Originally Posted by CPRich
In motion? Yes, I believe the procedure for opening the door at the gate will work while taxiing, and potentially during early climb.
Once above an altitude equal to the pressure inside the plane (8000 ft, iirc), the post above is correct about pressure differential preventing opening.
Depends on the airplane. For example, the Boeings are pressurized to 1/8 psi for takeoff and landing, which means that sometime during taxi-out the cabin becomes slightly pressurized and isn't depressurized (at least on purpose) till sometime during taxi-in.
That doesn't sound like much, but on a door that's roughly 30" x 70" (~2100 sq. in.), that works out to about 260 lbs of force holding the door closed.
That's why the first item on the evacuation checklist is to depressurize the airplane.
Once the plane starts climbing, the cabin pressure also starts to increase. So the force holding the door closed starts to increase immediately also.
Jim