WASHINGTON (AP) — The Osprey, a workhorse aircraft vital to U.S. military missions, has been approved to return to flight after an "unprecedented" part failure led to the deaths of eight service members in a crash in Japan in November, Naval Air Systems Command announced Friday.

The crash was the second fatal accident in months and the fourth in two years. It quickly led to a rare fleet-wide grounding of hundreds of Ospreys across the Marine Corps, Air Force and Navy.

Before clearing the Osprey, which can fly like an airplane and then convert to a helicopter, officials said they put increased attention on its proprotor gearbox, instituted new limitations on how it can be flown and added maintenance inspections and requirements that gave them confidence it could safely return to flight.

The entire fleet was grounded Dec. 6, just a week after eight Air Force Special Operations Command service members were killed when their CV-22B Osprey crashed off Yakushima island.

Before lifting the flight restrictions the military also briefed officials in Japan, where public opinion on the Osprey is mixed, on the crash findings and new safety measures. In a statement Friday, Japan defense minister Minoru Kihara said his nation would likewise return its 14 Ospreys to flight status following an "adequate" analysis of the cause of the crash, extremely detailed information on the accident and the steps to mitigate the issue in the future.

Kihara said Japan and the United States will closely coordinate the timeline for resuming flights in Japan, to give the government time to "thoroughly" explain the issue to its citizens.

However, Okinawa Gov. Denny Tamaki did not support the return to flight. Okinawa is home to Marine Corps Air Station Futenma and its 24 MV-22B Ospreys and is where the public has been most vocal in its opposition to the aircraft.

"It would be best if they stay on the ground, as we have all along requested scrapping of the Osprey deployment," Tamaki said.

Officials who briefed reporters Wednesday ahead of the flight restrictions lifting said that they quickly grounded the entire fleet in December because it became clear that the way the Osprey part failed in that crash was something they had not seen before on the tiltrotor aircraft.

While the officials did not identify the specific component, because the Air Force's crash investigation is still not completed, they said they now have a better — but not complete — understanding of why it failed.

"This is the first time that we've seen this particular component fail in this way. And so this is unprecedented," said Marine Corps Col. Brian Taylor, V-22 joint program manager at Naval Air Systems Command, or NAVAIR, which is responsible for the V-22 program servicewide.

However, the decision by the Department of Defense to return to flight before separate congressional investigations on the Osprey program are complete drew criticism from the chair of the House Oversight Committee.

"DoD is lifting the Osprey grounding order despite not providing the Oversight Committee and the American people answers about the safety of this aircraft," said Rep. James Comer, a Kentucky Republican. "Serious concerns remain, such as accountability measures put in place to prevent crashes, a general lack of transparency, how maintenance and operational upkeep is prioritized, and how DoD assesses risks."

A former Osprey pilot familiar with the investigation confirmed that the component in question is part of the proprotor gearbox, a critical system that includes gearing and clutches that connect the Osprey's engine to the rotor to turn it.

The services have done a "deep dive" into the proprotor gearbox, and the new safety measures "will address the issues we saw from that catastrophic event," the head of Air Force Special Operations Command, Lt. Gen. Tony Bauernfeind, said Wednesday.

"I have confidence that we know enough now to return to fly," he said.

The proprotor gearbox system as a whole is a recurring trouble spot for the Osprey. Service safety data obtained by The Associated Press show dozens of instances among the Marine Corps and Air Force Ospreys in which power surges, sudden loss of oil pressure due to leaks, engine fires or chipping — when the metal components inside the gearbox shed sometimes dangerous metal chips — have damaged the proprotor gearbox in flight, sometimes requiring emergency landings.

Other components of the proprotor gearbox, including the sprag clutch and input quill assembly, have been factors in previous crashes, and the services have made changes, such as replacing those parts on a more frequent basis.

The services are also looking closely at the material that the failed part is made of and how it is manufactured, Bauernfeind said. NAVAIR is also running further tests to give the services more insight into why the component failed.

"It was a single component that failed in such a way that led to catastrophic consequences," Bauernfeind said.

After that testing is complete, he said, some of the operational safety controls now placed on the Osprey may be lessened "to give us greater flexibility with the platform."

The investigation, known as an accident investigation board, will be made public and is expected to be completed within the next two months.

The proprotor gearbox failure was first reported by NBC News.

Crews have not flown now for more than 90 days — a factor that will make their return to flight more dangerous. The services said Wednesday they are taking a cautious approach that could last from 30 days to several months to retrain their crews before their Osprey squadrons are back to normal flight operations.

The Osprey has been in development for four decades but only became operational in 2007. The U.S. military has flown the Osprey about 750,000 hours and relied on its ability to fly long distances quickly like a plane and then convert to a helicopter to conduct operations in the Middle East and Africa, where some Marine Corps squadrons received an exemption to the flight ban because it was so critical to the mission.

In future needs to counter China, the military has planned on using the Osprey in the Indo-Pacific to operate throughout islands that lack the airfields necessary for traditional aircraft.

But it has also been a controversial, first-generation design of military tiltrotor technology that has recorded more than 14 major accidents that have killed 59 people and in some instances led to the loss of the aircraft, which costs between $70 million and $90 million depending on the variant.

None of the services is planning on new production orders of the V-22, which is produced by a joint venture between Bell Flight and Boeing. The Army has contracted with Bell Flight to buy the Osprey's successor, the Bell V-280 Valor, which is a tiltrotor like the Osprey but smaller and with an important design change — the engines stay in a fixed, horizontal position. On the Osprey, the rotors and entire nacelle that houses the engine and proprotor gearbox tilt to a vertical position when it flies in helicopter mode.

The Marine Corps operates the vast majority of the Ospreys, with more than 240 currently assigned to its 17 squadrons. Its aviation mission is dependent on the aircraft returning to flight, and the Marine Corps is committed to the Osprey remaining in its fleet through the 2050s, said Marine Corps assistant deputy commandant for aviation Brig. Gen. Richard Joyce.

"There is no taking our eye off of V-22 and the years of service life that it has in front of us," Joyce said.

The Air Force, which has the second most Ospreys in the fleet, with about 50 assigned to its special operations mission, however, suggested on Wednesday it may start to consider other options.

The early concepts for the Osprey date back to the 1980s, when the Iran hostage crisis exposed a need to have an airframe that could move fast and hover or land like a helicopter, Bauernfeind said.

And it's met that need quite well, but it is still an older platform, he said. "I do think that it's time for us to start talking about what is that next generation of capability that can replace what the V-22 does."

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Yamaguchi reported from Japan.