After more than a dozen MV-22 Osprey incidents involving what is known as a hard clutch engagement, or HCE -- including one that claimed the lives of five Marines -- military officials now say that they have finally made progress in understanding the deadly issue.
"While the ultimate root cause has not yet been verified, the HCE team has narrowed down the scope of the investigation to a leading theory," Neil Lobeda, a spokesman for the Osprey's Joint Program Office, told Military.com in an email.
That theory, according to Lobeda, is something called "out of phase engagement."
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The Osprey is unique in that it is a tilt-rotor aircraft, meaning it lifts off like a helicopter but flies like a traditional airplane. With that design, it also means that the Osprey is limited in its ability to recover from an issue, should one occur.
Unlike fixed-wing aircraft, it is not able to be equipped with ejection seats or glide to a landing. It also isn't able to make use of "autorotation" -- a safety maneuver helicopters use where the engine is disengaged and the rotors spin as air moves past them while descending.
As a result, it is equipped with a complex system of clutches and linkages that are designed to address a situation where the aircraft loses power in one engine. That system enables the second engine to still turn both propellers and keep the aircraft flying.
In a hard clutch engagement, according to an Air Force official, a slip in one engine's clutch, "causes the engine [system] to transfer all the power into one of the [two] engines to keep the Osprey in the air."
The result is that, sometimes, the system seems to shred itself from the inside, creating a situation that can lead to a hard landing or, in the case of the Marine flight in 2022, a deadly crash .
According to Lobeda, the system uses "sprag clutches" -- a mechanism like a ball bearing, but instead of spheres, there are oblong "sprags" located between the two rotating components. That enables the free rotation in one direction and the application of force when one component wants to rotate in the opposite direction.
The idea is that all of the sprags catch at once, but according to Lobeda, "if some of the sprags engage before or after the rest of the sprags in the clutch, the engagement is then 'non-uniform.'"
"Non-uniform sprag operation could result in slipping and/or rapid disengagement and re-engagement," Lobeda said when asked about how an "out of phase engagement" may lead to a hard clutch engagement.
An official and definitive identification of the root cause of the clutch issues aboard the Osprey is key because the military's current fix -- replacing a clutch component after a certain number of flight hours -- appears to be based on an analysis done in 2023 on what happened in the 16 clutch incidents known at the time, rather than a full understanding of what actually causes HCEs.
Despite the limitation, the office that oversees the Osprey had proclaimed last year that the replacement would prevent 99% of future hard clutch engagements.
During a congressional hearing last week, Vice Adm. Carl Chebi, head of Naval Air Systems Command, said there have now been a total of 19 HCE incidents during the life of the Osprey program, which began in the late 1980s. The fleet became fully operational by the mid- to late 2000s.
The earlier claim -- that virtually all HCE incidents could be avoided by replacing the clutch component -- was met with skepticism from experts and some families of the five Marines killed in the June 2022 Osprey crash in California.
Some of those family members filed a wrongful death lawsuit last month against the companies that manufacture the aircraft, pointing to failures with the Osprey's interconnect drive system, the issues with the hard clutch engagements, and flaws in the aircraft's engine-controlling computer, Military.com reported.
While officials were pretty public with the 99% claim last year, it has not been touted recently, and none of the military representatives mentioned it during the congressional hearing last week into the safety of the aircraft.
Lobeda told Military.com that the office still stands by the claim, but he said there is "ongoing testing and clutch wear analytical investigations ... that may or may not inform an update to the existing analysis."
While Chebi did not repeat the 99% figure during the congressional hearing, he mentioned that "there have been no hard clutch events" since implementing the time restriction on the clutch component replacement.
In the summer of 2022, the Air Force grounded its fleet of Ospreys because of the hard clutch engagements and lifted it two weeks later without a permanent fix. An undisclosed number of Ospreys were grounded again briefly in 2023 due to clutch issues as the clutch component, known as the input quill assembly, on the aircraft was being replaced.
Shortly following an Air Force Osprey crash off the coast of Japan in November 2023 -- which killed eight airmen -- all the military services that fly the Osprey grounded the troubled aircraft.
While investigations into that crash are still ongoing, officials said the cause of the November incident was due, seemingly, to a parts failure but added that it was an issue that had not been seen before.
By March, the military lifted the grounding, and the Marine Corps and Navy quickly resumed flight operations. The Air Force is slowly resuming flight operations, telling Military.com last week that "several" of the aircraft in their fleet are back in the sky.
Military.com reported last month that the services cannot fly the Osprey more than 30 minutes from a safe landing field in the wake of the deadly crash as part of a Joint Program Office restriction.
Chebi said during last week's hearing that he did not expect the Osprey to resume full unrestricted operations until he was confident there had been property safety fixes, saying "this will not occur before mid-2025."
