Justin Bieber's Pilot -- Weed Smell Was SO BAD... I Needed Oxygen!

Justin Bieber is officially a member of the mile high club ... TMZ has learned.

Sources high up in the sky tell TMZ ... Justin and a group of his boyz got ridiculously high in their SUV on the way to the airport for a coast-to-coast flight on a private jet. When the doors opened on the tarmac, the pilot tells us smoke billowed out, adding, "I almost had to put on an oxygen mask."

The pilot said, "It was like 'Cheech and Chong Up In Smoke."

The Biebs didn't toke during the flight ... but some members of his posse lit up Swisher Sweet blunt cigars -- it's perfectly legal since they were flying private.

The pilot says Justin was "baked out of his mind" on the flight ... so much so his head spun when he was asked to sign a credit card authorization, saying "huh, what? Who am I signing this for?'"

By the way ... the cost of that flight ... more than $60K.