US Air Force and Navy ground V-22 Osprey fleets

The US Air Force said that the Bell Boeing CV-22B involved in the mishap was an aircraft from the 353rd Special Operations Wing. The unit operates its CV-22Bs from Yokota Air Base near Tokyo. (US Air Force)

The US Air Force (USAF) and US Navy (USN) grounded their respective V-22 fleets on 6 December, the services said in separate statements. The grounding follows the 29 November crash of a USAF CV-22 near Yakushima, Japan.

“Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time,” Air Force Special Operations Command (AFSOC) said about the 29 November crash in a 6 December statement.

“Out of an abundance of caution, following the AFSOC operational stand down, NAVAIR [Naval Air Systems Command] is instituting a grounding bulletin for all V-22 Osprey variants Dec. 6,” wrote the USN branch that oversees naval aviation, including the USN's and US Marine Corps' CMV- and MV-22s. “While the mishap remains under investigation, we are implementing additional risk mitigation controls to ensure the safety of our service members.”

The V-22 fleet has suffered several recent accidents leading to groundings. Most recently, in February 2023 the V-22 Joint Program Office grounded all MV-22s with input quills – part of the gearbox that transmits power between the engines and the proprotor – that had flown for more than 800 hours following a series of hard clutch engagements, in which the engine clutch disengages from the proprotor and suddenly re-engages, sending an impulse through the engine assembly and causing the aircraft to lurch. The root cause of hard clutch engagements has yet to be determined.