FILE – In this photo provided by the Japan Coast Guard, wreckage believed to be from a U.S. military Osprey aircraft is seen off the coast of Yakushima, Kagoshima Prefecture, Japan, Nov. 29, 2023. company, document)
Japan Maritime Security Agency/Associated Press
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Japan Maritime Security Agency/Associated Press
WASHINGTON — An Osprey crashed in the sea off Japan in November because a metal gear developed a crack and the pilot decided to continue flying rather than heed multiple warnings to land, an Air Force investigation released Thursday showed.
The CV-22B Osprey crash killed eight Air Force Special Operations Command service members and led to a military-wide grounding of the fleet for several months. Four fatal Osprey crashes in the past two years have fueled an investigation into the Osprey’s safety record and created divisions among the services over the future role of the unique aircraft, which can fly like an airplane but fly like an airplane. Can land like a helicopter.
For months, the Air Force said only that the crash was caused by an unprecedented parts failure. On Thursday, the company said the culprit was a piece of gear called a pinion – a key part of the Proprotor transmission. The propeller gearbox serves as the transmission of the aircraft. Within each gearbox, five pinions spin violently to transfer the engine’s power to turn the Osprey’s mast and rotor blades.
While the Air Force is convinced it was a pinion gear failure, it still doesn’t know the cause.
But lead investigator Lt. Gen. Michael Conley told reporters Wednesday before the report was officially released that the Pentagon program office responsible for the V-22 Osprey knew that if those propeller gearbox parts failed, “there could be a total loss of the aircraft and crew.” The investigation also took a rare step in rebuking the office, saying it failed to share safety data that would have informed the crews of the severity of the risks.
A pilot’s instinct to get the job done
Conley told The Associated Press in an interview that he believed the pilot’s instinct to complete military maneuvers drove his decision.
“In a way, it’s a way of life here. I mean, we want people in this command who are biased towards ‘yes’ and biased towards getting the mission done,” Conley said. “As we investigated, I saw people who were confident about the aircraft, but not arrogant.”
On the day of the crash, the Osprey was en route to Okinawa along the coast of mainland Japan when the first signs of trouble appeared.
In aircraft, vibrations are monitored as a sign of potential failure. The data logger noticed vibrations on the left side of the driveshaft that connects the two engines and acts as a failsafe if one of the engines loses power.
Then a second vibration occurred. This time, one of the five pinion gears in the left propeller gearbox was vibrating.
But pilot Maj. Jeff Horneman and his crew never knew about the vibrations because the data could only be downloaded at the end of the flight.
Five minutes after the first vibration, a warning about a burnt chip in the left propeller gearbox was posted in the cockpit. The warning let the crew know that there was metal peeling off the Osprey’s transmission, another sign of stress.
Shattering is common in spinning flight, so a safety net was designed into the Osprey. A chip detector burns away chips so they don’t move around in the oil and damage the transmission.
If the burn is successful, the warning will clear.
Six missed opportunities
The crew received six chip warnings that day. Holnemann was each offered the opportunity to heed the warning and land as a precaution, but he failed to do so, a decision that investigators found was a factor in the crash.
When the third chip burn warning was issued, the crew was approaching mainland Japan, just 10 miles (16 kilometers) from the nearest airport. The official guidance after three chip burnouts is “land as quickly as possible,” which remains at the pilot’s discretion.
According to the voice data recorder, Horneman and the crew were looking for minor signs of problems, such as an overheating propeller gearbox, but found none. Therefore, Holnemann instead instructed his co-pilot to continue monitoring the situation and elected to continue the 300-nautical mile overwater flight to Okinawa.
The investigation found that Horneman likely balanced different priorities in his decision-making. He led the airborne portion of the military exercise and spent months planning it.
The investigation found that until the final minutes of the flight, his primary focus was on completing the drill rather than on the changing aircraft conditions. He rejected the first officer’s suggestion to use alternative airborne mapping tools to determine the nearest airport. According to the recovered voice data, the investigation found that the first officer also did not directly express “his uneasiness about the evolving issues” throughout the flight.
The fourth and fifth chip burn warnings came quickly. Then comes the sixth point, upgrade: only chips. This means the Osprey cannot burn them. “Land ASAP” became “Land ASAP”. Despite this, the crew took no emergency action.
The last minutes of a doomed flight
In the final minutes of the flight, they began adjusting the plane’s landing position. The Osprey is located half a mile (0.8 kilometers) from Yakushima Airport and flies at an altitude of approximately 785 feet (240 meters).
But they chose to wait for local air traffic to take off, although Horneman confirmed over the radio that they had encountered an in-flight emergency.
Osprey issued its final chip-related warning three minutes before the crash: the chip detector had failed. Horneman told the crew that he was no longer concerned and that he now believed the previous warning was an error due to a malfunctioning chip detector.
Investigators later discovered that the fault message was because the detector “had too many chips on it and it couldn’t keep up,” Conley said.
Inside the propeller gearbox, the pinion gear broke. At least one part became wedged in the teeth of the larger drive gear system, jamming and breaking off the gear teeth until the left propeller gearbox could no longer turn the Osprey’s left propeller mast.
Within six seconds of the failure of the propeller gearbox, the Osprey transmission and interconnected drive system suffered catastrophic damage. The investigation found that the crew was unable to take any steps to save themselves or the aircraft.
The Osprey rolled violently, flipped over twice, caught fire in the left engine room, and fell into the water, killing everyone on board.
Operations changed after accident
After the accident, crews are now instructed to land as quickly as possible on the first wafer burnout and as quickly as possible on the second wafer burnout. The JPO is also developing a new system that can instantly communicate vibration data to pilots, giving them greater awareness during flight.
Japanese Defense Ministry officials said one of their Ospreys first reported a chip burn warning last August and made a precautionary landing. Japan grounded its fleet after the November accident. It has restarted flight operations under the stricter flight restrictions imposed by the US military – flying within 30 minutes of the landing site and conducting more frequent chip inspections and maintenance.
Japanese Defense Minister Minoru Kihara told reporters on Friday that while the root cause of the crack had not been determined, the new preventive measures were sufficient.
“I believe there are no safety concerns with the Osprey,” Kihara said, but added that Japan would continue to cooperate with the U.S. military “to ensure the highest level of safety measures.”
Kihara said that while the cause of the gear damage was not yet clear, Japan did not plan to conduct its own investigation or ask the United States to investigate further because the two sides had shared “unprecedented confidential information” about the accident. Japan expects further improvements in Osprey parts, he said.
The V-22 Osprey is jointly produced by Bell Flight Company and Boeing.
The accident killed Maj. Eric V. Spendlove, 36, of St. George, Utah. Maj. Luke A. Unrath, 34, Riverside, Calif.; Capt. Terrell K. Brayman, 32, Pittsford, N.Y.; Tech. Sgt. Zachary E. Lavoy, 33, Oviedo, Fla.; Staff Sgt. Jake M. Turnage, 25, of Kennesaw, Ga.; Senior Airman Brian K. Johnson, 32, of Reynoldsburg, Ohio; Staff Sgt. Jacob M. Galliher, 24, of Pittsfield, Mass.; Hoernemann, 32, of Andover, Minn.;
