What is the Osprey, the aircraft at the center of multiple tragedies?

More questions than answers remained Thursday about a U.S. Air Force CV-22 Osprey crash off the southern coast of Japan Wednesday that has left at least one airman dead.

The status of the other seven airmen who were aboard the aircraft, as well as what caused the incident, remained unclear. The crew was assigned to the Air Force’s 353rd Special Operations Wing at Yokota Air Base outside Tokyo.

The mishap is the latest fatal incident involving a U.S.-owned Osprey, in which dozens of service members have been injured or killed in accidents around the world over the past three decades.

Four fatal Osprey crashes, including Wednesday’s accident, have claimed the lives of at least 13 American troops in the past two years.

This is the first fatal incident involving an Air Force-owned CV-22 since 2010, and potentially the service’s deadliest accident since 2018, when nine Puerto Rico Air National Guard troops died in a WC-130 weather reconnaissance plane crash.

What is the CV-22 Osprey?

The CV-22 Osprey that went down Wednesday is a variant of the V-22 model produced through a partnership between Boeing and Textron’s Bell.

The tiltrotor aircraft — known for its towering nacelles that allow it to launch and land like a helicopter, and speed forward like a fixed-wing plane — began entering the U.S. inventory in 1999 with the Marine Corps MV-22.

That program started as a behind-schedule, over-budget invention that claimed dozens of lives during development. In the decades since, Ospreys have proven valuable for their speed and versatility, flying faster than conventional helicopters to approach enemies from unexpected angles or rush wounded troops to emergency care.

The Air Force received its first combat-ready CV-22 in 2007, and the Navy’s CMV-22 variant took flight in 2020 and deployed with carrier air wings in 2021. None of the sea service’s Ospreys have been involved in fatal mishaps. Japan’s military operates its own Ospreys as well.

The U.S. military now owns hundreds of Ospreys, largely operated by the Marines. Air Force special operations units use the CV-22 to slip in and out of areas without established runways, where fixed-wing planes may not be able to land with troops and supplies.

Ospreys can carry nearly three dozen service members or 10,000 pounds of cargo at a time, according to an Air Force fact sheet. Each is armed with a .50-caliber machine gun, and cost $90 million apiece as of 2020.

The Yokota-based unit involved in the most recent mishap activated squadrons in 2019 to operate and maintain CV-22 Ospreys as part of the Pentagon’s growing footprint in the Pacific.

But the technological advancements that the V-22 fleets brought to combat and training are still shadowed by concerns about the platform’s longevity and safety.

“CV-22 readiness keeps me up at night,” then-Col. Dale White, who oversaw acquisition and sustainment for the Air Force’s special operations platforms, said in 2019. “It’s not what it needs to be. It’s a tough platform to maintain.”

What problems has the Osprey faced lately?

A combination of pilot error and mechanical failures have contributed to periodic Osprey crashes that, when fatal, have killed multiple troops at once. The military has recently turned its focus to tackling a problem known as a “hard clutch engagement.”

Last year, Air Force Special Operations Command temporarily grounded its Ospreys following back-to-back safety incidents in which the Osprey’s clutch temporarily slipped and then re-engaged, causing an uneven distribution of power to its massive rotors. Such slips can cause the aircraft to lurch dangerously.

A hard clutch engagement on both sides of a Marine Corps MV-22 Osprey in June 2022 led to a crash that killed five Marines in California.

Then in February, the V-22 Joint Program Office — which manages the tri-service fleet — grounded an undisclosed number of Ospreys with more than 800 flight hours to replace aging clutch-related components.

The office claimed in July it had reduced the likelihood of hard clutch engagements, or HCEs, by 99%.

“The V-22 community executed 22,258 flight hours between Feb. 3, 2023 and July 19, 2023, with zero HCE events,” the Joint Program Office said at the time.

The following month, three Marines were killed when their Osprey crashed in northern Australia. The Marine Corps is still investigating what caused the accident.

“I wouldn’t right now apply a sweeping broad stroke across every incident linking them together,” Pentagon spokeswoman Sabrina Singh said when asked in August whether a clutch malfunction was to blame for those fatalities.

It’s unclear whether Wednesday’s crash involved a hard clutch engagement.

According to the Associated Press, Japanese news outlet NHK quoted a local resident as saying he saw the aircraft turned upside down, with fire coming from one of its engines, and then an explosion before it fell to the sea.

A non-exhaustive history of Osprey accidents

• October 2023: A Marine was injured after a Marine Corps MV-22B Osprey experienced a hard landing during a training event in Nevada. “Initial assessments indicate the incident was likely not mechanical in nature,” a Marine Corps spokesman said in October.

• September 2023: Three Marine Corps Ospreys diverted from scheduled flight paths in a single week due to “caution indications” in the cockpit. The service said the warnings did not appear to be prompted by clutch issues.

• August 2023: Three Marines were killed in an MV-22 Osprey crash in Australia during a multinational training exercise.

• October 2022: The engine of an Marine Corps Osprey caught fire during training at Air Station Miramar, California. No one was injured.

• Summer 2022: Air Force Special Operations Command’s CV-22 Osprey aircraft face two hard clutch engagement problems.

• June 2022: A problem with the clutch of an MV-22 Osprey aircraft caused the June 2022 crash that killed five Marines, according to a Marine Corps investigation released this July.

• March 2022: Marine Corps aviation investigators have determined that a fatal March 18 MV-22B Osprey crash near Bodo, Norway, that killed four Marines was pilot error.

• September 2017: Two service members were injured when a Marine MV-22B made a hard landing in Syria.

• August 2017: Three Marines were killed when their MV-22B Osprey crashed off Queensland, Australia.

• May 2015: Marine Corps officials determined that decisions made by pilots in low-visibility conditions contributed to an MV-22B Osprey crash in Hawaii that left two dead and 20 more injured.

• October 2014: One Marine died when he jumped from an MV-22B Osprey that nearly crashed shortly after taking off from the flight deck of an amphibious assault ship. The Defense Department reclassified his death as one that occurred in support of the military’s effort to combat the Islamic State group, making him the first official casualty of Operation Inherent Resolve.

• June 2012: Five airmen were injured when a CV-22 was destabilized by the wake of another Osprey near Hurlburt Field, Florida.

• April 2012: Two Marines died and two were severely injured when an MV-22 Osprey crashed in a Moroccan military training area while participating in a bilateral exercise.

• April 2010: A CV-22 Osprey accident near Qalat, Afghanistan, killed four people and injured 16 of the 20 onboard.

• April 2000: An MV-22 Osprey crash at Marana Regional Airport in Arizona killed 19 Marines.

Marine Corps Times reporter Irene Loewenson contributed to this story.