Grades 6–8 · Systems
How does an airliner maintain cabin air high above Earth?
At cruising altitude, the outside air is at much lower pressure than the air near sea level. Its oxygen fraction is still about the same, but each breath at that low pressure would contain fewer oxygen molecules. An airliner therefore maintains a higher pressure inside its cabin.
In the diagramCabin pressure needs managed flowcabinoutside aircompressfilter + recirculateoutflow valveSupply and outflow regulate pressureCabin air mixes fresh supply with filtered recirculated air.
The aircraft supplies compressed outside air and conditions it before distribution. Many aircraft take this supply from engine compressors; others use electrically driven compressors. Pressurizing air does not mean burning fuel inside the cabin or adding pure oxygen for normal breathing.
Air continually enters and leaves. Controlled outflow valves regulate how quickly it escapes, helping set cabin pressure. Some cabin air is also recirculated through filters and mixed with the fresh supply. Recirculation and pressurization perform different jobs.
The cabin is not normally held at exactly sea-level pressure. A controlled pressure difference limits structural loads while keeping conditions suitable for the flight. The aircraft shell, air supply, outflow control and monitoring all contribute; sealing the doors alone would not provide continuing ventilation.
About this explanation
Starting knowledge: Understand air pressure, gas flow and the need for oxygen.
Sources
- Federal Aviation Administration — Cabin Air Quality Educational Materials