Kim Kardashian's BFF -- Threatened with a Knife on Airplane

Kim Kardashian's bff Jonathan Cheban -- a regular on "Keeping Up With the Kardashians" -- found himself in the middle of an airport emergency when a woman on his plane threatened to stab him with a knife.

Cheban was flying from JFK to LAX -- when a woman on his flight tweeted at him, "Jonathan Cheban is on my flight, should I cut him with my weapon?"

She added in another tweet, "I gotta pretty intense pocket knife with mehhh."

Cheban, a former PR exec, responded, "Police?"

When the plane landed at LAX, the woman was escorted off by police and subsequently detained. We're told cops were deeply concerned about her claim that she got a knife through airport security.

She later tried to bury the hatchet, tweeting back at Cheban, "It was a bad joke ... you look really hot babe." But airport police weren't taking any chances.

Police searched the woman outside the plane and determined she did NOT actually have a knife. She was subsequently released without further incident.

Cheban tells TMZ, he never personally alerted authorities -- someone else reported the tweets -- but later in Bev Hills he admitted the incident was very "scary."