How is the air pressure maintained inside airplane cabins?

In the 1930s, aviation giant Boeing introduced the revolutionary Model 307 Stratoliner, which featured an advanced cabin pressure system. This innovation allowed the aircraft to cruise above the weather, at higher altitudes, while ensuring passengers and crew could breathe comfortably despite the thinner air at 20,000 feet (6,096 meters).

Since then, cabin pressurization has become a standard technology, one that most of us likely overlook when flying.

Airplane Cabin Pressure Systems Are Simple

Cabin pressurization functions so seamlessly that passengers hardly notice it. This is because it gradually adjusts the cabin pressure as the plane ascends, and again as it descends, explains Chuck Horning. He has been an associate professor at Embry-Riddle Aeronautical University in Daytona Beach, Florida, since 2005, and previously worked as a mechanic and instructor at Delta Airlines for 18 years.

"It’s not a particularly complicated system," says Horning, who notes that while the technology itself hasn’t changed much over the years, the introduction of computerized controls has improved its accuracy. The airplane uses some of the excess air drawn in by its engine compressors. "The engines don’t require all that air for combustion, so some of it is diverted for both air conditioning and pressurization."

The excess air from the compressors is cooled and pumped into the cabin. This process is controlled by an air cabin pressure controller, which Horning calls "the brain of the pressurization system."

"This controller automatically manages the pressurization," explains Horning. "It receives data from the flight crew regarding the cruising altitude and adjusts the pressure accordingly as the plane climbs and the outside air pressure decreases."

Over-pressurizing an aircraft can place excessive stress on its fuselage due to the difference in pressure as the plane ascends, says Horning. To prevent this, airliners don’t aim to replicate sea level pressure. Instead, at a cruising altitude of 36,000 feet (10,973 meters), most commercial jets simulate the pressure found at an altitude of 8,000 feet (2,438 meters), roughly equivalent to that of Aspen, Colorado.

The Boeing 787 Dreamliner, with its ultra-durable carbon fiber airframe, is able to maintain cabin pressure equivalent to that of 6,000 feet (1,829 meters). "This is beneficial, as higher cabin altitudes result in less oxygen in the bloodstream," explains Horning. "That’s why you may feel fatigued after a flight."

The amount of air needed for pressurization depends on the cabin volume, says Horning. Since the pressurization system works in tandem with the air conditioning system, it continuously circulates air through the cabin, recirculating some and venting the rest while drawing in fresh air from the engine compressors.

Most aircraft replace the air inside the cabin every three to five minutes, according to Horning.

Gradual Pressurization Is Essential

Airlines must ensure a gradual pressurization as they climb to higher altitudes and similarly depressurize at a steady rate when descending toward their destination, as humans are particularly sensitive to air pressure changes — something anyone who has experienced airplane ear can attest to. This is one reason why air pressurization systems are equipped with automated controls.

Horning explains that if the pressurization controller malfunctions, the pilot of the aircraft can manually depressurize during descent, but this might lead to a less comfortable experience for passengers and crew, as achieving a smooth transition manually is more difficult.

Safety Mechanisms Against Depressurization

The aircraft's pressurization system includes built-in safety mechanisms designed to prevent issues. For instance, if the internal pressure becomes too high, the positive pressure release valve will open to serve as an outflow valve, allowing the excess air to escape and preventing over-pressurization. Additionally, the negative pressure valve protects the aircraft in case external pressure exceeds the internal pressure, a situation that could occur during a rapid descent, as described by Aerosavvy.

"Airplanes aren’t built to function like submarines," Horning says. "They are designed to maintain a higher pressure inside than outside. That’s why the negative pressure relief valve is much more responsive." As a result, during a descent, passengers may occasionally hear a loud rush of air, which is the sound of the negative pressure valve operating to adjust the cabin pressure.

In the unlikely scenario where depressurization fails during a flight, there are alternative safety measures in place, as noted by Horning. A sensor is used to detect when cabin pressure drops to the equivalent of 12,000 feet (3,658 meters) in altitude. When this occurs, the sensor automatically activates oxygen masks connected to supplementary oxygen systems, ensuring that passengers can continue to breathe easily. In some aircraft, oxygen is supplied from cylinders, while others use generators that release oxygen via a chemical reaction.