At some point or another, most frequent flyers have experienced some unease at the idea of someone opening the aircraft doors half way through a flight, and sucking them out. This is, to a certain extent the fault of the film industry, which frequently portrays misleading scenes in which the doors are shown to open with relative ease whilst the plane is 7miles or so up in the sky. Even watching someone leaning against an aeroplane door can be enough to make people nervous about flying. The fact of the matter is, however, that it is not possible for a person to open the door of a commercial aeroplane once it's airborne.
The main "cabin" doors on most commercial aeroplanes are classed as plug-type, which means that in the closed position the door is physically larger than the external aperture of the opening. Whilst this may, at first, seem to suggest that the door would be permanently inoperable, it should be noted that the doors are able to open and close by entering and leaving the opening at a slight angle. Generally, the doors are opened by rotating a handle, and pulling the door inward slightly first. The door can then be swung outward, either sideways or vertically depending on aircraft model, in such a way as to allow it to pass through the smaller opening and open fully. Likewise, using the reverse technique the door can be closed.
This simple mechanism means that no matter how hard someone pushes against the door, they would be unable to open it, as they wouldn't be able to force it through the smaller opening. For similar reasons, it would be impossible to push out the emergency exit doors/windows too, as these are generally designed to open inwards and have openings that are smaller than the door/window itself.
The doors being larger than their openings is, however, only one reason why they can't be opened mid-flight, and it may lead to the idea that they could simply be opened by pulling them inwards. This isn't, however, the case.
Commercial aeroplanes, tend to fly at cruising altitudes around 12,000m (35,000ft), where the outside air is very thin. In order to overcome this and allow passengers to breathe normally, the cabin is pressurized. This higher cabin pressure means that if an opening occurred in the fuselage, such as if a door were "theoretically" opened, the rush of air would be strong enough to suck people out.
In practice, however, it's due to the large pressure difference between the pressurized fuselage and the outside air, that the doors would be un-openable. As has already been stated, emergency doors/windows almost always open inwards, and main "cabin" doors need to initially open inwards before they can be rotated outward. The large pressure differential present, however, creates an outward acting force on the doors of such magnitude that even a very strong person, or even several would be incapable of pulling inward sufficiently to overcome it and allow the door to open, even if they rotate the door handle.
In other words, the doors are not only larger than the door opening, but they are also forced and sealed shut by an outward pressure caused by the pressurized cabin. As the cabin only becomes pressurized when the aircraft is moving, the doors are openable with relative ease when the plane is parked at an airport, as there is little or no pressure difference.
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