You cannot de-pressurize an aircraft below the outside pressure for a departure. There is no mechanism for sucking air out of the cabin. The only thing you can do is blow air into the cabin and let air leave the cabin on its own.

The cabin air pressure depends on the rate at which air is being pushed into the cabin and the rate at which air is allowed to leave the cabin through the outflow valves. More air coming in than going out means increasing air pressure, and less air coming in than going out means decreasing air pressure.

I believe you could have lower cabin pressure than outside pressure when landing, if you close the outflow valves when descending. It shouldn't be a big difference -- maybe 0.25 psi.

I suspect that if pressurization begins before take-off, the pressure differential is very small. Maybe it has something to do with conserving engine power during take-off.


About a year or so ago, an AA (A300?) landed at MIA. The FA opened the door too soon, and the plane suddenly depressurized. He was thrown out of the plane onto the ground, and I think he was killed.

[This message has been edited by JS (edited 07-04-2002).]