Steve Dirickson writes after me:
>> Generally in pressurized aircraft,
>> it's not possible to open an emergency exit in flight because
>> all the exit doors and windows open "in" and are held in place
>> by the air pressure differential.
>
> Say what? The forward or aft entry door of every 7x7, MD80, and other
> pressurized-cabin plane I've ever walked into was opened and lying back
> against the outside of the aircraft as I went through it.

If you watch the opening sequence of these doors, you'll see that the first
motion of the door is inward, after which they rotate slightly and pass back
out of the door frame.

I don't know that all airliner doors use the pressure differential as an
in-flight safety measure but all over-the-wing window exits obviously do,
and most cabin doors that I've observed certainly appear to work this way.

I believe this kind of door is generally referred to as a "plug type".

Clearly you want these doors to be impossible to open in flight, mostly
*because* of the pressure differential, so using that differential force to
oppose the opening of the door seems logical, especially since you have to
oppose that force anyway to keep the aircraft from coming apart.

Even at only 1500 feet alititude where the pressure differential is only
about 0.3 psi, this will still result in 900 pounds of force on a typical
747 door.  At higher altitudes, typical pressure differences get up to
around 8 psi, and that same door will then feel a force of around 24,000
pounds, at which point the phrase "sucked out" will be more than appropriate
should the door fail.

G.