WASHINGTON — A broken gear that led to a fatal accident of a V-22 Osprey last year may have been caused by weaknesses in a metal used to produce the part, according to documents obtained by The Associated Press.
The November crash killed eight Air Force Special Operations Command service members. It was the second time in less than two years that a catastrophic failure of a component in the Osprey’s pro-protor gearbox, which serves as a transmission, caused a fatal accident. In June 2022, five marines were killed when another part of the pro-rotor gearbox system failed.
The crashes have prompted an aggressive effort by the V-22 program office and manufacturer Bell Flight to find solutions for the critical system, some of whose components were wearing out sooner than the military had anticipated. This latest finding may hold some clues.
There is no other aircraft like the Osprey in the fleet. It can race to a target like an airplane and then rev its engines to land like a helicopter. Program leaders have pointed out that the Osprey vital in special operations and combat missions and has flown hundreds of thousands of hours successfully.
But the plane also has a troubled crash history and the pro rotor gearbox is a persistent problem.
Data collected by The Associated Press through the Freedom of Information Act shows that 609 proprotor gearboxes have been removed for repair in the past 10 years. In the past five years, the Marine Corps, Navy and Air Force have reported 60 incidents involving the proprotor gearbox.
Last week the Air Force identified cracks in a pinion, a part about the size of a large jar lid, as one of two factors that caused the crash near Japan. The Air Force also blamed the pilot and crew, saying the Osprey sent six warnings during their flight that the proprotor gearbox was in trouble.
But additional crash report documents obtained by the AP show that this is not the first time this metal has failed in Osprey proprotor gear components, though it was the first time it failed in this particular gear. There have been seven previous cracking incidents in related gears that were likely caused by the same metal weakness, investigators reported.
It is not clear whether that information was shared with the services earlier. That could have led them to be much more restrictive in how pilots responded to pro-rotor gearbox warnings.
In a rare move, the Air Force criticized the V-22 program office in the accident report made public last week for failing to share data that could have better informed crews about the severity of the risk.
The pinions are located in the pro-rotor gearboxes on each wingtip. The gearboxes take power from the Osprey’s engines and process it to turn the Osprey’s masts and rotor blades.
To do this, the gears spin rapidly under extreme pressure. They can overheat and break off metal flakes, called chips, which can travel through the transmission and destroy it. Losing a proprotor gearbox is dangerous and can result in the loss of an aircraft and crew.
In the November crash, investigators believe the first of six chip warnings indicated that a crack had already developed in the pinion gear, and that it was shedding small flakes of metal as it continued to spin. The warnings increased as the gear shed more debris and eventually broke apart, leading to rapid failures of the Osprey’s entire drive system and the fatal crash.
In the additional crash documents, investigators said analysis of the recovered pinion pieces revealed multiple inclusions. An inclusion is a microscopic weak spot in metal caused by foreign matter mixed in during the manufacturing process. Those weak spots can lead to fatigue cracks.
The specific alloy used to make the Osprey’s pinion gears is called X-53 VIMVAR. Crash investigators found multiple inclusions in the failed pinion gear and similar inclusions in a second pinion gear on the plane, the report said. While the inclusions appeared to be within the allowable microscopic size limits, investigators noted that “the initiation of a fatigue crack is dependent on the size of the inclusion and its location in the gear material.”
Investigators concluded that they could not determine whether the inclusions led to the crack formation. But they left open the question of whether there might have been larger inclusions that could have caused the crack formation and were lost when the pinion broke apart. “If the pinion broke through an inclusion, the evidence was obscured by the secondary damage,” the report said.
Of the 60 incidents reported over the past five years, at least 41 involved evidence of chipping, according to AP data.
The transmission is a sealed system, meaning ground crews at the base can’t open it to inspect the gears for inclusions. And even if they could, they don’t have the machining necessary to detect the microscopic defects, Lt. Gen. Michael Conley, head of the Air Force’s Special Operations Command, told the AP in an interview.
“So there was nothing we could have done in the field to detect this,” Conley said.
And Bell Flight can’t test the entire equipment for inclusions without making multiple cuts, which would destroy the part. The primary safeguard is process control during production, the report said.
It is not clear whether other Osprey parts, including the feed pin that caused the 2022 Marine Corps crash, are also made from X-53 alloy.
Bell referred all questions about the proprotor gearbox to Naval Air Systems Command, or NAVAIR, which has overall responsibility for the V-22 Osprey program.
In a statement to the AP, Col. Brian Taylor, head of the Pentagon’s V-22 program office, said he could not comment on specific changes to the proprotor gearbox that are underway, but said that “as improved materials become available, they are being evaluated for use in all of our systems.”
Conley said the Air Force has made flying the Osprey more restrictive now while it does long-term engineering analysis. “Finding out if there’s a better way with the gearboxes, better manufacturing methods, better material. That’s with NAVAIR and Bell right now,” he said.
Until at least mid 2025The Osprey is expected to remain under flight restrictions, which require the aircraft to remain within 30 minutes of its landing site, in addition to other security checks.
Air Force Special Operations Command has just 51 Ospreys, but has had to remove 132 proprotor gearboxes for repair in the past 10 years, according to data obtained by the AP. The Marine Corps bought 360 Ospreys and currently operates about 270 of them. In the past 10 years, it has removed 464 proprotor gearboxes. The Navy, which has 27 in its fleet, has removed proprotor gearboxes 13 times.
Although the Osprey has been designed since the 1980s, the Marine Corps’ MV-22 version has only been in service since 2007, the Air Force’s CV-22 since 2009, and the Navy’s CMV-22 version since 2021.