The Twilight Zone - Nightmare At 20,000 Feet

Bob Wilson is on a flight when he sees a creature of some sort out on the wing of the aircraft. He's only recently recovered from a nervous breakdown and isn't sure that what he is seeing is real. Every time someone else looks out the window, the creature hides from view. When the creature begins to tamper with one of the engines he begs him wife to tell the pilots to keep an eye on the engines. If they see nothing, he agrees to commit himself to an asylum when they arrive at their destination. His paranoia drives him to a desperate act.