Hi BOAC,
I concede that KBSen accurately states in post #8 that there is no pressurisation on the ground with packs off.
The avoidance of the Take Off "pressure surge" with Packs ON is accomplished by motoring the outflow valve away from its fully open position to a point where it maintains a differential pressure of 0.1 psi. Any movement of the valve from this point will immediately control the differential pressure. After lift off, the valve moves smoothly to control cabin ROC by increasing differential pressure.
Edit after reading AmpClamp below. This 0.1 psi diff pressure will prevent ram air entering the fuselage on rotation via the outflow valve. End Edit.
With packs OFF - the outflow valve will move to a fully closed position in an attempt to obtain 0.1 psi diff pressure. The differential pressure will build during the climb for the reason Field in Sight made in post #13.
Hi The Chiltern Flyer - you are spot on - but I guess you are also.
Last edited by rudderrudderrat; 27th September 2010 at 10:10.
Reason: extra explanation