explanation of a cabin pressure control system.

[LewisU_Pilot]

Can someone explain to me the basics behind the controls. Such as pressure regulator, pressure relief valve, vacuum relief valve, dump valve etc.. Dont have to go into too much detail just looking for a basic english explanation.

 

[avbug]

Cabin pressurization works by pumping air into the cabin to maintain a particular cabin pressure altitude. In order to make this work, the cabin needs to be relatively air tight, and strong enough to hold a minimum value of pressure. The part of the airplane that holds the pressure, which is reinforced and sealed, is called the pressure vessel.

To prevent the pressure vessel from popping or bursting when pressurized, some way must be designed to allow excess pressure out. This is accomplished in two ways. The first basic protection is called the cabin safety valve; this will vent at a given pressure, and bleed off the excess pressure. Then it will close itself. You can think of it as a sort of spring loaded safety cap; it vents until the pressure drops to a safe level, and then closes again.

The safety valve doesn't work strictly on pressure in the cabin, however. An important element to pressurizing the airplane is differential pressure. This means the difference between the pressure inside the pressure vessel, and the pressure outside. If the pressure inside is 5 psi, for example, we're not going to do any damage to the pressure vessel if the pressure outside is also 5 psi. The pressure inside and outside is the same, and therefore you have a 0 pressure differential. If the outside pressure is only 4.0 psi, then you have a 1.0 pressure differential.

The cabin safety valve works by venting at a particular differential pressure, rather than an absolute value. It compares the air pressure outside the cabin by the air pressure from within, and does it without any electronics or special equipment. Only when the pressure difference from the inside of the pressure vessel is great enough can the safety valve be forced open, and it should automatically close when the differential pressure is no longer great enough to hold it open.

The negative pressure relief valve works the opposite way; it's function is to prevent the pressure vessel from having an internal pressure less than that outside. The pressure vessel is designed to be strong when holding pressure inward, but not the other way around. An aircraft that has a lower cabin pressure at altitude might descend to sea level. If the cabin pressure altitude it maintained at altitude were seven thousand feet, and it's landing at sea level, this means that there would be less pressure in the cabin than outside; a negative pressure differential. The negative pressure relief valve automatically vents any excess pressure outside the cabin, into the cabin, thus relieving the negative pressure situation.

The most important valve from a pilot perspective is the outflow valve. As air is pumped into the cabin, to control the internal pressure, some way must be established of letting the air back out in a controlled method. Slow down the rate at which the air leaks out of the cabin, and the cabin differential pressure rises; the cabin pressure altitude drops. Open the controlled leak to a bigger setting, and more air leaves the cabin, and the cabin pressure altitude climbs as cabin differential pressure drops.

This valve is called the outflow valve, and it's controlled by a pressure controller. The pressure controller generally works two ways. One involves electronic solenoids at various points in the system that may seve to provide protection (by running the outflow valve shut in the case of too high a cabin pressure altitude), or control. The other method is usually employed when no power is available, and serves as strictly a manual method of controlling the aircraft cabin. It usually involves a rougher cabin control.

The cabin presure controller employes various reference lines that are vented inside and outside the cabin. These lines may be used to directly control the outflow valve, or to control valves that control the outflow valve. The outflow valve is opened and closed by air pressure at acts mechanically to control the valve. The airflow to do this usually comes from bleed air from the engines...the same bleed air used to pressurize the cabin. The bleed air is usually run through a small venturi device called a jet pump, and the airflow from this pump is often used to actuate the outflow valve. The outflow valve may have other methods of control, as well.

The pilot usually merely sets his or her cruise altitude in the pressure controller,and the controller does the rest. The pilot may also slect a particular differential pressure, having arrived at that value from charts and graphs supplied by the manufacturer. For takeoff or landing, one will typically set in the controller the takeoff or landing field elevation, plus five hundred feet, with the intend that the aircraft remains unpressurized for takeoff and landing. When beginning a flight, one looks at the departure elevation, the cabin pressure altitude expected at maximum differential pressure during cruise (based on anticipated cruising altitude), and the landing elevation. One then sets in the controller the highest cabin pressure altitude to be expected during that trip, and doesn't worry about it after that except to check the system to ensure it's working properly.