Ospreys grounded

The United States military has grounded its entire fleet of Bell Boeing V-22 Ospreys following a deadly crash that killed eight crew off the coast of Japan. Although the accident is seen as the catalyst for the grounding, the aircraft has a long list of problems throughout its history. The 29 November crash involved an AFSOC CV-22 assigned to the 353rd Special Operations Wing, based at Yokota Air Base west of Tokyo. The tiltrotor went down in the water off Yakushima Island, some 80km south of the island of Kyushu.

The unique design of the Osprey allows it to take off and land like a helicopter, then tilt its propellors horizontally to fly like an aeroplane. This has enabled the Osprey to speed troops to any battlefield. The U.S. Marine Corps, in particular, operates a great many of the aircraft in service. The unique and complex design has allowed the Osprey to speed troops to the battlefield. The Ospreys grounding will affect missions carried out by the Marines, but spokeswoman Air Force Special Operations Command, Lt. Col. Becky Heyse, has said that it will work to mitigate the impact on operations, training and readiness. The command will continue to fly other aircraft and Osprey crews will continue to train on simulators. The Osprey was in the process of being adopted by the U.S. Navy to replace its C-2 Greyhound aircraft which transports personnel on and off at-sea aircraft carriers.

A preliminary investigation of the crash indicated that a material failure led to the accident. There have been questions regarding the safety of the Ospreys for more than a decade, including problems with the sprag clutch wearing out faster than it should. This has been known about for several years but not rectified to satisfaction, but whether all parts of the aircraft have been manufactured according to safety specifications. Questions also arise as to whether the strength of the aircraft parts over time, will stand up to the significant forces created by the unique structure and dynamics of the Osprey.

Japan is the only international U.S. partner flying the Osprey and the Japanese government grounded their aircraft after the crash.

Osprey engines and flight

The Osprey's two engines are linked by an interconnected drive shaft that runs inside the length of the wings. On each tip, by the engines, a component called a sprag clutch transfers torque, or power, from one proprotor to the other to ensure both rotors are spinning at the same speed. That keeps the Osprey's flight in balance. If one of the two engines fails, the sprag clutch is also a safety feature: It will transfer power from the working side to the failing engine's side to keep both rotors going.

Since 2010, Osprey's clutches have slipped at least 15 times.

“As the system re-engages, hard clutch engagement occurs. In just fractions of a second, an HCE event creates a power spike that surges power to the other engine, which can throw the Osprey into an uncontrolled roll or slide. A power spike can also destroy a sprag clutch, essentially severing the interconnected drive shaft. That could result in the complete loss of aircraft control with little or no time for the pilots to react and save their Osprey or crew,” explained Rex Rivolo, a retired Air Force pilot who analysed the Osprey for the Pentagon's test and evaluation office from 1992 to 2007 as an analyst at the Institute for Defence Analyses

In a 2022 crash of a Marine Corps MV-22 in California that killed five Marines, hard clutch engagement created an "unrecoverable, catastrophic mechanical failure," the investigation found. The fire was so intense it destroyed the Osprey's flight data recorder — another issue the Marines have pushed to be fix, by requiring new flight data recorders to be better able to survive a crash.

Two hard clutch engagement incidents within six weeks in 2022, the commander of Air Force Special Operations Command, Lt. Gen. James Slife, grounded all of its Ospreys for two weeks. A further grounding occurred again in February 2023 as work began on clutch replacements.

In the recent crash, Japanese media outlet NHK reported that an eyewitness saw the Osprey inverted with an engine on fire before it went down into the sea. If these accounts are correct, Rivolo said, clutch failure and a catastrophic failure of the interconnected drive shaft should be investigated as a potential cause.

After it investigated the 2022 crash, the Marine Corps made several recommendations, including designing a new quill assembly, which is a component that mitigates clutch slippage and hard clutch engagement, and requiring that all drivetrain component material be strengthened.

Whistle-blower

In September 2023, Boeing settled with the Justice Department for $8.1 million. Two former Boeing V-22 composites fabricators had come forward with allegations that Boeing was falsifying records certifying that it had performed the testing necessary to ensure it maintained uniform temperatures required to ensure the Osprey's composite parts were strengthened according to DOD specifications.

A certain temperature was needed for uniform molecular bonding of the composite surface. Without that bond, "the components will contain resin voids, linear porosity, and other defects that are not visible; which compromise the strength and other characteristics of the material, and which can cause catastrophic structural failures," the lawsuit alleged.

In its settlement, the Justice Department contended Boeing did not meet the Pentagon's manufacturing standards from 2007 to 2018; the whistle-blowers contended in their lawsuit that this affected more than 80 Ospreys that were delivered in that time frame.

In a statement to The Associated Press, Boeing said it entered into the settlement agreement with the Justice Department and Navy "to resolve certain False Claims Act allegations, without admission of liability”.

Boeing said while composites are used throughout the V-22, the parts that were questioned in the lawsuit were "all non-critical parts that do not implicate flight safety. Boeing complies with its curing processes for composite parts," the company said. "Additionally, we would stress that the cause of the accident in Japan is currently unknown. We are standing by to provide any requested

A separate incident in 2022 saw an AFSOC Osprey make an emergency landing on a remote Norwegian island after experiencing a mechanical issue. That aircraft was found weeks later.

Between 1991 and 2022, V-22s suffered 36 accidents, according to the Aviation Safety Network, a service of the U.S. non-profit Flight Safety Foundation. Many of those incidents occurred during the early years of the development and fielding of the tiltrotor, the first operational aircraft of its kind.