Justin Bieber -- The Idiot Knew He'd Get Searched And Still Smoked Up the Jet

Justin Bieber is not only out of control ... he's hard-core stupid. He knew last week -- before he took off for New Jersey -- his plane would get searched by U.S. Customs yet he and his friends still hotboxed on the private jet.

TMZ has confirmed .... the pilot radioed in the air Justin and his gang were smoking weed on the plane and the flight attendant couldn't get them to stop. The pilot complained the smoke in the cabin was thick. NBC News says the pilot actually had to don a gas mask because it was so bad.

Now here's what's incredible ... TMZ has previously reported Justin is on a Homeland Security watch list because of 2 pending criminal cases.

Multiple sources connected with Bieber tell TMZ ... Justin's people called federal officials before the plane took off and they were informed Justin was on the list. Justin's people told him before he got on the jet that he would be searched in New Jersey and interrogated, yet Bieber STILL smoked up the aircraft.

We're also told Bieber and his buds were abusive to the flight attendant because she ordered them to stop smoking ... in vain.

And to add icing to the cake ... Bieber's father was on the plane.

By the time the plane landed the weed was gone, but drug-sniffing dogs detected trace amounts.

One person who was on the plane 3 hours after it landed says it continued to reek of marijuana.