The US military has grounded part of its fleet of Bell Boeing V-22 Osprey tiltrotors, following continued issues with the problem-plagued craft’s clutch system.

The US Navy, US Air Force (USAF) and US Marine Corps (USMC) jointly said on 4 February they had issued a flight restriction on the type, following a recommendation from the Pentagon’s V-22 programme office.

CV-22B Osprey

Source: US Air Force

The fleet-wide alert, which took effect on 3 February, is based on a “progressive increase in hard clutch engagement events and ongoing engineering analysis”, the services say.

Hard clutch engagement refers to incidents in which the Osprey’s clutch releases from the rotor system and suddenly re-engages, according to the V-22 joint programme office (JPO). The hard engagement sends an impulse through the drive train, which can potentially damage the system.

The JPO notes the unwanted clutch release events are the result of some force from the aircraft’s engine, but did not elaborate. However, the office does reveal the source of the Osprey’s latest problem to be an element of the proprotor gearbox – known as the Input Quill Assembly (IQA) – that houses the clutch.

The recent flight restrictions specifically affect a “subset” of the V-22 fleet, according to the Pentagon. Aircraft with an IQA above an unspecified number of flight hours are grounded until the component has been replaced.

Once a new IQA has been installed, the affected aircraft will return to flight status, the JPO says. The office declines to reveal how many V-22s are on restricted status or provide the flight-hour threshold for IQA replacement, citing security concerns.

The latest grounding is the second time in less than six months that a portion of the Osprey fleet has been put on flight restriction over the clutch issue.

The USAF grounded its fleet of 51 CV-22s for two weeks last August, citing repeated issues with hard clutch engagement.

The USMC at the time declined to follow suit, saying it had previously identified the issue and trained pilots on new flight procedures to mitigate the problem within its fleet of 287 Ospreys.

No mechanical solution to the problem was identified by the USAF when it lifted the 2022 grounding. The service instead released modified guidelines for aircrew meant to avoid situations likely to cause hard clutch engagement and added the scenario to simulator training curricula.

The USAF said at the time those measures were necessary until a permanent solution could be identified.

The current grounding is just the latest incident over some three decades of often-fatal mechanical problems for the V-22. Between 1991 and 2022, V-22s suffered 36 accidents according to the Aviation Safety Network, a service of the US non-profit Flight Safety Foundation.

Two fatal and one non-fatal Osprey crashes occurred in 2022 alone. Four marines died in a Norway incident attributed to pilot error, while five marines were killed last June when their V-22 went down in Southern California.

The USMC has a substantially higher rate of serious aviation accidents compared to the other US military services, according to analysis by the National Commission on Military Aviation Safety.

The JPO, USMC and USAF did not immediately respond to an inquiry from FlightGlobal regarding their previous response to the hard clutch issue and whether it was adequate to ensure the safety of flight crews.