Why do we need Pressurisation?
As we climb higher, air molecules are spread farther apart. When we breathe, our lungs take in less air, and less oxygen.
At 18,000 feet, the atmospheric pressure is down to 7.3 psi, about half the sea-level pressure. There just isn’t enough oxygen in a breath of air to adequately supply the brain. At this pressure, a healthy adult has only 20-30 minutes of useful consciousness.
Airliners fly between 30,000 and 43,000 feet. At those altitudes the atmosphere provides less than 4 psi of pressure. If you tried breathing at that altitude, your useful consciousness would be less than a minute (followed soon after by death).
To survive at high altitudes, occupants of an aircraft need help in breathing. The solution is to pump air into the airplane so the interior pressure is high enough to keep the humans happy.
As we climb in altitude, the amount of air pressure acting on us decreases rapidly. You notice the decrease when your ears pop while driving up a mountain or riding a fast elevator. Although the atmosphere is 300 miles thick, most of the air molecules are squashed down to within a few thousand feet of the earth’s surface.
Airplanes can certainly fly below 10,000 feet where the atmospheric pressure is a comfy 10 psi or higher, but it has some drawbacks:
- It’s tough to cross a 14,000 foot mountain range if you fly at 10,000 ft.
- Bad weather is generally at lower altitudes.
- Turbofan engines are very inefficient down low.
- Aircraft ground speeds are slower at lower altitudes.
If you want a fast, smooth ride in a fuel efficient airplane that can fly over a mountain range, then we need to pressurise!
The airplane body (fuselage) is a long tube capable of withstanding a fair amount of differential air pressure; think of it like a big plastic soda bottle. In theory, we could seal the bottle so, as the airplane climbs, the interior air pressure would stay the same. However, we can’t do that because it’s hard to perfectly seal a huge airplane fuselage. Even if we could, the passengers would quickly use up the available oxygen, and just imagine the smell inside a perfectly sealed tube on a long flight! Clearly, a big sealed soda bottle won’t work for us without some modification.
A fuselage is a bit like a soda bottle with a hole in the back.
To solve the problems, pressurisation systems constantly pump fresh, outside air into the fuselage. To control the interior pressure, and allow old, stinky air to exit, there is a motorised door called an outflow valve located near the tail of the aircraft. It’s about the size of a briefcase and located on the side or bottom of the fuselage. Larger aircraft often have two outflow valves. The valves are automatically controlled by the aircraft’s pressurisation system. If higher pressure is needed inside the cabin, the door closes. To reduce cabin pressure, the door slowly opens, allowing more air to escape. It’s one of the simplest systems on an aircraft.
Pressurised areas of A320:
- Avionics Bay
- Cargo Compartment
The Cabin Press System has four general functions:
- Ground function: Fully opens the Outflow Valve on ground
- Pre-pressurisation: During T/O it increases the cabin press(0.1PSI) to avoid sudden surge in cabin pressure during rotation
- Pressurisation: Adjusts the cabin altitude, and rate of change to provide pax with comfortable flight
- Depressurisation: After touchdown, gradually releases the residual cabin overpress before the Grund function fully opens the outflow valve.
The System consists of:
- Two Cabin Press Controllers (CPC’s)
- An Outflow valve with an actuator that incorporates the three motors (two for automatic operation, one for manual operation)
- One control panel
- Two safety valves
- One RPCU
The Outflow valve is on the right side (f/o side) of the fuselage, behind the AFT cargo compartment and below the floatation line.
It is an inward and outward opening flaps linked to an electric actuator and this actuator contains the drive for the two automatic electric motors and one manual motor.
An Outflow valve is used to regulate the amount of air allowed to escape from the pressurised areas.