Tokyo (CNN) The sudden crash of a Japanese F-35 stealth fighter into the Pacific Ocean in April this year was likely caused by the "spatial disorientation" of its pilot, the country's defense minister said Monday.

The Japan Air Self-Defense Force jet, among the world's most sophisticated aircraft, disappeared from radar while on a training mission with three other F-35s off northern Japan on April 9. There was no indication from the jet's pilot, Maj. Akinori Hosomi, of any problems with the aircraft before contact was lost.

The Ministry of Defense said Monday that Hosomi, a 41-year-old with 3,200 hours of flight experience, essentially flew the stealth fighter straight into the ocean during the night training mission.

Spatial disorientation is defined as "a situation in which a pilot cannot sense correctly the position, attitude, altitude, or the motion of an airplane," according to a 2009 study on Japan's Air Self-Defense Force in the journal Military Medicine.

The effects are worse at night, according to the study, which said at the time that 12% of Japanese military air accidents were caused by spatial disorientation.

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