MELBOURNE, Australia — Japan is citing spatial disorientation by the pilot as the likely cause of last April’s F-35 fighter jet crash.

A news release by the Japan Air Self-Defense Force said the pilot’s last radio transmission, 15 seconds before the time of the crash, indicated he was conscious at the time, making it unlikely that a loss of consciousness due to G-forces was responsible for the crash.

It also said there were no indications of mechanical or other issues with the Lockheed Martin-made aircraft

The release also retraced the last moments of the aircraft, with the pilot of the F-35, 41-year-old Maj. Akinori Hosomi, declaring that he had scored simulated kills against two of the three other aircraft in his flight at 7:25 p.m. local time on April 9th. He was taking part in an air combat training mission with three other F-35s, as they trained in airspace approximately 85 miles east of Misawa Air Base in the northern part of Japan’s main island of Honshu, from where the aircraft had taken off just before 7 p.m.

At 7:26 p.m., Hosomi was ordered to descend for airspace deconfliction purposes due to U.S. military aircraft operating nearby. He acknowledged the order and complied, descending in a left-hand turn to 15,500 feet from an altitude of 31,500 feet in 15 seconds, reaching speeds of approximately 560 miles per hour.

It was at that point when Hosomi, who was the leader of the flight of four F-35As, called “knock it off,” indicating the end of the training exercise.

He meanwhile continued his descent, which ended when the F-35A crashed into the Pacific Ocean not more than a minute after the order to descend was given. He had been traveling at an estimated speed of 690 MPH.

The report also concluded that Hosomi, whose remains were recently found, was unlikely to have attempted to eject from his aircraft. The bulk of the aircraft’s wreckage remains on the seafloor, approximately 5,000 feet underwater. Although it was previously reported by local media that an extensive underwater search recovered the aircraft’s flight recorder, it was too badly damaged for any data of the flight to be retrieved.

Instead, Kyodo News, citing anonymous officials, had earlier reported that the crash investigation had most likely gathered data from land-based radars and the information gleaned from the data links on board the other F-35s in the flight.

The news release also said the Japan Air Self-Defense Force will implement additional training to recognize spatial disorientation and G-force-induced loss of consciousness for its F-35 pilots as part of remedial actions following the crash. The service also carried out additional inspections of its F-35s, even though technical issues were unlikely to have been a factor in the crash.

Mike Yeo is the Asia correspondent for Defense News.

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