WASHINGTON—A tragic Osprey aircraft accident near Japan last November was caused by fractures in a metal gear and the pilot's choice to continue flying despite several warnings to land, according to an Air Force report revealed Thursday.
The CV-22B Osprey disaster killed eight Air Force Special Operations Command service members and triggered a months-long military-wide suspension of the aircraft. Four tragic Osprey crashes have occurred in the last two years, prompting probes into the Osprey's safety record and causing divisions within the services regarding the future role of the unusual aircraft that can fly like an airplane but land like a helicopter.
For months, the Air Force would only admit that an unusual component failure caused the tragedy. On Thursday, it stated that a toothed piece known as a pinion gear—a vital component of the propeller gearbox—was to blame. The proprotor gearbox functions as the aircraft's transmission. Inside each gearbox, five pinion gears rotate rapidly to send the engine's power to the Osprey's masts and rotor blades.
The Air Force is convinced that the pinion gear broke, but it does not know why.
However, the Pentagon program office in charge of the V-22 Ospreys was aware that "total loss of aircraft and crew were possible" if the proprotor gearbox components failed, according to chief investigator Lt. Gen. Michael Conley, speaking to reporters Wednesday before the report's formal publication. In an unusual step, the probe chastised that agency for failing to provide safety data that may have educated personnel on the gravity of the risk.
In an interview with The Associated Press, Conley stated that he felt the pilot's impulse to finish the military exercise guided his judgments.
“To a degree, it’s a way of life here. I mean, we we want people in this command that that are biased toward ‘yes,’ biased toward getting the mission done,” Conley said. “As we went through the investigation, I saw someone that was confident in the aircraft but not cocky.”
On the day of the incident, the Osprey was flying along the coast of mainland Japan, heading for Okinawa, when the first signs of problems appeared.
Vibrations in airplanes are examined as indicators of future problems. A data recorder detected vibrations on the left side of the driveshaft, which connects the two engines and serves as a failsafe if one loses power.
A second tremor occurred. This time, one of the five pinion gears in the left propeller gearbox was vibrating.
However, because vibration data can only be obtained after a flight, pilot Maj. Jeff Hoernemann and his crew were unaware of them.
Five minutes after the initial tremor, a left proprotor gearbox chip burn alert was shown in the cockpit. The notice informs the crew that metal was peeling off the Osprey's gears, another sign of stress.
Chipping is a regular occurrence in rotary flight, hence the Osprey has a safety net. The chip detector may burn off the chips so that they do not get into the oil and ruin the transmission.
If the burn is successful, the warning disappears.
The crew received six chip warnings that day. Each gave a chance for Hoernemann to heed the warning and land as a precaution, but he did not, and investigators determined that choice was a cause of the catastrophe.
When the third chip burn alert was issued, the crew was close to mainland Japan and only 10 miles (16 kilometers) from the nearest airstrip. After three chip burns, the official advise was to "land as soon as practical," although this choice is still left to the pilot's discretion.
According to the speech data recorder, Hoernemann and the crew searched for secondary signs of a problem, such as the proprotor gearbox overheating, but found none. So Hoernemann instructed his co-pilot to continue monitoring the situation and decided to continue the 300-nautical-mile flight over sea to Okinawa.
According to the inquiry, Hoernemann's decision-making was most likely influenced by competing priorities. He was in charge of the aerial element of the military exercise and had spent months arranging it.
According to the inquiry, he remained focused on completing the exercise rather than the deteriorating aircraft situation until nearly the end of the flight. He rejected his copilot's idea of utilizing a different onboard mapping program to find the closest airstrip to land. Based on the retrieved voice data, the inquiry discovered that the co-pilot was not open about "his discomfort with the evolving issues" during the flight.
The fourth and fifth chip burn warnings arrived quickly. Then comes the sixth escalation: only chips. It meant the Osprey could not fire them away. "Land as soon as practical" became "land as soon as possible." However, the crew members did not respond quickly.
They had started preparing the aircraft for landing in the last minutes of the flight. The Osprey was flying approximately 785 feet (240 meters) over the ocean, half a mile (0.8 kilometers) from Yakushima's runway.
Despite the fact that Hoernemann acknowledged over the radio that there was an in-flight emergency, they chose to wait for nearby air traffic to take off.
The Osprey sent its final chip-related alert three minutes before the crash: the chip detection failed. Hoernemann informed the crew that he was no longer concerned, and that he now believed the previous warnings were mistakes caused by a malfunctioning chip detector.
Investigators eventually discovered that the fail indication happened because the detector "had so many chips on it, it could not keep up," Conley explained.
The pinion gear in the proprotor gearbox was falling apart. At least one piece became lodged in the teeth of the bigger transmission gearing system, blocking and breaking off gearing teeth until the left proprotor gearbox was unable to move the Osprey's left propellor mast.
Within six seconds of the proprotor gearbox collapsing, catastrophic damage ripped through the Osprey gearing and associated drive system. According to the study, the crew members had no way of saving themselves or the aircraft at that moment.
The Osprey rolled violently, flipped twice, with the left engine housing on fire, and plunged into the ocean, killing everyone on board.
Following the incident, personnel are now instructed to land as soon as practicable on the first chip burn and as soon as feasible on the second. The joint program office is also developing a new technology that will send vibration data to pilots in real time, allowing them to be more attentive during flight.
According to officials from the Japanese Defense Ministry, one of their Ospreys reported a first chip burn alert in August and landed as a precaution. Following the November disaster, Japan grounded its fleet. It has resumed flying operations in accordance with the US military's tougher flight limitations, which include operating within 30 minutes of a landing spot and completing more regular chip checks and maintenance.
Japanese Defense Minister Minoru Kihara told reporters on Friday that, while the fundamental cause of the breaking has yet to be established, the new preventative measures are sufficient.
“I believe Ospreys have no safety problems.” Kihara said, but he added that Japan will continue to cooperate with the U.S. military “to ensure highest levels of safety measures are taken.”
Kihara stated that, while the reason of the gear damage is unclear, Japan does not intend to initiate its own study or request that the United States conduct a further investigation because the two countries have exchanged "unprecedented levels of classified information" regarding the disaster. He stated that Japan anticipates more improvements to Osprey components.
Bell Flight and Boeing collaborate to create the V-22 Osprey.
The accident killed 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.
