Has anyone ever opened the door midflight?
The first such incident occurred in the USA in 1971. A passenger flying under the name Dan Cooper hijacked a Boeing 727, extorted a US$200,000 ransom and ordered the pilot to fly at less than10,000 feet and just above stall speed.
Cooper then strapped on a parachute, opened the rear door, deployed the aft airstair and jumped into the night sky somewhere over Washington state, never to be seen again.
The reason Cooper was able to open the door was that he had ordered the pilot not to pressurise the cabin. Once the cabin is pressurised, it is impossible to open the door on a modern jet aircraft.
Aircraft doors are larger than their opening. Before the door can open outward, it must be pulled inward and turned.
As the aircraft climbs, the pressure outside the cabin decreases relative to that inside the cabin, and this difference in pressure keeps the door sealed.
To open the door, you would need to overcome the difference between those two pressures and this is a physical impossibility, even for several very powerful people working together.
In a slightly more chaotic parallel, an armed Filipino man hijacked a Philippine Airlines Airbus A330, relieved passengers of their cash, ordered the pilot to descend to 6000 feet and put on what appeared to be a home-made parachute before opening the rear door and jumping.
The aircraft landed safely and the hijacker - who blamed his distress on his wife's affair with a policeman - was never seen again.