Robert Moore wrote in message .6...
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

Thanks for the excellent explanation, Bob. Back when I had the
misfortune of having to fly commercially, I used to notice that my
altimeter watch would drop about 200 ft. as we taxied away from the
gate. I always wondered how that would impact getting the door open
in case of emergency.

John Galban=====N4BQ (PA28-180)