The MV-22B Osprey crash in California last June, which killed five Marines, was caused by a gear problem the military had long known, according to a 436-page redacted command dossier released Friday.

In a statement, the Marine Corps said a “dual hard clutch engagement” caused the engine and interconnected drive system to fail, leading to a “catastrophic loss of thrust” in the right proprotor. “The drivetrain degradation caused by the dual HCE events and subsequent single engine/ICDS failure resulted in an irreversible deviation from controlled flight, resulting in the tragic crash that occurred on June 8, 2022.”

In a statement, the V-22 Integrated Programs Office said the pilot, crew and aircraft mechanic were not responsible for the crash.

“At the time of the catastrophic, unpreventable and unexpected mechanical failure, it was determined that the pilot and crew were conducting routine flight duties in accordance with applicable regulations,” the agency said.

The Marine Corps has been aware of the V-22 aircraft’s hard clutch engagement issue since 2010 and has changed protocols to mitigate it if it occurs. In 2022, the Air Force Special Operations Command grounded its fleet of tiltrotor aircraft for several weeks, citing “increased safety incidents” involving hard clutch engagements. The Marines did not fly the Osprey.

Officials said they are working with manufacturer Bell-Boeing on a redesign of the Proproter gearbox input quill assembly, including the clutch, to solve the problem.

The V-22 Consolidation Program Office confirmed that this was the 16th time a hard clutch had occurred since 2010. defense one.

According to the compiled investigative report, the MV-22B was participating in a “tail firing exercise” alongside another Osprey at a training range in California when it radioed that it had a “hotbox,” meaning the oil temperature in at least one gearbox was too hot. Pilots said they needed to go into airplane mode and fly the plane higher to cool it down. According to eyewitness testimony from another Osprey officer, this was the second time the Osprey had to deal with a “hot box” indicator during flight.

Engagement of the dual hard clutch occurred during that climb. The Marines on the other plane did not see the Osprey crash, but noticed smoke and fire coming from the crash site minutes later. The fire was so violent that the aircraft’s flight data recorder was destroyed.

According to reports, the June 8 crash was the first known accident in which both the engine and the interconnect drive system that synchronizes the aircraft’s pro-promoter failed in a V-22. Additionally, according to the report, no Marine died as a result of a hard clutch engagement or related emergency, and the crash is only the third time in the program’s history that dual hard clutch engagement has occurred, the V-22 Integrated Programs Office said.

Since most hard clutch engagement incidents occur within seconds of takeoff, Marine Corps pilots are trained to perform a “hover check” to make sure everything is fine before continuing the flight.

In February, the JPO set flight time limits for aircraft input quill assemblies. Defense officials at the time declined to provide a flight time limit, but the report notes that aircraft with more than 800 flight hours were grounded.

The Osprey, which suffered a fatal crash in June 2022, had the input quill assembly flying for more than twice that amount of time. Before the crash, he had 2,437 flight hours in the left pro-promoter gearbox input quill assembly of the aircraft and 2,007 flight hours in the right pro-promoter gearbox input quill assembly.

The program secretariat said recent efforts to prevent accidents, such as replacing the input quill assembly, have reduced hard clutch engagement by “more than 99 percent.”

The investigators recommended a new input quill assembly and improvements to the aircraft’s “drivetrain and flight control system software, material strength of drivetrain components, and inspection requirements.” They also recommended adding a “crash-proof, high-temperature, fire-resistant flight data recorder” to all MV-22Bs.

The Marine Corps said in a press release on Friday that it is already working with manufacturers to take action on all three cases.