Originally Posted by
SBA320
Greetings everyone,
I have a question which I feel only this website can answer.
I was going through the A320 FCOM and was in the pressurisation chapter.
As I was reading about its pre-pressurisation function, the FCOM went on to say that the system on the take off roll pressurises the cabin at a rate of -400 fpm.
Is there someone who could help me out to understand why that happens. If the system maintains cabin rate of -400 fpm, the pressure differential would increase and make the job of the pressurisation system harder and make it more uncomfortable for the passengers.
As always, thank you.
FCOM DSC 21-20-10
TAKEOFF (TO)
To avoid a pressure surge at rotation, the controller pre-pressurizes the aircraft at a rate of 400 ft/min, until the ΔP reaches 0.1 PSI.
At liftoff, the controller initiates the climb phase.
As the vast majority of operators use Packs OFF takeoffs this pressure surge is avoided by selecting Packs ON after takeoff with a 10 seconds delay between each other.
Originally Posted by
SBA320
If the system maintains cabin rate of -400 fpm, the pressure differential would increase and make the job of the pressurisation system harder and make it more uncomfortable for the passengers.
Actually it is quite the opposite. The job of the pressurization is to increase the DeltaP as the airplane climbs and to decrease it steadily as the airplane descends. With a pre-pressurized airplane the job is already started. The reason why You have a negative V/S is because during the takeoff run the airplane is not climbing, i.e. airplane v/s is zero, so the only thing the cabin can do is to descend to increase the DeltaP.