The US military has grounded its entire fleet of Bell Boeing V-22 Ospreys following a deadly crash that killed eight crew off the coast of Japan.

Three variants of the tiltrotor aircraft are operated by the US Air Force, US Navy (USN) and US Marine Corps (USMC).

US Air Force Special Operations Command (AFSOC), which flies the CV-22 special-operations variant, issued the grounding order on 6 December to “mitigate risk” while the air force investigates the fatal crash.

“Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time,” AFSOC says.

The grounding order will remain in place indefinitely, AFSOC told FlightGlobal on 7 December. 

“The CV-22s will not be returned to flight until they are deemed safe to fly for all our air crews,” the command says.

The 29 November crash involved an AFSOC CV-22 assigned to the 353rd Special Operations Wing, based at Yokota Air Base west of Tokyo. The tiltrotor went down in the water off Yakushima Island, some 50 miles (80km) south of the island of Kyushu.

CV-22 take-off Yokota air base Japan

Source: US Air Force

A Bell Boeing CV-22 Osprey from the US Air Force’s 353rd Special Operations Wing crashed on off the coast of Japan on 29 November, killing all eight crew aboard

AFSOC says the grounding will allow for a “thorough investigation” to determine the causes of the crash and to provide recommendations to safely return the tiltrotors to service.

Shortly after the air force’s decision, the USN announced a grounding bulletin for both the CMV-22 carrier-resupply Ospreys and the MV-22 medium-lift variant used by the USMC.

“While the mishap remains under investigation, we are implementing additional risk mitigation controls to ensure the safety of our service members,” Naval Air Systems Command says.

That grounding order also took effect on 6 December.

While the navy says it issued the flight restriction out of an “abundance of caution”, the service has experienced its own deadly Osprey mishaps. In August 2023, a USMC MV-22 crashed near Darwin, Australia, killing three American personnel and prompting the service to order an aviation safety review.

CV-22 Osprey recovery Norway 2022

Source: US Air Force

Another US Air Force CV-22 had to be recovered by barge in 2022, after making an emergency landing in a remote area of Norway

Multiple USMC Ospreys went down in 2022, including one in Norway that killed four marines and one in Southern California that killed five.

A separate incident in 2022 saw an AFSOC Osprey make an emergency landing on a remote Norwegian island after experiencing a mechanical issue. That aircraft was recovered via barge weeks later.

The Pentagon previously grounded Ospreys across all three US operators in February 2023, citing ongoing issues with the complex aircraft’s clutch system. The V-22 procurement office said at the time that a technical solution for the problem had been identified and retrofits were being rolled out.

Between 1991 and 2022, V-22s suffered 36 accidents, according to the Aviation Safety Network, a service of US nonprofit Flight Safety Foundation. Many of those incidents occurred during the early years of development and fielding of the tiltrotor, the first operational aircraft of its kind.

Despite scrutiny, Bell, Boeing and the Pentagon have repeatedly insisted the Osprey is safe, citing a large fleet and high flight-hour count.