Police say Jernnard, of the western French town of La Rochelle, was wearing a white shirt with an Air France logo and had a black jacket with epaulets on the shoulders.
Police offficer Christine O'Brien told the AP news agency that the crew of an outbound US Airways flight found Jernnard in the jump seat behind the pilot at about 6:20pm Wednesday.
He became argumentative and was escorted from the plane, she said.
He was found to have a counterfeit Air France crew member identification card.
Jernnard was arraigned on state charges. Police say they expect federal charges to be filed on Friday.
The incident resembles one of the scams of US conman Frank Abagnale, the subject of Stephen Spielberg's filmCatch Me if You Can.
He made millions of dollars from cheque fraud and impersonations, including posing as a Pan Am pilot.
Spielberg is to head this year's Cannes Film Festival jury but there is no indication that his impending arrival in France influenced Jernnard's behaviour.