Brandon T. Jackson -- Inflight Pain In the Ass ... Says Airline

"Tropic Thunder" star Brandon T. Jackson was an obnoxious, drunken a-hole during a flight Monday ... and that's why he was booted from his plane, so says US Airways.

A rep for the airline tells TMZ ... crew members adamantly deny using the N-word during an inflight altercation with the actor, despite the actor's allegations.

As we previously reported, Brandon went on a Twitter rant last night -- claiming a flight attendant hurled racially-charged epithets at him for no reason ... but now, the airline is calling BS.

The rep explains, "The disruptive behavior started with loud music being played by Mr. Jackson at his seat. He was asked several times to turn down the music. Additional erratic behavior was observed and that is when the crew made the decision to stop serving [alcohol to Brandon]."

US Airways says it stands behind the crew -- claiming they handled the situation appropriately.

When Jackson's flight landed in Phoenix (where he was supposed to catch a connecting flight to Burbank) he was escorted off the plane and interviewed by Phoenix PD.

Cops determined no crime was committed and he was allowed to leave -- but US Airways banned him from getting on the plane to BUR.