- not really! KBPsen in post #8 hit it fair and square.

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.

BOAC you are correct in that he/she answered the original question.
No packs no pressurisation, full stop.

CJ driver.. from the Boeing 737 engineering training notes...abbreviated to a degree.

flt/gnd s/w to flt, OFV driven partially closed to take cabin to -189' .As aircraft taxis and takes off cab press is held at -189' .As airplane rotates the OFV is nearly closed.Ram air effect on the valve will not cause a pressure bump.(a sharp rise in cabin pressure).

I posted -387' in a prior post.My error. I did the course well over 20 years ago and memory failed but the principle is well remembered.

From the A320 eng course notes...
Cabin is pre pressurised as power is set for t/off to prevent pressure surge at rotation.

Note that both mention rotation not weight off wheels.The surge being prevented is from a change in attitude and ram air effects & airflow at the OFV.

bio161 further to the a320 question. as answered no pre press if no packs on but the 320 (older ones do anyway) have a ram air inlet purely for ventilation in the event of dual pack failure.operated by a s/w in the cockpit.Aircraft must descend to -10,000', when diff pressure is less than 1psi the OFV will open allowing ram air in to ventilate the cabin.

rrat- the reason for my pointing out the succinct answer at #8 was that it was almost instantly obvious that the OP had very little understanding of aircraft pressurisation - for everyone to launch off into complicated explanations of systems is all very interesting, but wasted on the OP. To me the question 'how can an aeroplane pressurise with the packs off' is a pretty broad hint.

This is a recurring event here - we recently had a question from someone who all could see plainly had not much idea of the effect of a moving air mass on an aircraft in flight but was presented instantly with a trigonometric solution to the query about the effect of wind on a curved flight path, when all that was needed was 'do you actually understand drift'?

bio and fred were plainly 'not understanding' hence my early replies to probe the level of knowledge.

Hi BOAC,

fred's knowledge has been probed before - he has very little experience but is extremely keen and able to point to FCOM references. We all started with a confused understanding. If they are brave enough to ask on an open forum, then I think they deserve some help.

Field in Sight explained very well how it is possible in post #13.