Cracks in Gear and Pilot's Decision Cited in Fatal Japan Osprey Crash

A deadly crash involving an Osprey aircraft last November off Japan was attributed to cracks in a metal gear and the pilot's decision to continue flying despite multiple warnings, according to an Air Force investigation released on Thursday. The CV-22B Osprey crash resulted in the deaths of eight Air Force Special Operations Command members and led to a months-long grounding of the fleet.

The crash is one of four fatal Osprey incidents in the past two years, prompting investigations into the aircraft's safety record. This has caused a division among military services regarding the future use of the unique aircraft, which can fly like an airplane but land like a helicopter. Initially, the Air Force only mentioned an unprecedented component failure as the cause of the crash.

Identifying the Cause

On Thursday, it was revealed that a toothed piece called a pinion gear, a crucial part of the proprotor gearbox, was responsible. The proprotor gearbox functions as the aircraft's transmission, with five pinion gears inside each gearbox working hard to transmit engine power to turn the Osprey's masts and rotor blades. While confident about the pinion gear failure, the Air Force still doesn't know why it happened.

Pentagon leadership overseeing the V-22 Ospreys were aware that "total loss of aircraft and crew were possible" if these proprotor gearbox components failed, lead investigator Lt Gen Michael Conley told reporters on Wednesday. The investigation also criticized this office for not sharing safety data that could have informed crews about the severity of the risk.

Flight Details and Warnings

On the day of the crash, vibrations were first detected along the coast of mainland Japan as the Osprey headed to Okinawa. Vibrations are monitored in aircraft as signs of potential trouble. A data recorder noted vibrations on the left side of the driveshaft linking the two engines, acting as a fail-safe if one engine loses power.

A second vibration followed from one of the five pinion gears inside the left proprotor gearbox. However, pilot Maj. Jeff Hoernemann and his crew were unaware of these vibrations since such data can only be downloaded post-flight. Five minutes after detecting vibrations, a left proprotor gearbox chip burn warning appeared in the cockpit, indicating metal flaking off due to stress.

Chipping is common in rotary flight, and there's a safety mechanism in place within the Osprey to burn off these chips so they don't travel in oil and damage the transmission. If successful, this clears the warning. The crew received six chip warnings that day, each providing an opportunity for Hoernemann to land as a precaution. However, he did not heed these warnings.

Pilot Decisions and Consequences

When the third chip burn warning occurred, they were still close to mainland Japan and just 10 miles (16 km) from an airfield. Official guidance after three chip burns is to "land as soon as practical," leaving discretion to pilots. According to voice data recordings, Hoernemann and his crew sought secondary indications like gearbox overheating but found none.

Hoernemann directed his co-pilot to monitor while continuing their 300-nautical-mile flight over water to Okinawa. Balancing split priorities, he focused on completing their military exercise rather than addressing evolving aircraft issues. He dismissed his co-pilot's suggestions for using onboard mapping tools to find nearby airfields.

The fourth and fifth chip burn warnings came quickly, escalating with just chips that couldn't be burned off. "Land as soon as practical" turned into "land as soon as possible," yet there was no urgency from the crew. In final minutes before crashing, they began positioning for landing but deferred again due to local traffic instructions from Japanese controllers.

Final Moments

The Osprey gave its final chip-related warning three minutes before crashing: chip detector fail. Hoernemann assumed earlier warnings were errors due to a faulty detector. Investigators later found it failed because it had too many chips on it. Hoernemann then instructed his co-pilot for one more loop before landing.

Inside the proprotor gearbox, however, pinion gear was breaking apart. A piece wedged into larger transmission gearing teeth until it couldn't turn left proprotor mast anymore. Within six seconds of gearbox failure, catastrophic destruction spread through Osprey gearing system.

The Osprey rolled violently, inverted twice with its left engine housing on fire before crashing into water, killing all onboard. Following this crash, crews are now directed to land immediately on first chip burn warning and as soon as possible on second warning.

New Safety Measures

The joint program office is developing a new system that will communicate vibration data in real-time to pilots for better awareness during flight. The accident claimed lives of Maj Eric V Spendlove (36), Maj Luke A Unrath (34), Capt Terrell K Brayman (32), Tech Sgt Zachary E Lavoy (33), Staff Sgt Jake M Turnage (25), Senior Airman Brian K Johnson (32), Staff Sgt Jacob M Galliher (24), and Maj Jeff Hoernemann (32).