Prepressurization during TO and LDG is equivalent to 50 ft to avoid pressure bumps.
And in school I had physic, and I learned something about closed systems.....
or in other words: when we carry birds on board and during TO they start to swing their wings, does the plane become lighter ? because the bird dont stand on the floor or heavier because the wings press the air to the ground ;-))
Neither, in a closed system the weigt is the same.
enjoy
