Cracks in a gear, pilot’s decision to keep flying caused Japan Osprey crash: Air Force

The Osprey aircraft crash that killed all eight service members on board last November off the coast of Japan was caused by “catastrophic failure” of a rotor gearbox and the pilot’s decision to keep flying after multiple warnings to land, an Air Force investigation released Thursday revealed.

The Nov. 29 accident near Yakushima involving a CV-22B Osprey — one of four fatal Osprey crashes in the last two years — led to a months-long grounding of the aircraft across the military and kicked off a major investigation into the crash. 

Questions remain as to whether the aircraft, which takes off and lands like a helicopter but flies like a plane, is safe and if it should remain in the military. 

“By conducting a thorough review and accident and safety investigations, we hope to provide answers to the families of the Airmen that lost their lives and prevent future occurrences and tragedies,” former Air Force Special Operations Command head Lt. Gen. Tony Bauernfeind, the official who convened an accident investigation board on the incident, said in a statement.

The investigation found that the mishap was “caused by a catastrophic failure of the left-hand prop rotor gearbox that created a rapidly cascading failure of the aircraft’s drive system,” causing the Osprey to violently roll over and crash into the water.

Decisionmaking by the crew also played a part in the crash, as it was too “casual” during the military exercise, with the pilots “removing any consideration of an earlier landing at a different divert location.”

Even with the issues, officials in charge of V-22 Ospreys knew that “total loss of aircraft and crew were possible” if the gearbox components failed, lead investigator Lt. Gen. Michael Conley told the media Wednesday, as reported by The Associated Press. 

Conley also told the outlet that he believes the pilot’s instinct to finish the exercise led him to not land sooner.

In addition, the V-22 Osprey office did not share safety data that could have educated crews on how severe the risks of not landing were, according to the investigation. 

Specifically, a major part of the proprotor gearbox, known as a pinion gear, was to blame. The box is the aircraft’s transmission and contains five pinion gears that spin to send energy from the engine to the Osprey’s masts and rotor blades. The Air Force does not know why the piece failed. 

The Osprey was flying along the coast of Japan heading to Okinawa when two vibrations in the aircraft happened, one in the driveshaft that links the aircraft’s two engines followed by one in one of the pinion gears.

Vibrations are seen as signs of potential issues, but the pilot, Maj. Jeff Hoernemann, and his crew were unaware it was happening as such data can only be downloaded and inspected at the end of a flight.

Then Hoernemann proceeded to get six warnings that chips — metal flakes coming off the Osprey’s gearing and a sign of stress — were happening, sending a warning to the pilot each time. Pilots are instructed to “land as soon as practical” after three such incidents, according to official guidance.

Investigators found that the decision not to land after each warning played into the crash.

Because Hoernemann and his crew could not find any other problems, including overheating, he told his co-pilot to monitor the situation and chose to continue the over-water exercise, according to report, which obtained a voice data recorder from the day of the accident. 

The Osprey also has a chip detector that can burn off the tiny metal pieces and prevent them from traveling in the oil and/or potentially ripping through the transmission.

Up until the flight’s final minutes, Hoernemann was focused on finishing the drill and rejected his co-pilot’s suggestions to identify the closest airfield to land. 

After the sixth chip warning, with the indication that the Osprey was no longer able to burn them off, the aircraft should have landed “as soon as possible,” but it seemed the crew members did not heed it as urgent. 

Conley told reporters that three minutes before the crash, the Osprey gave a final “chip detector fail” warning as the detector “had so many chips on it, it couldn’t keep up.” 

Hoernemann mistook the warning and the earlier ones as errors caused by a faulty chip detector and said he wasn’t worried. 

Right before the crash, the Osprey was preparing to land and was only half a mile from an airfield in Yakushima, but held off after Japanese traffic controllers told them to wait for local traffic to take off.

Hoernemann directed his co-pilot to “do one more big, right-hand loop and come in and just set up for landing” as the pinion gear was breaking.

Roughly six seconds after it broke, the Osprey’s gearing and interconnected drive system suffered massive failure and the aircraft went down. There was nothing the crew could have done to save themselves or the aircraft, per the investigation.

Several changes have come out of the crash, including new instruction that pilots land as soon as practical after a first chip burn and as soon as possible after the second. The Osprey program, which resumed operations last month, is also creating a new system that would send vibration data to pilots in real time.