Japan Osprey crash caused by cracks in a gear and pilot’s decision to keep flying, Air Force says

WASHINGTON — A deadly crash of an Osprey plane off the coast of Japan last November was caused by cracks in a metal gear and the pilot’s decision to continue flying instead of heeding warnings to land, an Air Force investigation released Thursday found.

The crash of the CV-22B Osprey killed eight Air Force Special Operations Command service members and sparked months of fear military-wide grounding of the fleet. It is one of four fatal Osprey crashes in the past two years that have prompted investigations into the Osprey’s safety recordIt has created a divide among the services over the future role of the unique aircraft, which can fly like an airplane but land like a helicopter.

For months, the Air Force would say only that an unknown component failure had caused the crash. On Thursday, it was determined that a toothed piece, called a pinion gear — a critical part of the proprotor gearbox — was the culprit. The proprotor gearbox acts as the plane’s transmission: Inside each gearbox, five pinion gears spin hard to transmit power from the engine to turn the Osprey’s pylons and rotor blades.

The Air Force is certain that the pinion broke, but still does not know why.

But the Pentagon leadership responsible for the V-22 Ospreys knew that “total loss of aircraft and crew was possible” if those propulsion gear components failed, lead investigator Lt. Gen. Michael Conley told reporters in a phone call Wednesday ahead of the report’s release. In a rare move, the investigation also criticized that office, saying it failed to share safety data that could have informed flight crews of the severity of the risk.

Conley said in an interview with The Associated Press that he believed the pilot’s instinct to complete the military exercise influenced his decisions.

“To some extent it’s a way of life here. I mean, we want people in this command who are biased toward ‘yes,’ biased toward accomplishing the mission,” Conley said. “In the investigation, I saw someone who confidence in the plane but not overconfident.”

On the day of the crash, the Osprey was flying along the coast of Japan en route to Okinawa when the first signs of trouble emerged.

In aircraft, vibrations are monitored as a sign of potential problems. A data recorder recorded vibrations on the left side of the drive shaft that connects the two engines and acts as a failsafe in case one engine loses power.

A second vibration followed. This time one of the five pinions in the left pro-rotor gearbox vibrated.

But pilot Major Jeff Hoernemann and his crew knew nothing about the vibrations, because that data could only be downloaded at the end of the flight.

Five minutes after the first vibration, a left proprotor gearbox chip burn warning sounded in the cockpit. The warning alerted the crew to metal shavings coming from the Osprey’s gearing, another indication of stress.

Chipping is a common occurrence in rotary flight, so a safety net is built into the Osprey. The chip detector can burn the chips so they don’t end up in the oil and destroy the transmission.

If the burn is successful, the warning will disappear.

The crew received six chip warnings that day. Each gave Hoernemann the chance to heed the warning and land as a precaution, but he did not, and investigators found that decision to be a causal factor in the crash.

When the third chip burn warning was issued, the crew was still close to mainland Japan and only 10 miles (16 kilometers) from the nearest airport. The official directive after three chip burns was to “land as soon as possible,” a directive that still leaves that decision to the pilot’s discretion.

According to the voice data recorder, Hoernemann and the crew looked for secondary signs of a problem, such as overheating of the proprotor gearbox, but saw none. So Hoernemann ordered his co-pilot to monitor the situation and decided to continue the 300-nautical-mile flight over water to Okinawa.

Hoernemann was likely balancing split priorities in his decision-making, the investigation found. He led the airborne portion of the military exercise and had spent months planning it.

Until nearly the final minutes of the flight, he remained focused on completing the exercise, not on the changing aircraft situation, the investigation found. He rejected his co-pilot’s suggestions that he use an onboard mapping tool to identify the nearest airport to land at. But the co-pilot also was not forthcoming about “his discomfort with the changing issues,” the investigation found, based on the recovered voice data.

The fourth and fifth warnings of chip burn came quickly. Then with the sixth, escalation: chips only. It meant the Osprey could not burn them. “Land as soon as practical” became “land as soon as possible.” Still, the crew did not act with urgency.

In the final minutes of the flight, they began positioning the plane for landing. The Osprey was half a mile (0.8 kilometers) from an airfield in Yakushima, flying about 785 feet (240 meters) above the water.

But they waited again and postponed it, because Japanese controllers told them to wait until local traffic could leave. Hoernemann confirmed by radio that there was an emergency on board.

The Osprey gave its last warning about the chip three minutes before the crash: chip detector failure. Hoernemann told the crew that he was no longer concerned, he now assumed that the earlier warnings were errors due to a faulty chip detector.

Instead, researchers later discovered that the error occurred because the detector “had so many chips in it that it couldn’t keep up,” Conley said.

Hoernemann then instructed his co-pilot to “do one more big right loop, come in and prepare for landing.”

But inside the proprotor gearbox, the pinion broke apart. At least one piece jammed into the teeth of the larger transmission gear system, causing the gear teeth to seize and break off, 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 swept through the Osprey gearbox and its interconnected drive system. At that point, there was nothing the crew could have done to save themselves or the aircraft, the investigation found.

The Osprey rolled violently, flipped twice, igniting the left engine housing, and crashed into the water, killing all occupants.

After the crash, crews are now tasked with landing on a first chip burn as quickly as possible and on a second as quickly as possible. The joint program office is also working on a new system that would communicate vibration data to pilots in real time, to make them more aware during flight.

Killed in the crash were Maj. Eric V. Spendlove, 36, of St. George, Utah; Maj. Luke A. Unrath, 34, of Riverside, California; Capt. Terrell K. Brayman, 32, of Pittsford, New York; Tech. Sgt. Zachary E. Lavoy, 33, of Oviedo, Florida; Staff Sgt. Jake M. Turnage, 25, of Kennesaw, Georgia; Senior Airman Brian K. Johnson, 32, of Reynoldsburg, Ohio; Staff Sgt. Jacob M. Galliher, 24, of Pittsfield, Massachusetts; and Hoernemann, 32, of Andover, Minnesota.