61-year-old Brian Jokat gave the blow-by-blow to CNN -- saying he'd fallen asleep on the plane only to awaken to see bodies flying past him and hitting the roof ... a scene he likened to "The Exorcist," as the plane dropped around 500 feet on Monday, according to him.

At first, he says he thought it was a dream -- but then a fellow passenger suddenly dropped from the roof, hitting his armrest ... and that's when he realized he was in a living nightmare.

After the flight ... an airline spokesperson said there was a technical issue that had caused the drop, but left everyone in the dark about what it actually was. However, Brian has given insight into what really went down ... telling CNN the pilot revealed he'd lost control of his instruments on a dime, with everything just going dark.

He says the shocked pilot came back to check in on everyone ... telling them he'd essentially lost control of the jet 'cause the gauges went blank on him. Absolutely terrifying, no doubt.

After a few seconds, the gauges returned and reengaged to their normal flight pattern ... with the plane ultimately landing safely at Auckland airport, much to the relief of Brian. He also told The Post he was certain they were all gonna go down and had made peace that it was all out of his control.

BTW, Brian shared pictures of people's injuries as they were being tossed around in the cabin -- and they look absolutely brutal, to say the least.

Roughly 50 passengers were treated for mild to moderate injuries, and one person was in serious condition.

This is just the latest incident with Boeing of late -- their aircrafts have come under incredible scrutiny due to problems in the sky ... and doubly so amid John Barnett's sudden death.