U allway's hear in movies that airplanes are pressurized, this makes sence because airpressure decreases the higher u get, but what about airconditioning? Why is that important?

Airplanes operate at a high altitude wich means that the air pressure is lower and it is very cold. So cold in fact that a human being could not possibly survive there. Knowing this we can look at how an airplane creates a liveable environment inside the cabin and we see the importance of proper airconditioning. The air not only needs to be kept at pressure but also must be refreshed, imagine a 100 people breathing the same air over and over again during the whole flight. This refreshing of the air is done by two systems, the airconditioning and the air cycle machines.

If the airpressure at high altitude is so low, where does the airplane get the pressure needed to keep the cabin pressure up and refresh the air inside the cabin? The answer is even easier then the question, the engines.

Engines provide more than thrust, they provide the aircraft with electricity, pneumatic's and hydraulic pressure too.

The pnuematic's that the engines provide is tapped from the compressor. Two different compressor stages are used wich both differ in pressure and temperature wich are used alone or mixed depending on the situation. (information on how this is done is widely available on the internet but I will now only talk about the basic principles).

Both these pneumatic air sources are very hot coming straight from an engine. The hottest one being the closest to the combustion chamber (hottest due to higher pressure). These pneumatic sources are then routed through a pressure regulating shut-off valve wich keeps the air pressure at around 35 psi on most aircraft.

This hot air is used for starting engines, de-icing, putting a head pressure on the hydraulic tanks and on the potable water tank.

There is another jet engine on larger aircraft wich is called the A.P.U. (Auxilliary Power Unit), this is a small engine usually mounted in the tail of the aircraft wich also can supply the aircraft with pneumatics and electrics but for now I will talk only about the main engines.

There are many different airconditioning systems available but all rely on the same basic principle.

The air is offered at around 35 psi it is a very hot source so it must be cooled. An air cycle machine (ACM) cools the air for the cabin. This ACM works just like any airconditioning. pressurize the air (air gets hotter) cool the air (wich cools much faster when the air is very hot) then expand the air (releasing the energy that was put in by pressurizing the air in the first place) and colder air comes out. ofcourse the air u used to cool the high pressure hot air goes right overboard like any house or car airconditioner does. On aircraft this cooling air is taken directly from outside so it is allready very cold. but it is not the air used for the cabin that air came from the engine.

Then this cooled air will enter a cool air duct and is usually mixed in a mix manifold with hot air coming directly in the airconditioning pack. Having this, we can supply the aircraft with pressurized and comfortable air and because we mix it on demand we can select how hot or how cold we want it per section of the aircraft.

If aircraft are pressurized, why do I feel my ears pop when we go at higher altitude?

This is because aircraft are not pressurized to the same pressure as take off. The differential pressure between the inside and the outside of the aircraft is kept minimal. This means that the actual cabin pressure decreases when the aircraft climbs (when the cabin pressure decreases the cabin altitude will increase). The cabin pressure is controlled by the cabin pressure controllers wich modulate a outflow valve. Air is pumped into the cabin at a regulated rate and at a regulated temperature. The pressure is now controlled by opening a modulating valve wich can be commanded to more close when the pressure gets too low and more open when the aircraft needs to get rid of some pressure.

U may wonder why, isn't it possible to just keep the cabin pressure to a comfortable ground level?

Sure it is possible but an aircraft is a metal cilinder and a metal cilinder that expands and retracts many times get's fatigued. Therefore the pressure differential is limited to the airframe structure, if the aircraft manufacturer chose to keep the pressure at sealevel then the fuselage would have to be much heavier wich translates to more weight wich in turn translates to more fuel and fuel is money.

We've now established that the air used for airconditioning comes from the engines but anything that's extracted from the engines costs energy, just like in a car with the headlights on, the radio playing and the airconditioning full cold, it consumes more fuel....

This is why in airplanes we use recirculation fans. There are minimum requirements for fresh air per minute per passenger, the airconditioning packs are usually selected to a 'low flow' mode to save fuel. In this low flow mode the fresh air rate is achieved but not the minimum for odors and bacteria's fresh air rate. These recirculation fans re-use air from the cabin, it is sent through filters and mixed with fresh air coming from the airconditioning pack in the cabin, this way the minimum airflow is compliant for both requirements and hence we need less air from the engines and so burn less fuel.

All filters are replaced at regular intervals ofcourse and during the S.A.R.S. period these recirculation fans were prohibited to be used in asian countries (so packs were used in 'high flow' mode to comply with the requirements).

The max differential pressure differs from airplane to airplane but mostly it's around 7 to 8 psi difference wich doesn't seem like much pressure but when u consider the volume of air it has a lot of power.

Aloha airlines suffered an explosive decompression and it ripped the forward top of the fuselage open, the flight attendant that was working there got sucked into the opening and the whole forward fuselage exited the aircraft including the flight attendant.

Back to:

How do aircraft work ?