When you think of passenger airline travel, it’s impossible to imagine it happening in an unpressurized cabin. Even more than being able to fly long distances at great speed, pressurization has changed the face of traveling by air for the general public. Although generally unavailable in entry-level general aviation airplanes, many large piston twin-engine GA airplanes and virtually all jets and turboprops incorporate a pressurization system.
A cabin that is pressurized has a higher air pressure than the outside ambient pressure, and therefore feels lower to the passengers and crew. This is done quite simply by pumping lots of air into the cabin with bleed air from the compressor section of turbine engines, superchargers (older jets), or flow limiter (piston), and then allowing it to escape through an outflow valve. This outflow valve ensures the inside pressure doesn’t become too great, and thus cause structural damage to the fuselage. As altitude increases and the outside ambient pressure decreases, the air source must work harder to pressurize the cabin. At a certain point, it will no longer be able to keep up, and the cabin altitude will not increase beyond a certified maximum pressure differential, which corresponds to a cabin altitude (usually somewhere around 8,000 feet).
Further complicating the system is the fact that the heating and air conditioning system often is integrated with pressurization. The bleed air from turbine engines is obviously hot, so it is run through a heat exchanger and mixed with cooled air to keep the cabin a comfortable temperature.
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