crashes operating or testing the aircraft. More than 20 of those deaths came after the V-22 entered service in 2007.
In August, three U.S. Marines died in an Osprey crash off the coast of northern Australia while transporting troops during a routine military exercise.
In 2022, four U.S. personnel were killed when an Osprey crashed in a remote part of northern Norway during a NATO training exercise.