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ATA 21 Air conditioning

ATA 21

Air conditioning.

The air conditioning system is provided to keep the environment which contain passengers or cargo as well as all other pressurized compartments at a pre-selected pressurization level, also the temperature, humidity and cleanliness is kept at a comfortable level for the passengers and crew. And this during the compete flight as well as on the ground. The air conditioning system also ventilates; the avionics compartment, the lavatories and galleys.

Where does the pneumatics come from?

The conditioning air is taken from the following possible sources :
The engine compressors (on the ground or in flight)
The APU, mainly on the ground. Due to the fact that the APU can’t be used at all flight levels usually. In case that the APU is inoperative there is a ground air supply unit connected to the plane, for engine starting mainly. Somewhere at in the belly surface of the plane there is a special connection build in. Here the ground compressor is connected with large long hoses to supply around 20 to 40 psi pneumatic air depending upon which airliner you talk about.

What does the air conditioning system do?

The air conditioning blows the air into the cabin after it is checked on temperature and pressure. The air conditioning system is also known as “the packs” being “pack #1” and “pack#2” or left and right pack. Eventually the conditioned air gets dumped overboard after passed trough the cabin creating a continuous air flow so their always fresh air coming into the airplane.

But it also keeps the cabin pressure inside the plane at a certain level, think of your popping ears!

How does the crew keep it so nice in the cabin?

To supply the passengers the highest possible comfort during flight all variations are controlled within the system. This can be done automatically, or manually.

What if the air conditioning system fails to operate during flight?

In this case the most airplanes have a device called a “RAM AIR INLET”, this is a little door somewhere in the fuselage, equipped with a actuator that will open/close the ram air door. Ram air is the air that due to the forward movement of the airplane will be “forced” into the intake. So in fact the ram air inlet can only be used during flight because of the movement required to “ram” the air inside the inlet. It will only be used in case of emergency.

Inside the electronic racks below the floor there is also ventilation provided to cool the electronic boxes. In case the plane is on the ground without any bleed supply from neither APU or engine or ground supply for that matter, the airplane is  sometimes equipped with another device called a ground blower, like the A300 I worked on. This will take over from the air conditioning system to cool the electronic boxes, because else they will overheat and possibly cause a fire down below.


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