Flying is stressful for a lot of reasons. From long queues at the airport to screaming children on the plane, flying pushes our buttons and makes us stressed. If this wasn’t enough, airplanes are a standard movie setting for when something goes wrong. Media coverage of aircraft incidents and terrorism have exacerbated people's fears of flying, and we often look at our fellow passengers like they might snap and try to unlock one of the doors, dooming us all.
So what would happen if you tried to open the door of a plane while the aircraft is in flight? Nothing. Well, you would be restrained by flight attendants and arrested as soon as the plane lands, but nothing would happen to the aircraft. Why? Because you wouldn't be able to open the door. The pressure would stop you from doing so.
“Think of an aircraft door as a drain plug, fixed in place by the interior pressure," pilot Patrick Smith blogged on the subject. "Almost all aircraft exits open inward. Some retract upward into the ceiling; others swing outward, but they open inward first, and not even the most musclebound human will overcome the force holding them shut."
The pressure inside the cabin is much greater than the outside. So you would need to be able to pull the doors with a superhuman amount of force. At cruising altitude, the inside of the cabin is at a pressure of about 0.7 or 0.8 atmospheres, which is equivalent to about 5-5.4 kilograms (11-12 pounds) of force per square inch. This is what you are trying to overcome!
Now, you might want to know what happens at lower altitudes. Again, this is the science so please don't take it as a challenge. Even a difference of 15 percent of standard atmospheric pressure would be too strong for humans to overcome. Doors can only be opened when near the ground. And that means very near. In case of an emergency during takeoff or landing, the doors should open easily and deploy the inflatable slides.
If you can open the door in the air, you have bigger problems.