The U.S. military’s V-22 fleet will receive a slate of improvements aimed at safety as the fallout from a deadly U.S. Air Force Osprey crash off the coast of Japan in late 2023 continues to reverberate across the fleet.
The primary focus of the confirmed changes are improvements to the gearbox, which had a catastrophic failure during an Air Force Special Operations Command flight in November 2023. That failure caused the Osprey to crash into the ocean, killing eight Airmen.
The military plans to install new sensors to monitor for failures, improve the quality of steel used in parts of the gearbox, and field a redesigned input quill assembly, an element of the proprotor gearbox that houses the aircraft clutch, according to the Marine Corps 2025 Aviation Plan issued Feb. 4.
“These modifications will be fleet-wide,” a spokesperson for Air Force Special Operations Command added in an email to Air & Space Forces Magazine. “All variants of the V-22 will receive them.”
The Osprey is known as a tiltrotor aircraft, because its unique proprotor gearbox (PRGB) allows it to fly like a helicopter or a plane. But the gearbox has also been the cause of many safety concerns and several mishaps, including the fatal 2023 crash. The complexity of the aircraft’s transmission has been a known issue since it debuted in the 1990s. The engine, weight, and vibration have to rotate, which puts enormous stress on the gears and driveshaft.
“We are pursuing several improvements to the PRGB to enhance aircraft safety and improve component reliability and durability,” the Marine Aviation plan said.
The changes outlined by the Marines are coming just months after the Air Force, along with the Marines and the Navy, issued an operational pause for the Osprey fleet after what AFSOC described as another “materiel failure” that had not been seen in the fleet before during a Nov. 20 flight out of Cannon Air Force Base, N.M. That Osprey made a safe emergency landing.
AFSOC controls the Air Force’s fleet of around 50 Ospreys, though it rotates roughly 15 through “flyable storage” as part of previously announced improvements to the fleet.
In the November 2023 crash, metal chips built up in the gearbox fluid, causing the gearbox and drivetrain to fail when a pinion gear cracked. That failure led to a loss of power and caused aircraft to become uncontrollable. The military grounded the Osprey fleet for months as officials investigated what was described at the time as a “materiel failure.”
On Dec. 20, 2024, Naval Air Systems Command (NAVAIR), which controls Marine and Navy aviation, and works with AFSOC on the Osprey through the V-22 Joint Program office, lifted the operational pause caused by the Cannon emergency for some aircraft. But “based on engineering analysis” NAVAIR issued a fleet-wide fleet bulletin directing the inspection of flight hours on each gearbox, and some aircraft were not cleared to resume flying. NAVAIR said “specifics of the V-22 flight-hour threshold, number of aircraft affected, and additional flight controls will not be released” due to “operational security concerns.”
It is unclear when all the changes will be fully implemented, and the V-22 Joint Program Office could not immediately provide a projected timeline.
Among the changes outlined in the Marine Corps Aviation Plan:
- “Osprey Drive System Safety and Health Instrumentation (ODSSHI pronounced ‘Odessey’) will install sensors in critical areas of the PRGB and drive train to provide vibration signature data that will allow maintenance to forecast the failure of parts and plan to remove those parts prior to failure.”
- “A more refined Triple-Melt steel will be the source material for the internal components of the PRGB, which will drastically reduce the likelihood of material defects in critical gears and bearings.”
- “A redesigned Input Quill Assembly (IQA) will reduce the incidence of the wear-out mode observed in previous IQA failures that led to aircraft Hard Clutch Engagement (HCE) occurrences.”
The quality of the steel used in pinion gear was suspect for years, according to a November 2024 report by Military.com, which reviewed internal Air Force documents. The company that made the part that failed in the fatal crash, Universal Stainless, was sued in 2001 for allegedly producing defective steel for aircraft parts. Now, the military says it is improving the quality of the steel it uses to “drastically reduce the likelihood of material defects in critical gears and bearings” that have caused crashes.
The input quill assembly is an element of the proprotor gearbox, which houses the aircraft clutch, and a program to replace the part has been underway on some aircraft since 2023. Over the life of the program, there have been at least 19 cases of hard clutch engagement, officials say. There was a notable rise around 2022, which prompted AFSOC to stand down its fleet and led the Marine Corps and Navy to implement mitigation measures.
Former AFSOC commander and current Air Force Vice Chief Gen. James C. “Jim” Slife previously said such incidents result in a “kind of a Christmas tree of lights, caution lights, in the cockpit, and some pretty squirrely flight control inputs” which prompted him to briefly ground the fleet in 2022.
The services later put a flight hour limit on the input quill assembly as officials determined the clutch would wear out over time and had a higher susceptibility to slipping after 800 flight hours. In June 2024, the V-22 Joint Program Office said a newly designed clutch would be fielded around the middle of this year.
“Osprey must continue to evolve,” the Marine Aviation plan states.