Photo/IllutrationAn F-35A stealth fighter jet (Asahi Shimbun file photo)

The pilot of the F-35A stealth fighter jet that crashed off Aomori Prefecture likely suffered from spatial disorientation and did not realize he was diving toward the ocean at breakneck speed.

The pilot, Akinori Hosomi, a 41-year-old major of the Air Self-Defense Force, was killed in the crash in the Pacific Ocean on the night of April 9.

The Defense Ministry, which released the results of its investigation into the accident on June 10, said Hosomi apparently lost a sense of equilibrium and veered toward the sea.

The aircraft, stationed at the ASDF’s Misawa air base in the prefecture, was taking part in a combat exercise involving three other F-35A fighters. Hosomi was the flight leader that night.

Data from the ground radar and other sources showed that Hosomi’s aircraft descended almost vertically toward the sea at an abnormally fast speed of about 10,000 meters in 30 seconds, according to the investigation results.

Hosomi exchanged communications by radio with the controller on the ground. His voice was calm, and there was very little evidence that the pilot knew he was in trouble or attempted an emergency escape during the descent, according to the ministry.

Based on these circumstances, the ministry concluded that the pilot was in a state of spatial disorientation and was unaware the fighter was nosediving into the ocean.

It was the first-ever crash of an F-35A.

The ministry has grounded 12 other F-35As. No particular mechanical problems have been found or reported on these other jets.