Failed gearbox, crew decisions caused fatal Osprey crash, Air Force says

A deadly Osprey crash that killed eight airmen in November off the coast of Japan was caused by the “catastrophic failure” of a prop-rotor gearbox and the pilot’s decision to keep flying despite warnings, the Air Force says. 

The incident, one of four fatal Osprey crashes in the past two years, occurred after a metal gear broke apart inside the Air Force Special Operations Command CV-22’s left prop-rotor gearbox, according to an Air Force investigation released today. 

This is the first time this specific part, called a “pinion gear,” has failed, the head of the Accident Investigation Board and commander of AFSOC, Lt. Gen. Michael Conley, told reporters Wednesday ahead of the investigation’s release. The service still doesn’t know why the part failed, he said. 

The Osprey was flying in a “fairly routine” exercise when the crew got notified of a “chip burn” about 40 minutes after takeoff from Marine Corps Air Station Iwakuni, according to the report. A chip burn advisory pops up on the flight engineer’s display when the system has detected metal chips in the gearbox and burned them off. 

The crew received two more chip burn notifications, and despite protocol stating the crew should “land as soon as practical” after three chip burns, the crew decided to press on with the training mission, Conley said. After five total chip burns, the crew received a “chips indication,” which means the chip detectors have found pieces of metal that can’t be burned off. Flying guidance says that after a chips indication, crew members need to “land as soon as possible.”

Following that notification, the aircraft commander directed the crew to turn to Yakushima Island, about 60 miles away, despite other closer options for landing. The crew then contacted the ground operator on the airfield and said they were an emergency aircraft, but didn’t ask to be prioritized for landing, Conley said. They went into a holding pattern while waiting for the runaway to open, during which they received a final warning: one of the chip detectors had stopped working. 

A few minutes later, the pinion gear, which had been slowly cracking and chipping, completely failed and a piece came off and lodged itself into the spinning gears, stripping the teeth off one of the gears and causing the left proprotor to stop spinning. 

This sent the Osprey into a roll. The aircraft crashed into the water with such a violent impact that all crew members were killed, he said. 

Once the gearbox failed, no human could have done anything to save the aircraft, Conley said.

But leading up to that, there were multiple places “where the crew had the opportunity and the guidance to divert and they chose not to,” he said. For example, if the crew had landed after the third chip burn, the crash would have been prevented, he said. 

“I can't prove that with 100% certainty, but the investigation indicates that time mattered in this, that the failure progressed over time. If an earlier decision to land had been made, I do believe that there was an alternative ending,” he said. 

Asked why an experienced pilot would’ve made these decisions, Conley said they likely faced “internal pressure” to complete the mission they had spent months planning. 

“We ask crew members to make a million decisions, and sometimes seemingly mundane decisions or easy ones end up being consequential, and in this case, a series of decisions resulted in them extending the flight longer than they should have,” he said.  

The investigation also found the V-22 program office was at fault, because the program didn’t share information widely enough on the risks associated with parts in the gearbox. 

The “chip burn” protocol has changed since the crash, Conley said. Now, the crew is directed to land as soon as practical after a single chip burn, and as soon as possible after a second.

Before the investigation was released publicly, Conley said he and his team briefed all of the family members on the findings.

“This has been a hard eight months. We lost eight air commandos [who] were valued members of this command and the small, elite group of crew members that fly CV-22s. They were out there doing the mission we asked them to do that day,” he said. 

Tim Loranger, a lawyer representing two families of airmen who died in the crash, said in a statement that while it’s natural to scrutinize the actions of the pilots and crew, the crash was primarily caused by the failure of the left proprotor gearbox.

“The report concludes that this was a catastrophic mechanical failure that no amount of skill or experience could have fully overcome. While it does mention decisions made by the crew during the incident, it is crucial to understand that these decisions were made under extreme pressure and what became a rapidly deteriorating situation,” said Loranger, who is also representing family members of Marines killed in a 2022 Osprey crash. Those families are suing Boeing, Bell Textron, and Rolls Royce—accusing the companies of knowing the aircraft was unsafe and not disclosing it to the Pentagon. 

The Osprey has had more than a dozen deadly crashes; Vice Adm. Carl Chebi, head of Naval Air Systems Command, said in June that 64 service members had died in Osprey mishaps since the aircraft entered service. 

Following the crash in November, the Pentagon grounded its entire Osprey fleet for months, then cleared all V-22s to return to “restricted” flight operations in March. U.S. officials have said Ospreys won’t return to full flight operations until mid-2025, but Conley said AFSOC is “getting close” to providing combat capability again. 

“I don't want to discuss the specifics of operational deployments, but we're getting back in the ballpark where I think we will be supporting combatant commanders this year, this calendar year,” he said.