By Idrees Ali and Tim Kelly
WASHINGTON/TOKYO (Reuters) -The U.S. military said on Wednesday it was grounding its fleet of V-22 Osprey aircraft after a crash last week off the coast of Japan that killed eight people onboard.
Tokyo grounded its small fleet of the tilt-rotor aircraft the day after the fatal incident, which reignited controversy over its deployment. Critics in Japan have said the Boeing (NYSE:BA) and Bell Helicopter developed Osprey is prone to accidents, although U.S. and Japanese governments reject that claim.
"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 latest crash happened during a routine training mission on Nov. 29 off Yakushima Island, about 1,040 km (650 miles) southwest of the capital, Tokyo.
At least 400 multipurpose Ospreys have been delivered and are mainly used by the U.S. Air Force, Marines and Navy in Japan and elsewhere, according to Boeing. The U.S. Navy aircraft carrier deployed to Japan, the USS Carl Vinson, relies on them to deliver some supplies and personnel.
"If the Osprey grounding keeps going for a week or more, the inconvenience starts to become something more. And without the Osprey, training can be affected, and that affects readiness," said Grant Newsham, a retired U.S. Marine Corps colonel and research fellow at the Japan Forum for Strategic Studies.
Immediately after the crash, Japan, the only other country to operate them, grounded its 14 Ospreys and asked the U.S. to suspend flights of V-22s operating in the country. The U.S. initially halted flights from the doomed aircraft's unit but said other Ospreys would continue to fly after safety checks.
It is not unusual for the U.S. military to ground entire fleets after fatal accidents. A spokesperson for Japan's defence ministry said on Thursday its aircraft remained grounded.
"It goes without saying that ensuring flight safety is the highest priority in the operation of aircraft," Japan's chief cabinet secretary, Hirokazu Matsuno, said on Thursday. "We will continue to request information sharing with the U.S. side to ensure flight safety."
The deployment of the aircraft in pacifist Japan has faced opposition, especially among residents of the country's southwest Okinawan islands, where there has been a large U.S. military presence since Japan's defeat in World War Two.
An Osprey crash there in 2016 also led the U.S. to ground its fleet of the aircraft in Japan.
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.