The cause of the Osprey aircraft crash that killed eight was released in a report by the Air Force Special Operations command. The report says the CV-22B Osprey mishap November 29, 2023, near Yakushima, Japan was caused by a cracked gear and the decision of the pilot to continue the flight. The report says there were multiple warnings that should have lead to the decision to end the mission and land. The crash led to a month-long grounding of the Osprey fleet.
The aircraft, assigned to the 21st Special Operations Squadron, 353rd Special Operations Wing at Yokota Air Base, Japan, was participating in a joint inter-operability exercise when the mishap occurred. The aircrew was comprised of personnel from the 21 SOS, 1st Special Operations Squadron and 43rd Intelligence Squadron.
An Accident Investigation Board was convened by Lt. Gen. Tony Bauernfeind, then AFSOC Commander, to assess the facts and circumstances of the crash to determine its cause. From December 6, 2023, through May 30, 2024, a team of multi-disciplinary subject matter experts examined multiple sources including interviews, maintenance logs, flight recorder data, briefing materials, and inspection of aircraft wreckage. After an extensive engineering and human factors analysis, the team assembled a detailed sequence of events surrounding the CV-22 mishap to determine cause.
“The purpose of the investigation was to identify the cause and contributing factors that led to this mishap,” Bauernfeind said. “By conducting a thorough review and accident and safety investigations, we hope to provide answers to the families of the Airmen that lost their lives and prevent future occurrences and tragedies.”
The board president found, by a preponderance of evidence, the mishap was caused by a catastrophic failure of the left-hand prop rotor gear box that created a rapidly cascading failure of the aircraft’s drive system, resulting in an instantaneous asymmetric lift condition that was unrecoverable by the mishap crew. Additionally, the board president found decision-making was causal, prolonging the mishap sequence and removing any consideration of an earlier landing at a different divert location.
The crew got six chip warnings that day. Each presented an opportunity to heed the warning and land as a precaution. Investigators found that decision was a causal factor in the crash.
When the third chip burn warning posted, the crew was close to mainland Japan. The official guidance after three chip burns was to “land as soon as practical,” but still leaves the decision to the pilot’s discretion.
According to the voice data recorder, the pilot and crew were looking for secondary indications of a problem but saw none. There was no proprotor gearbox overheat. They opted to keep monitoring the situation and continued the flight over water to Okinawa. Throughout the flight, the co-pilot was also not direct about “his uneasiness with the evolving issues,” the investigation found, based on the recovered voice data.
The fourth and fifth chip burn warnings came quickly. Then with the sixth, just chips, meaning the Osprey could not burn them off. The Osprey gave its final chip-related warning three minutes before the crash: chip detector fail. The pilot told the crew he was no longer worried, and now assumed the earlier warnings were due to a faulty chip detector. Investigators later found the fail message occurred because the detector “had so many chips on it, it couldn’t keep up.”
“Inside the proprotor gearbox, the pinion gear was breaking apart. At least one piece wedged into the teeth of the larger transmission gearing system, jamming and breaking off gearing teeth until the left proprotor gearbox could no longer turn the Osprey’s left proprotor mast. Within six seconds of the proprotor gearbox failing, catastrophic destruction splintered through the Osprey gearing and interconnected drive system. At that point, there was nothing the crew members could have done to save themselves or the aircraft, the investigation found. The Osprey rolled violently, inverted twice with its left engine housing on fire and crashed into the water, killing all on board,” a report from Iowa Public Radio says.
The full USAF Accident Investigation Board report is available here.