How Aircraft Cabins are Pressurized

A Pennsylvania State University graduate, John Moss Wathen IV holds a BS in aerospace engineering and has completed projects such as the design of model aircraft. The interests of John Wathen IV include the equipment and aerodynamics of aircraft.

One of the essentials of flight at high altitude is cabin pressurization, which keeps the interior pressure the same as at sea level. Pressurization allows the plane to cruise at an altitude where the air is smooth, thin, and ideal for an uninterrupted fight.

In an earlier era, piston-powered commercial aircraft used air compressors to pump air from outside into the cabin. Compressors added significant weight and were discontinued with the advent of jet engines that draw in fresh air through spinning blades. This air is compressed at an extremely high temperature, at which point it combines with fuel in the combustion chamber of the engine. The expanding gasses from this process are expelled through the turbine blades and deliver thrust as they leave the engine.

“Bleed air” is hot, clean air removed during engine compression before mixing with fuel or exhaust gasses occurs. It’s used for pressurizing cabins as well as ice protection for the engine and wing. It also provides power to engine starter motors and air-driven hydraulic pumps. The bleed air that passes into the cabin is first cooled by an intercooler (which functions similarly to a car radiator). This connects to a refrigeration unit, or air cycle machine, which brings the air to the correct temperature for passengers in the plane.