The Horror At 37,000 Feet (1973)
An architect and his wife are flying from London to L.A. with an altar from an ancient abbey secured in the plane's cargo hold. The other crew and passengers come into jeopardy when an invisible demon escapes from the altar mid-flight and threatens the plane in an effort to destroy the architect's wife.
REVIEW: The Horror At 37,000 Feet is a very low budget 1970s Made-for-TV movie, and said low budget on this one definitely stands out as a sore point, especially during one moment where a character gets sucked out of the plane. However, if you're able to get past that aspect of it, this is actually a nifty little in-flight set horror movie, filled with likable characters, William Shatner hamming it up like his entire life has led to this one specific movie, and a few genuinely creepy moments.
It also helps that it's only an hour and 13 minutes long, including the credits, so the entire thing... (I hate to make this pun, trust me) ...flies by... pretty quickly.