Brandon T. Jackson -- Passenger Backs Him Up ... He Wasn't Drunk!

"Tropic Thunder" star Brandon T. Jackson was kicked off his US Airways flight for no reason -- at least according to the passenger sitting next to him on the plane, who tells TMZ, Jackson was NOT drunk as the airline claims.

As we previously reported, Brandon was escorted off his flight from NYC to LA on Monday during a layover in Phoenix ... after flight attendants accused him of being drunk and disorderly.

Brandon went off on Twitter after the incident, claiming a flight attendant accused him of being drunk, hurled a racial epithet at him -- and for good measure added, "I don"t care if you're Obama's son -- get in your seat!!"

We spoke to Darrell Howard, who was sitting next to Brandon, and he says he doesn't recall hearing the Obama comment ... but insists Jackson was NOT drunk -- and had no more than two drinks (Jack and gingers) during the 5-and-a-half hour flight from NYC to Phoenix.

Howard also says Brandon shared a few scripts with him and another passenger -- backing up the actor's claim that he told the angry flight attendant he was just "rehearsing lines" with the people next to him.

Howard added Brandon was nothing but nice, and he was shocked to see the flight attendant snap the way she did.