The U.S. Marine Corps has since 2010 known of a critical safety issue on its V-22 Osprey fleet—a gearbox flaw that can cause the tiltrotor’s clutch to slip, severely impacting the safety of flight.

For years, the Corps says it has trained its crews to be aware of and work around the issue. While the Air Force grounded its V-22 fleet last year because of the issue, the USMC kept flying, saying it is confident in the safety of its aircraft.

On June 8, 2022, a hard clutch engagement (HCE) hit a V-22 as it flew over the deserts of Southern California, causing the Osprey to violently crash and killing all five on board. Despite USMC claims that its crews could work around the issue, an investigation released July 21 states there was nothing the expert pilots could do. They could not have known that the problem was going to happen, and they did not have time to react.

Aviation Week reporting shows the newly released investigation was one of at least four such gearbox problems that occurred in serious crashes of both Marine Corps and U.S. Air Force V-22s last year. While the new accident investigation board report into the June 2022 incident states the HCE was the primarily cause of that crash, an investigation into another 2022 fatal MV-22 crash in Norway states a gearbox problem did occur but blamed that mishap on pilot error.

For the U.S. Air Force, at least two proprotor gearbox problems forced down V-22s, according to information on the Class A mishaps by Aviation Week---incidents defined as causing at least $2.5 million in damage or severe injuries or deaths. One high-profile incident in August 2022 in Norway prompted the Air Force to stand down its operations. This came after another incident on May 17, 2022. Investigations into these incidents have not been released, and there were multiple other similar incidents, including another USAF CV-22 forced to land on July 8, 2022, and an MV-22B experienced an engine fire while landing in October 2022.

While the Air Force stood down operations and was the first to commit to retrofitting the input quill assemblies of its fleet, the Marine Corps and Navy kept flying after the June 2022 crash in California.

The mishap crew was from the “Purple Foxes” of Marine Medium Tiltrotor Squadron 364 (VMM-364)---Capt. Nicholas P. Losapio, Capt. John J. Sax, Cpl. Nathan E. Carlson, Cpl. Seth D. Rasmuson and Lance Cpl. Evan A. Strickland. 

According to the accident investigation board (AIB) report, the Osprey was one of two flying near Glamis, California, for a live-fire tail gun training mission from Camp Pendleton. After the third pass, the V-22 crew reported via radio it had “hot boxes,” meaning the aircraft’s gearboxes were running at a high temperature. The crew then climbed to a higher altitude to cool the gearboxes. After coming down for another weapons pass, the Osprey’s wingman lost visual of the V-22 during a turn because of the angle of bank. Seven seconds after the last radar contact, the V-22 crashed. 

The AIB states the cause of the crash was a dual HCE, creating a single engine and interconnect drive system failure. This caused a “catastrophic loss of thrust” on the right proprotor, creating an unrecoverable departure from controlled flight. 

“It is clear from the investigation that there was no error on the part of the pilots and aircrew and nothing they could have done to anticipate or prevent this mishap,” Headquarters Marine Corps says in a statement. “They were conducting routine flight operations in accordance with applicable regulations when this catastrophic and unanticipated mechanical failure occurred.”

In the statement the Marine Corps states it has since 2010 made “numerous actions associated with defining, mitigating or eliminating HCEs.”

In February, the V-22 Joint Program Office (PMA-275) announced the whole fleet would undergo input quill assembly (IQA) replacement even though the full root cause of the HCE has not been determined. The AIB report states IQAs are being replaced every 800 hours. Since then, there have been 22,258 flight hours with no reported HCE events. 

“The completion of this investigation does not close the HCE effort within PMA-275,” says Col. Brian Taylor, PMA-275 program manager, in a statement. “The implemented IQA life limit, which reduced overall V-22 HCE risk by greater than 99 percent, was not a result of this investigation but is certainly reinforced by its findings.” 

The HCE issue is one of 13 Category 1 deficiencies on the V-22 fleet, issues that are defined as possibly affecting safety of flight. The program office has declined to identify the other deficiencies. 

After the grounding was first announced, Air Force Special Operations Command was the first command to look back at past V-22 mishaps to see if what is now understood about the HCE phenomena would have changed the findings. The Navy and Marine Corps followed suit. AFSOC said at the close of that investigation that what is known would not “materially” change---while HCEs could not completely be ruled out of some, it did not merit re-opening the investigations. 

One of the most notable incidents was the April 2010 crash of an Air Force V-22 in Afghanistan that killed four and injured 16. The official account of the crash did not identify a main cause since key pieces of evidence, such as the flight data recorder and an entire engine, were missing. However, then-AIB president Brig. Gen. Donald Harvel wrote there was an “abnormal engine response” and the proprotor’s speed was low when it attempted a rolling landing. AFSOC at the time disagreed with the findings, and issued a statement blaming the pilots.

Harvel, who retired shortly after and died in 2020, wrote his account of the investigation in a book titled Rotors in the Sand. In a chapter called “My Unofficial Opinion of What Really Happened,” Harvel wrote that gearbox problems were likely a cause.