SOME WORDS OF THEORY :
1) The atmospheric pressure decreases as the altitude raises. This has been carefully measured and the following curve represents the pressure evolution according to the altitude:
Accordingly a particular pressure corresponds to a specific altitude.
2) Human body needs a minimum air pressure level. Moreover to feel comfortable the air pressure needs to remain to a minimum value in the area of the atmospheric pressure at 8000 Feet ( 2438 meters). This is only the height of a medium size mountain.
3) An aircraft cabin is similar to a closed volume. The cooling air required for passenger's comfort and breathing, is injected inside this closed volume. Accordingly, it is necessary to control and maintain the inside pressure at the required level. This is done through a kind of tap which is carefully opened or closed by an automatic system: the cabin pressure control system.
HOW DOES THE CABIN PRESSURE CONTROL SYSTEM WORKS:
The purpose of the pressurization control system is to fully automatically control the pressure in the fuselage . The pressure level and change rate are controlled to provide satisfactory pressure values for comfort and safety for all the passengers and crew.
This is made by the control of the quantity of air that flows out of the fuselage through one or several outflow valves ( " the taps ") installed on the fuselage surface or on the aft bulkhead as on the following figure.
These outflow valves are controlled on modern commercial airliners by computers. These computers measure the outside pressure (at the altitude where the aircraft flies ), the current pressure inside the fuselage and then drive the opening or closing of the outflow valves according to programmed laws to get the proper pressure inside the fuselage.
These laws take account of the flight phase : take-off, climb, cruise, descent, landing, takiing. The general system behaviour is as per the following scheme :
As the aircraft climbs, the pressure inside the cabin generally decreases at a lower speed than outside.
What is important during this phase is the pressure rate of change, this is controlled in reasonnable limits for the passenger's comfort by the pressurization system which mainly takes account of the aircraft vertical speed . If a too large deviation from the required pressure rate of change occurs in the cabin , this may have an important and direct possible consequence : ear-ache
The human physiology dictates that the pressure rate of change must not exceed 18 mbar per minute during a climb phase. This speed allows air to flow inside human head (nose, throat, internal ear ) so that we may have the same pressure on each side of our ear drum. For example, if the aircraft climbs too fast for the cabin pressure control system, or if this system makes the pressure in the cabin to change at a too high speed there will be a pressure difference between both sides of the ear drum : ear-ache.
Then, when the cabin pressure decreases down to the pressure equivallent to an altitude of 8000 feet , the cabin pressure control system will keep this pressure in the cabin at this selected level during the remaining climb phase and the complete cruise phase.
All previous informations provided are given in the background of the passenger's comfort. This fuselage pressure control system is also an important aircraft design criteria as the aircraft structure design must take account t of he pressure loads generated by the fuselage pressure differences between the outside and the inside of the cabin .
It can be understood from the hereabove flight profile curve, that for example, during cruise, there is a differential pressure between the outside air and the inside of the cabin. The fuselage is consequently inflated, resulting in mechanical pressure loads on its structure.
These repeated variations generate another kind of loads : fatigue strength.
MAIN SITE NAVIGATION :
bleed air system / air cooling system / zone temperature control system / cabin pressure control system