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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.

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    $\begingroup$ Related: Why is cabin pressure increased above ambient pressure on the ground?. The part you may have not understood could be "This ground pressurization of the cabin makes the transition to pressurized flight more gradual for the passengers and crew, and also gives the system better response to ground effect pressure changes during takeoff." (236 ft below the runway) $\endgroup$ – mins Jul 4 '17 at 17:34
  • $\begingroup$ @mins That answer has a lot of technical detail to get through along the way to that basic, conceptual answer. $\endgroup$ – Ralph J Jul 4 '17 at 18:06
  • $\begingroup$ @Ralph J can you please explain what is this pressure bump I have come across this term before but no one could exactly answer what it is? $\endgroup$ – Jai Jul 5 '17 at 18:10
  • $\begingroup$ @Jai It is a relatively small but very abrupt change in the cabin pressure. Closest analogy outside of pressurized environments would be a fast elevator that makes your ears pop. Noticeable, perhaps startling, and generally undesirable in the airlines world where passenger comfort is important. $\endgroup$ – Ralph J Jul 6 '17 at 4:36
  • $\begingroup$ I'm confused. Given that most airports are below 6-8K ft, wouldn't pressurizing the cabin on the ground actually mean depressurizing it somewhat? $\endgroup$ – jamesqf Jul 17 '17 at 18:38
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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.

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    $\begingroup$ It's also an opportunity to check that the pressure can actually increase and there are no leaks. $\endgroup$ – ratchet freak Jul 5 '17 at 12:42

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