When flying, many passengers assume the atmosphere in the cabin is the same as the atmosphere on the ground. After all, you can breathe normally and there’s really no noticeable difference other than dryness (cabin air has low humidity). Airplane cabins, however, are pressurized to achieve a specific cabin altitude.
Cabin Altitude Explained
The term “cabin altitude” refers to the pressure level of an airplane’s cabin, expressed as the equivalent above-sea-level altitude. It’s not the actual altitude of an airplane. Rather, cabin altitude represents the pressure level inside of an airplane’s cabin.
Cabin altitude is somewhat confusing because, unlike other pressure-based measurements, it’s not expressed in pounds per square inch (PSI). Cabin altitude is expressed in feet.
Most commercial airliners have a cabin altitude of 6,000 to 8,000 feet. A cabin altitude of 6,000 feet indicates the cabin is pressurized to the equivalent of 6,000 feet above sea level, whereas a cabin altitude of 8,000 feet indicates the cabin is pressurized to the equivalent of 8,000 feet above sea level.
The Importance of Cabin Altitude
You might be wondering why cabin altitude is important. After all, couldn’t airplanes just maintain sea-level cabin pressure?
While passengers would be able to breathe comfortably with sea-level pressure, this isn’t feasible. As airplanes climb higher in the atmosphere, they are exposed to thinner, less dense air. The difference between the high-pressure cabin atmosphere and the low-pressure exterior atmosphere would place significant stress on the airplane’s body and structures. To preserve the structural integrity of airplanes, engineers specify cabin altitudes.
Other Types of Altitude Used in the Aerospace Industry
Cabin altitude is just one type of altitude used in the aerospace industry. Another common type is true altitude. True altitude represents the vertical height of an airplane in flight relative to the mean sea level (MSL) or the average height of the oceans. Indicated altitude, on the other hand, represents the altitude of an airplane as shown on an altimeter when set to local barometric pressure. There’s also absolute altitude, which is the vertical height of an airplane above ground level.
In Conclusion
Cabin altitude plays a crucial role in how airplanes keep passengers safe and comfortable at cruising heights. While it might feel like you’re breathing air at ground level, you’re actually in a cabin pressurized to simulate conditions much higher — usually between 6,000 and 8,000 feet. Cabin altitude represents this simulated atmosphere in an airplane’s cabin.
