"Geoffrey Barnes" wrote

So, just how would the emergency exits get opened when the aircraft is
on the ground and needs to be evacuated, if the cabin pressure is
significantly higher than the outside pressure?

I'm sure that every aircraft has some differences, but the Boeing B-707
and B-727 that I flew worked as follows:

It is desireable to have "some" pressurization differential at takeoff
for passenger comfort. This prevents the outflow valves from slamming
shut at liftoff with the engines at takeoff thrust causing a discomforting
pressure "bump". For obvious reasons, it is also desireable to have the
cabin de-pressurize after landing.

To accomplish these two goals, the pressurization controller has a switch
labeled "takeoff" and "landing". Normally left in the "landing" position,
the outflow valves are electrically driven open. Just before takeoff, the
switch is placed in the "takeoff" position and the automatic features of
the controller closes the valves somewhat to cause no more than a .125 psi
differential. This pressure limit is an FAA aircraft certification limit
and is sometimes stated as "no more than 250 ft below field elevation". The
reason for the limit is obvious....to permit an emergency evacuation.
Sometime during flight, the controller switch is placed in the landing mode
and upon touchdown, the "squat switch" on the landing gear signals the
controller to open the outflow valves.

At a maximum of .125 psi differential, the air load on an average emergency
exit would be less than 100 lbs.

Bob Moore
ATP B-707 B-727
PanAm (retired)