Can a plane door always be opened from the inside when on the ground?
The doors of the plane can almost always be opened from the inside, when the aircraft is on the ground. However after engine start on most aircraft types with pressurized cabin, the cabin starts to pressurize. Opening of an exit door may therefor be prevented by outflow valves and the door is locked by suction. To further explain this, outflow valves is used to control the pressure in the cabin. As altitude increases, pressure decreases, and the outflow valves are turned to close a little, to hold a cabin altitude of approx. 8000ft. This means that the cabin is pressurized, and therefore the doors cannot be opened mid air. It is the same case, if the cabin is pressurized on the ground. You will have a higher pressure in the cabin, than the air pressure outside. In these conditions the doors will not open and there could also be a function of a door locking mechanism which is also activated after engine start before takeoff. Of course if an evacuation is needed before takeoff, the pilots will shut down the engines which will start a quick process of depressurizing the cabin and you could then open the door. Also after landing, even with engines running, the compression of the main landing gear will typically make the out flow valves fully open and thereby equalize the pressure allowing you to open the door.
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