The Pentagon is allowing its fleet of V-22 Ospreys to return to the air — with some new guidelines in place — after temporarily pausing flights on the troubled aircrafts last month following an incident in New Mexico.
The unique aircrafts, which can rise vertically like a helicopter and fly horizontally like a plane, have crashed four times since 2022, resulting in the deaths of 20 service members.
The latest incident took place on Nov. 20, when an Air Force Osprey made a precautionary landing in New Mexico after pilots got warnings indicating they needed to land immediately.
No airmen were injured and the aircraft wasn't damaged, according to a spokesperson for the Air Force Special Operations Command. But the Pentagon put a temporary pause on Osprey training flights out of what it described as an abundance of caution.
Investigators determined there was a “materiel failure that had not been seen before leading to the recommendation for an operational pause,” according to the Air Force Special Operations Command spokesperson.
Naval Air Systems Command, which runs the Osprey program for the military, said in a Friday statement that it was lifting the pause with a new directive focused on a crucial component of the aircraft — the prop rotor gearbox.
Each of the aircraft's twin rotors has its own gear box, which is as large as a motorcycle and serves a function similar to a car’s transmission. A catastrophic failure of the prop rotor gearbox was determined to be the primary cause of an Osprey crash in Japan last December that killed seven airmen. A crash in June 2022 in California that killed five Marines was found to be caused by a faulty clutch connected to the prop rotor gear box.
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The new directive specifies that a prop rotor gearbox is only safe to fly if it has exceeded a certain number of flight hours. A gearbox with fewer hours will be required to get an upgrade before flying again. The threshold for flight hours was not disclosed.
"Due to operational security concerns, the specifics of the V-22 flight-hour threshold, number of aircraft affected and additional flight controls will not be released," Naval Air Systems Command, which runs the Osprey program for the military, said in a statement.
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A person familiar with the recommendation told NBC News that gearboxes with fewer hours are newer and may have the potential to fail more often.
“Ongoing engineering analysis suggest that [prop rotor gearboxes] under the predetermined flight-hour threshold are potentially more likely to succumb to a materiel failure," Neil Lobeda, spokesperson for the Naval Air Systems Command, wrote in an email.
The Air Force says they have resumed training flights in accordance with the new bulletin.
Some of the families of loved ones who have died in recent crashes expressed concern at the return to flight.
The family of Staff Sgt. Jacob Galliher, who died in the Japan crash, released a statement through their attorney that questioned the decision by Naval Air Systems Command, also known as NAVAIR.
“How did NAVAIR conclude that flight hours is the appropriate measure to prevent crashes, and why won’t NAVAIR disclose the cut-off number of flight hours utilized to ground some Ospreys, but not others?” the statement said.
Galliher’s family and others have filed lawsuits against the aircraft’s manufacturers.
Attorney Tim Loranger, who represents families who lost service members in the 2022 crash, said he believes there should be a comprehensive safety review.
“NAVAIR’s assurances ring hollow when juxtaposed with the history of mechanical failures and crashes that have repeatedly shown this aircraft to be a liability," Loranger wrote in an email. "These families are asking a simple question: What is being done — specifically and immediately — to ensure no one else suffers the same fate?”
NBC News' Courtney Kube contributed.
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