(Bloomberg) -- A major gearbox failure caused the crash of a US military Osprey aircraft off the coast of Japan last year that killed eight airmen, the US Air Force said, a cause similar to some previous incidents with the tilt-rotor aircraft.
An investigation by the Air Force, released Thursday, said the crash was caused by a “catastrophic” failure of the left-hand gearbox that destabilized the aircraft. The report also said the pilots contributed to the disaster by failing to quickly divert to an emergency landing location.
The aircraft went down off of Japan’s southwest island of Yakushima on Nov. 29, prompting the Pentagon to ground its fleet of Ospreys. Japan, the only other country to fly the aircraft, also paused its use. Both nations have since resumed flights.
After multiple cockpit warnings, the pilots attempted to land at Yakushima Airport, but on the approach, the mechanical failure sent the Osprey into a roll approximately 800 feet (245 meters) above the ground, according to the report. All the crew members sustained fatal injuries upon impact, the investigation assessed.
The incident also stirred long-running concerns among the public in Japan about the use of Osprey aircraft. The CV-22 aircraft involved in the crash last year was on a routine training mission before it went down.
Gearbox problems have triggered problems with the Osprey in the past, including two emergency landings in 2022. Since 1992, 11 crashes of the aircraft have killed more than 60 US service members and other passengers.
The Osprey, a workhorse of the US military to move troops and supplies, has two propeller engines on its wings that can be tilted to make it fly like a helicopter. The design allows it to land in tight spaces.
The aircraft is made by a unit of Boeing Co. and the Bell Helicopter unit of Textron Inc. The Pentagon has given the companies contracts to come up with new gearbox designs.
“Our top priority is safety,” Boeing said in a statement sent by email. “Bell Boeing will continue to work closely with the DOD and all operators of the Osprey aircraft on any additional actions that may emerge from the report’s findings to ensure the continued safe operation of the V-22 fleet,” it said.
Bell Helicopter did not immediately respond to a request for comment.
While Osprey flights have resumed since the crash last year, the Pentagon has limited them to shorter-distance missions.
A lawyer for the families of two of the crew killed in the crash in Japan criticized the highlighting of the role of those on board, noting that the report describes the mechanical failure as irretrievable.
“The loss of these brave individuals is a profound tragedy, and any suggestion that places blame on those who are no longer here to defend their actions only adds to the deep pain their families are already enduring,” Tim Loranger, aviation attorney and senior partner at Los Angeles’ Wisner Baum, said in a statement.
(Updates with statement from Boeing in paragraph 9.)
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