I know this question has been asked, but I still haven't understood the answer. In the A320 the cabin gets pre pressurised before takeoff as we apply takeoff thrust. What I don't understand is what is the exact reason behind this. A little technical answer will be much appreciated.
In general, you want the cabin to be
- pressurized in flight, and
- not pressurized on the ground
If the system followed only & exactly that logic, at the moment of liftoff it would start pressurizing immediately, with the goal of overcoming the sudden change in pressure due to the aircraft's rapid climb away from the ground, but not more than that so that you don't pressurize below ground level all at once. In practice, that ends up being a pressure "bump" right at (or slightly after) rotation, which is undesirable since it is at least noticeable and potentially uncomfortable or startling.
To prevent that pressure bump, the pressure controller is programmed to slowly pressurize the cabin slightly while the aircraft is still on the ground, so that at liftoff it is essentially maintaining the differential it has, rather than starting pressurization all at once.