Japan's air force said Monday "spatial disorientation" likely caused one of its pilots to fly his F-35 stealth fighters into the ­Pacific Ocean in April, hitting the water at more than 1,100 kilometers per hour.

The Lockheed Martin Corp jet disappeared from radar screens during an exercise with three other F-35s over the ocean off northwest Japan on April 9. The 41-year-old pilot was killed.

"We believe it highly likely the pilot was suffering from vertigo or spatial disorientation and wasn't aware of his condition," Japanese Defense Minister Takeshi Iwaya told a briefing.

"It can affect any pilot regardless of their experience."

The air force has yet to recover any intact data from the $126 million aircraft's flight data recorder to back its assessment, which is based on data and communication received by ground controllers and interviews with other pilots.

The pilot, who had only 60 hours' flying time in the F-35, gave no indication that he was in trouble and did not try to avoid a collision despite advanced instrumentation and a ground proximity warning system that should have alerted him to pull up. 

The air force did not find any indication he had attempted to eject.

His plane, which was less than 1 year old and part of a squadron that had just become operational, crashed 28 minutes after taking off. 

Debris from the plane is spread across the sea floor about 1,500 meters below where it slammed into the ­water.