The Osprey crash was caused by the pilot’s decision to continue flying despite cracks in the gearbox

WASHINGTON — The fatal 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 keep flying instead of heeding several warnings to land, according to an Air Force investigation released Thursday.

The CV-22B Osprey crash killed eight Air Force Special Operations Command personnel and grounded the entire fleet for months. There have been four fatal Osprey crashes in the past two years, prompting investigations into the Osprey’s safety record and leading to disagreements between the service over the future role of this unique aircraft that can fly like a plane but land like a helicopter.

For months, the Air Force simply claimed that the crash was caused by an unprecedented component failure. On Thursday, it said a gear, a critical part of the propeller gearbox, was to blame. The propeller gearbox serves as the aircraft’s power transmission mechanism. Five pinions rotate in each gearbox to transfer engine power to the Osprey’s masts and rotor blades.

Although the Air Force is convinced that the pinion has failed, the cause is still unclear.

But a Pentagon program office responsible for the V-22 Ospreys knew that “total loss of aircraft and crew” was possible if these propeller gear components failed, lead investigator Lt. Gen. Michael Conley told reporters on Wednesday before the report was officially released. In an unusual move, the investigation also criticized this office for failing to pass on safety data that could have informed the crews of the severity of the risk.

A pilot’s instinct to complete the mission

In an interview with the Associated Press, Conley said he believed his decisions were driven by the pilot’s instinct to complete the military exercise.

“To some extent, this is a way of life here. I mean, we want people in this command who tend to be ‘yes,’ who are dedicated to accomplishing the mission,” Conley said. “During our investigation, I saw someone who had confidence in the aircraft, but not overbearing.”

On the day of the crash, the Osprey was flying along the coast of mainland Japan toward Okinawa when the first signs of trouble appeared.

Airplanes monitor vibrations as a sign of potential problems. A data recorder detected vibrations on the left side of the driveshaft, which connects 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 pinions in the left promoter gear vibrated.

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

Five minutes after the first vibration, a warning of a chip fire in the left propeller gearbox appeared in the cockpit. The warning informed the crew that metal fragments had broken loose from the Osprey’s gearbox, another sign of overload.

Chippings are so common on rotary wing aircraft that the Osprey is equipped with a safety net. The chip detector can burn off the chips so they don’t get into the oil and destroy the gearbox.

If the burn is successful, the warning will be cleared.

Six missed landing opportunities

The crew received six chip warnings that day. Each of these offered Hoernemann the opportunity to heed the warning and land as a precaution. However, he did not do so, and investigators concluded that this decision was a causal factor in the crash.

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

According to the voice data recorder, Hoernemann and the crew looked for secondary indications of a problem, such as overheating of the propeller gear, but could not find any. So Hoernemann instead instructed his copilot to continue monitoring the situation and decided to continue the 300-nautical-mile over-water flight to Okinawa.

Hoernemann’s decision-making was likely a balancing of priorities, the investigation found. He led the airborne portion of the military exercise and had been planning it for months.

Until nearly the final minutes of the flight, his primary focus was on completing the exercise rather than on the aircraft’s evolving situation, the investigation found. He rejected his copilot’s suggestion to use an alternative boarding pass tool to find the nearest airfield. Throughout the flight, the copilot also did not directly express “his discomfort with the evolving problems,” the investigation found based on recovered voice data.

The fourth and fifth chip burn warnings came quickly. Then the sixth, the escalation: chips only. That meant the Osprey couldn’t burn them off. “Land as soon as possible” became “land as soon as possible.” Still, crew members didn’t act with the urgency.

The last minutes of the doomed flight

In the final minutes of the flight, they began to position the plane for landing. The Osprey was 800 meters from an airfield in Yakushima and flying about 240 meters above the water.

However, they decided to wait for local air traffic to depart, even though Hoernemann confirmed over the radio that it was an in-flight emergency.

Three minutes before the crash, the Osprey gave its final chip warning: chip detector defective. Hoernemann told the crew that he was no longer worried and now assumed that the earlier warnings were due to errors caused by a defective chip detector.

As investigators later discovered, the error message was due to the detector “having so many chips that it couldn’t keep up,” Conley said.

Inside the propeller gear, the pinion gear broke apart. At least part of it became wedged in the teeth of the larger gear, binding and breaking off the teeth until the left propeller gear could no longer rotate the Osprey’s left propeller mast.

Within six seconds of the propeller gear failure, the Osprey’s gearbox and associated propulsion system were catastrophically destroyed. At that point, there was nothing the crew members could have done to save themselves or the aircraft, the investigation found.

The Osprey overturned violently, rolled over twice, its left engine casing caught fire, and plunged into the water, killing all occupants.

After the crash, operational changes were made

After the crash, the crew will now be instructed to land as quickly as possible on the first chip burn and as quickly as possible on the second. The Joint Program Office is also working on a new system that will provide real-time vibration data to pilots so they are better informed during the flight.

Officials at Japan’s Defense Ministry said one of their Ospreys reported an initial chip burn warning last August and conducted a safety landing. After the November crash, Japan grounded its fleet. It has resumed flight operations under the U.S. military’s stricter flight restrictions — allowing flights within 30 minutes of the landing site and conducting more frequent chip checks and maintenance.

Japanese Defense Minister Minoru Kihara told reporters on Friday that the new precautions were sufficient, even though the actual cause of the cracks had not yet been determined.

“I believe there are no safety issues with the Ospreys,” Kihara said, but added that Japan will continue to work with the U.S. military “to ensure that safety measures are taken at the highest level.”

Kihara said that although the cause of the gearbox damage is still unknown, Japan does not intend to launch its own investigation or ask the U.S. to investigate further, as both sides have exchanged “an unprecedented level of classified information” about the accident. He said Japan expects further improvements to be made to the Osprey parts.

The V-22 Osprey is produced jointly by Bell Flight and Boeing.

Killed in the accident were Maj. Eric V. Spendlove, 36, of St. George, Utah, Maj. Luke A. Unrath, 34, of Riverside, Calif., 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.

Copyright: NPR