A problem with the clutch of an MV-22 Osprey aircraft caused the June 2022 crash that killed five Marines, a Marine Corps investigation found.

The “hard clutch engagement” problem with Ospreys prompted the V-22 joint program office in February to ground an undisclosed number of the tiltrotor aircraft across the services ― and the office said in a statement Friday that it has eliminated the risk of a hard clutch engagement by 99%.

In hard clutch engagements, the clutch momentarily slips from its position connecting the engine to one propeller’s rotor gearbox and then reengages, often severely damaging key components of the dual-engine aircraft and causing it to lurch, according to the Marine Corps’ investigation into the incident, released Friday.

The Marine Corps had not previously specified the cause of the June 8, 2022, Osprey mishap that killed Capt. Nicholas P. Losapio, 31; Capt. John J. Sax, 33; Cpl. Nathan E. Carlson, 21; Cpl. Seth D. Rasmuson, 21; and Lance Cpl. Evan A. Strickland, 19.

The command investigation, conducted by the 3rd Marine Aircraft Wing, found that there was nothing these Marines or the Marine maintenance team could have done to prevent the aircraft from crashing in eastern Southern California, near El Centro.

“A catastrophic, unpreventable and unanticipated mechanical failure occurred,” the joint program office said in the statement Friday.

No one directly witnessed the crash, and the post-crash fire was so severe that the “crash-survivable” data recorder wasn’t recoverable, according to the command investigation. But the investigators, themselves experienced Osprey pilots, relied on other sources of information that included an analysis of the flight path, a survey of the crash site and statements from Marines who witnessed the aftermath.

The investigation found that the five crew members were skilled at their jobs, had no major physical ailments and weren’t facing major life stressors.

The aircraft launched at 8:40 a.m., according to the investigation. In the publicly released version of the investigation, the description of the moments right before and during the crash are redacted, but the investigation notes that the crash occurred at 12:14 p.m.

Because of a hard clutch engagement on both sides of the Osprey, the single engine and interconnect drive system — which synchronizes the proprotors and transfers power to a rotor in case the engine on its side fails — itself failed, according to the investigation. The right-hand proprotor lost all thrust, leading to a thrust asymmetry that meant the aircraft couldn’t continue to fly in a controlled manner.

The Osprey crashed, killing the five Marines aboard.

Marines flying another Osprey as part of the same training flight soon saw smoke, according to the investigation. When they flew by the crash site minutes later, they came to the conclusion there were no survivors.

A sergeant in that other Osprey told investigators that his or her crew couldn’t land near the crash site in part because their right engine air particle separator, which is supposed to protect the engine from particulates like sand during landings, had failed.

“There was a high potential we would lose the engine on an attempted landing,” the sergeant said in the statement to investigators, which was included in the enclosures to the investigation.

When asked about the prevalence of engine air particle separator failures, the joint program office didn’t give any specifics, saying only that it continually evaluates the part and was working on improvements to it.

First responders showed up at 1:45 p.m., delayed by the remote location of the wreckage and the lack of roads leading to it, according to the investigation.

The five deaths came months after another high-profile tragedy involving the MV-22 Osprey. In March 2022, an Osprey crashed in Norway, killing four Marines. The Marine Corps would later say the mishap was caused by pilot error.

A slide deck prepared by the Marine Corps’ safety division on July 6, a month after the tragedy in California, expressed confidence in the safety of the aircraft.

The Osprey has average mishap rates lower than the AV-8 Harrier, the F/A-18 Hornet and the F-35B Lighting II, the slide deck stressed.

“No mishap is ever acceptable,” the slide deck reads. “That said, the gravity of these two mishaps is highlighted by the many hundreds of thousands of hours that our Ospreys have operated safely — in every clime and place — throughout the globe.”

Before the June 2022 mishap in California, no one had died from a hard clutch engagement in an American V-22, according to the command investigation.

The first public rumblings of trouble with the Osprey’s clutch came in August 2022, when Air Force Special Operations Command temporarily grounded its 52 Ospreys because of two hard clutch engagements in six weeks.

In February, the joint program office did the same for “a subset” of Ospreys across the services, which had seen an increase in hard clutch engagements, according to the office.

The office required replacements of the input quill assembly — which contains the clutch and provides the mechanism for the engine to drive the gearboxes and rotors — on Ospreys that had flown more than 800 hours, according to Marine spokesman Maj. Jim Stenger.

Those replacements reduced the likelihood of a hard clutch engagement by 99%, according to a statement Friday by the Marine Corps.

Although the root cause of hard clutch engagements is unknown, the problem has occurred in Ospreys in which the input quill assemblies have been flown more than 800 hours, according to the command investigation. In the Osprey that crashed in June 2022, the input quill assemblies on both sides had each accrued more than 2,000 flight hours, the investigation noted.

The Marine Corps has worked with the manufacturer, Bell-Boeing, to design and field a new input quill assembly, improve the drivetrain, and put a flight data recorder that can withstand fire and heat into all MV-22 Ospreys, according to the service’s statement.

The V-22 joint program office has taken other steps to reduce hard clutch engagements, including extensive assessments of risk, and updates to software and simulators, according to Stenger.

All Marine commands that fly the Osprey will present the investigation to pilots and aircrew to raise awareness about hard clutch engagements, the Marine Corps’ statement added.

“We will never forget Capt. Nicholas P. Losapio, Capt. John J. Sax, Cpl. Nathan E. Carlson, Cpl. Seth D. Rasmuson, and Lance Cpl. Evan A. Strickland, and their loved ones, as we continue with our quest to provide the safest, most lethal platforms to the men and women who fly them,” the Marine Corps’ statement reads.

Irene Loewenson is a staff reporter for Marine Corps Times. She joined Military Times as an editorial fellow in August 2022. She is a graduate of Williams College, where she was the editor-in-chief of the student newspaper.

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