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.
Understanding Aviation: Cabin Pressurization
The cabin air system begins with a source of compressed air. On many aircraft, this air comes from the engine compressor stages. It is very hot at first, so air conditioning packs cool it, remove some moisture, and mix it with recirculated cabin air.
Modern aircraft may use electric compressors instead of engine bleed air. Filters clean recirculated air before it returns to the cabin. Fresh air is supplied continuously, while a controlled amount leaves through outflow valves near the rear of the fuselage.
The cabin is therefore not a sealed container. It is a carefully managed flow system.
An automatic controller moves the outflow valves to follow a pressure schedule. During climb, the system raises cabin pressure gradually. A change that happens too quickly can make ears painful.
During descent, it lowers cabin pressure at a controlled rate so the pressure inside and outside the aircraft become closer before landing. The controller must account for the aircraft altitude, rate of climb, and the target landing airport elevation. Pressure matters because gas molecules push on every interior surface.
The ideal gas law says pressure times volume equals the amount of gas times a constant times temperature. In a fixed cabin volume, changing the amount of air or its temperature changes the pressure.
The fuselage must carry the force caused by the difference between cabin pressure and outside pressure. This force acts across a very large area, so even a moderate pressure difference creates large loads. Aircraft bodies are shaped like tubes because curved surfaces spread these loads more evenly than flat panels.
Repeated flights create repeated pressurization cycles. Over many cycles, tiny cracks can begin around windows, doors, rivet holes, and other openings.
Engineers inspect these areas closely and set limits on the life of parts. This is one reason aircraft maintenance is based on flight cycles as well as flight hours.
For passengers, the important body effect is oxygen partial pressure. Air still contains roughly the same fraction of oxygen at altitude, but each breath contains fewer oxygen molecules at lower pressure. Less oxygen can move from the lungs into the blood.
Early hypoxia may cause poor judgment, tiredness, headache, or slower reactions. A sudden loss of pressure is more serious. Oxygen masks can drop automatically when cabin altitude rises too high.
Passenger masks provide enough oxygen for a descent to a safer altitude, not for many hours of flight. When learning this topic, separate oxygen supply from pressurization.
Oxygen masks help people breathe for a short time. Pressurization protects the whole cabin environment and reduces the physical strain on people during normal flight.
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.