WASHINGTON (NEWSnet/AP) — A fatal Osprey aircraft crash last fall off Japan was caused by cracks in a gear and the pilot’s decision to keep flying despite a series of technical warnings, according to an Air Force investigation.

The CV-22B Osprey crash killed eight Air Force Special Operations Command service members and led to a months-long, military-wide grounding of the fleet as one of four fatal Osprey crashes in the last two years.

An Osprey is a unique aircraft in that it can fly like an airplane but land like a helicopter.

For months, the Air Force would only say an unprecedented component failure caused the crash.

On Thursday, it reported that a toothed piece called a pinion gear — a critical part of the proprotor gearbox — was to blame. The proprotor gearbox serves as the aircraft’s transmission: Inside each gearbox, five pinion gears spin hard to transmit the engine’s power to turn the Osprey’s masts and rotor blades.

While the Air Force is confident it was the pinion gear that failed, it still doesn’t know why.

But Pentagon leadership in charge of the V-22 Ospreys knew that “total loss of aircraft and crew were possible” should those proprotor gearbox components fail, lead investigator Lt. Gen. Michael Conley told reporters Wednesday in a call ahead of the report’s release. In a rare move, the investigation also faulted that office, saying it did not share safety data that could have educated crews on the severity of the risk.

On the day of the crash, the Osprey was flying along the coast of mainland Japan headed to Okinawa when the first indications of trouble began.

A data recorder noted vibrations on the left side of the driveshaft that links the two engines and acts as a fail safe in case one engine loses power.

A second vibration followed. This time one of the five pinion gears inside the left proprotor gearbox was vibrating.

That being said, the pilot and crew never knew about the vibrations, because that data can only be downloaded at the end of a flight.

Five minutes after the first vibration, a left proprotor gearbox chip burn warning posted in the cockpit. The warning lets the crew know there’s metal flaking coming off the Osprey’s gearing, another indication of stress.

Chipping is a common enough occurrence in rotary flight that there’s a safety net designed into the Osprey. The chip detector can burn the chips off so they don’t travel in the oil and destroy the transmission.

The crew got six chip warnings that day.

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

Until almost the final minutes of flight, he kept his primary focus on completing the exercise, not the aircraft situation, the investigation found. He rejected his co-pilot’s suggestions on using an onboard mapping tool to identify the closest airfield to land. But the co-pilot was also not direct about “his uneasiness with the evolving issues,” the investigation found.

The fourth and fifth chip burn warnings came fast. Then with the sixth, escalation: just chips. “Land as soon as practical” turned into “land as soon as possible.”

In the final minutes of flight, they’d begun to position the aircraft to land.

But they waited and deferred as Japanese traffic controllers told them to hold for local traffic to take off, even as Hoernemann confirmed over the radio they had an in-flight emergency.

The Osprey gave its final warning three minutes before the crash: chip detector fail. Hoernemann told the crew he was no longer worried, he assumed the earlier warnings were errors due to a faulty chip detector.

Instead, investigators later found the fail message occurred because the detector “had so many chips on it, it couldn’t keep up,” Conley said.

The pinion gear was breaking. 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.

As a result of this crash, crews are now directed to land as soon as practical on a first chip burn and as soon as possible on the second.

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