This 1974 tv-movie stars venerable Canadian actor Glenn Ford as Colonel Peter Moore, commander of the Whitney Radar Test Group. They've been having electrical glitches on their test flights, but Flight 412, piloted by a pre-Starsky and Hutch David Soul, encounters a UFO. 412 is ordered to a secret base for debriefing, but to the rest of the world, including Colonel Moore, it's as if they have disappeared off the face of the earth.
This film is not really about the UFO encounter as it is about the secret debriefing and interrogation of the crew. Moore knows that the plane was diverted -- what he doesn't know is where or why. The crew are subjected to a brutal questioning, while Moore bucks the bureaucracy to find his men.
Directed by Jud Taylor, who's had a long career in tv including directing five original Star Trek episodes, this film is an adequate, if standard movie. Interestingly, Edward Winter (Colonel Flagg from M*A*S*H) briefly plays an Air Force officer investigating the UFO incident. He will later star in the second season of Project UFO, playing an Air Force officer investigating UFO incidents.
The title is misleading, of course. Flight 412 never really disappears. And Flight 412 was a small Lear-type jet, not the wide-body passenger jet as seen in the poster