Suddenly there is a cracked window, and all the air in the airplane’s cabin is sucked outside, sometimes with a passenger or two. We’ve all seen this scene in Hollywood blockbusters. Did you like it ?

The air is sucked out (together with small objects like passengers) because the air-pressure in the cabin is under higher pressure than the air outside. As air tends to move from high pressure to low pressure, it is not possible to inflate tires with a knife, and if any breach opens in a pressurized airplane the air from the inside rushs out. You certainly know that. But do you know why the air inside is pressurized ?  No, this is not only for the fun of explosive depressurization.

The atmosphere is basically a layer of gas attracted by Earth gravity. When you sit on ground, the weight of the all layer presses on the air around you, making a pressure of about 0.9 to 1.03 bars. If you get at at altidude of 5′500 meters, there is less air above you, so less weight leading to less pressure. At 5′500 meters, the pressure is half what it is on ground level.

Our lungs are used to transfer oxygen (gaz) in our blood (liquid), and they are designed to work with air at a pressure of about 1.0 bar (ground level). If the air pressure is lower, it will be harder for oyxgen to get in our blood, and our organs will suffer from the lack of it. To make sure the blood of persons on board maintain an acceptable oxygen level while flying at high altitude, compressors are used to maintain an air pressure corresponding to an altitude of approximately 2′500 meters in the cabin.

If the pressure is lost for any reason, the well known “oxygen masks” will drop from the ceiling, distributing pressurized oxygen to the passengers. It is important to put your own mask quickly – even before helping others. At 37′000 feet (usual cruising altitude of commercial jets), you will loose consciousness within 20 to 30 seconds without oxygen ! This is also why pilots will dive quickly to lower level in case of depressurization.

In the next post about pressurization I will tell more about the effects of flying at altitude without pressurization or oxygen…