At cruising altitude, a passenger airplane may fly near 35,000 feet, where the outside air is too thin and cold for people to breathe safely. Cabin pressurization lets passengers and crew breathe comfortably even while the aircraft is high above most weather. Instead of making the cabin feel like sea level, aircraft usually maintain a cabin altitude of about 6,000 to 8,000 feet.
This balance protects people while reducing stress on the airplane structure.
Key Facts
- Atmospheric pressure decreases as altitude increases, so high-altitude air contains fewer oxygen molecules per breath.
- Typical cruise altitude: about 35,000 ft; typical cabin altitude: about 6,000 to 8,000 ft.
- Pressure difference = cabin pressure - outside pressure.
- Cabin pressurization is controlled by adding conditioned air and adjusting outflow valves.
- Density relation: ρ = m/V, where lower air density means fewer molecules in the same volume.
- Ideal gas law: PV = nRT, which links pressure, volume, amount of gas, and temperature.
Vocabulary
- Cabin altitude
- Cabin altitude is the altitude that has the same air pressure as the air inside the aircraft cabin.
- Pressurization
- Pressurization is the process of keeping the air pressure inside the cabin higher than the outside pressure at altitude.
- Bleed air
- Bleed air is compressed air taken from an aircraft engine and used as a source for cabin air after cooling and conditioning.
- Outflow valve
- An outflow valve is a controllable opening that lets cabin air leave the aircraft to regulate cabin pressure.
- Differential pressure
- Differential pressure is the pressure difference between the inside of the cabin and the outside atmosphere.
Common Mistakes to Avoid
- Thinking the cabin is pressurized to sea-level pressure, which is wrong because most aircraft use a higher cabin altitude to reduce structural stress.
- Ignoring the outflow valve, which is wrong because cabin pressure is controlled by both incoming air and how quickly air is allowed to leave.
- Assuming oxygen percentage drops at altitude, which is wrong because the percentage of oxygen stays about 21 percent but the total pressure and oxygen molecules per breath decrease.
- Confusing altitude with pressure, which is wrong because higher altitude means lower pressure, not higher pressure.
Practice Questions
- 1 An airplane cruises at 35,000 ft while its cabin altitude is 8,000 ft. What is the altitude difference between the airplane and the pressure felt inside the cabin?
- 2 If cabin pressure is 75 kPa and outside pressure is 25 kPa, what is the differential pressure across the fuselage?
- 3 Explain why a cabin pressurization system must control both incoming air and outgoing air instead of only pumping air into the cabin.