Differential pressure is an important concept in aviation. Air pressure, of course, changes in relation to altitude. As the altitude of an airplane increases, the air pressure decreases. This is due to the fact that air molecules are spread farther apart at high altitudes, making the air less dense. Differential pressure, however, is a different type of air pressure that’s commonly used in aviation.
The Basics of Differential Pressure
Differential pressure is the difference between the air pressure inside of an airplane’s cabin and the air pressure outside of the airplane. It represents the difference between the air pressure pushing against the airplane’s exterior (atmospheric air pressure) and the air pressure pushing against the airplane’s interior (cabin air pressure).
Airplanes have pressurized cabins to provide passengers with a safe and comfortable flying experience. At sea level, the atmospheric pressure is about 14.7 pounds per square inch (PSI). At 30,000 feet, the atmospheric pressure drops to about 4.4 PSI.
Humans can’t breathe with such low air pressure, so airplanes pressurize their cabins. Differential pressure is simply the difference between the air pressure of the cabin and the atmospheric air pressure outside of the airplane. If the cabin pressure is 8 PSI and the atmospheric pressure is 4.4 PSI, for example, the differential pressure will be 3.6 PSI.
The Importance of Differential Pressure
You might be wondering why pilots and airlines even care about differential pressure. After all, it doesn’t change the cabin pressure, which is necessary for passenger safety and comfort. Well, differential pressure is important because it ensures the airplane operates within its structural limits.
Airplanes are designed with structural limits for differential pressure. Some of them can handle a maximum differential pressure of 7 PSI, whereas others can handle a slightly higher maximum differential pressure of 9 PSI. If the actual differential pressure exceeds this limit, structural damage may occur to the fuselage or other parts.
How Airplanes Regulate Differential Pressure
While they can’t change the atmospheric pressure, pilots and airlines can change the cabin pressure, which in turn allows them to maintain a safe differential pressure.
Airplanes are equipped with systems that regulate the cabin pressure and, thus, the differential pressure. Known as environmental control systems (ECSs), they leverage bleed air. Some of the air bypasses the engines’ core and is rerouted to the ECS. From there, the bleed air is used for various purposes, including regulating the cabin pressure.
