In the 1930s, Boeing introduced a groundbreaking aircraft design called the Model 307 Stratoliner, marking a pivotal moment in the history of aviation. The Boeing 307 was equipped with a pressurized cabin, allowing the aircraft to fly faster and higher at altitudes of 20,000 feet (6,096 meters) above sea level without the risk of passengers and crew suffering from oxygen deprivation due to thin air.
Chuck Horning, an Associate Professor in the Aviation Maintenance Science Department at Embry-Riddle Aeronautical University in Florida since 2005, and a former mechanical and maintenance instructor at Delta Airlines for 18 years, explained that this cabin technology functioned so effectively that passengers and crew inside often struggled to notice changes in pressure. This was partly because it began adjusting the cabin air pressure as the aircraft changed altitude and readjusted it during descent.
The amount of air required for the pressurization system depends on the cabin volume.
Horning explained: “This is not a very complicated system,” noting that these basic technologies have remained largely unchanged for decades, although advancements in electronics and automated control systems have improved accuracy. Essentially, the aircraft utilizes some excess air from the compressor section of the jet engine. “The engines do not require all the compressed air for combustion, so this compressed airflow is harnessed and used for both cabin climate control and pressurization.“
A jet engine operates by compressing air and then igniting it. After compression, a small amount is pressurized and redirected before combustion, known as bleed air, which is at a temperature of 200°C. Once cooled, this air is pumped in to pressurize the cabin. This process is controlled by a device called the air pressure controller, which Horning describes as the “brain of the pressurization system.”
He further explained: “The controller will automatically adjust the pressure. It will have full information on the aircraft’s altitude during flight. It then schedules the pressurization to begin when the aircraft starts to ascend; as outside pressure decreases, it will start functioning.“
When ascending, a significant pressure differential can put considerable stress on the aircraft’s fuselage. To avoid this, planes typically do not try to double the pressure compared to sea level. Instead, for instance, at an altitude of 36,000 feet (10,973 meters), most commercial aircraft adjust their cabin pressure to that equivalent to 8,000 feet (2,438 meters) above sea level.
For newer models, such as the Boeing 787 Dreamliner, constructed from ultra-strong carbon fibers, the cabin pressure can be adjusted to the equivalent of 6,000 feet (1,829 meters) above sea level. Horning stated: “This adjustment level is better than older models, as higher cabin altitudes can result in lower blood oxygen levels. That’s why sometimes you feel very fatigued after disembarking from a flight.“
Additionally, the amount of air required for the pressurization system depends on the cabin volume. Since the pressurization system works alongside the air conditioning system, air is continuously cycled in and out of the cabin, some used for conditioning, while the rest is expelled when the system begins to draw air from the compressor. Horning noted that the entire cabin air is exchanged every 3-5 minutes.
Aircraft must precisely control pressurization during ascent and descent to ensure passenger and crew comfort, as humans are quite sensitive to changes in air pressure—those who experience ear issues while flying will understand this. This is why the system needs to be automated. According to Horning, in case of a malfunction in the control system, pilots can manually adjust the pressure, but this could create an uncomfortable experience for passengers and crew, as it is challenging for a manual process to achieve the same level of accuracy as an automated system.
The pressurization system also incorporates safety mechanisms to protect passengers and crew in case of emergencies. Positive pressure relief valves automatically open when internal pressure becomes too high due to excessive compressed air pumped into the cabin, reducing the pressure. There are also negative pressure relief valves to safeguard the aircraft from the pressure differential when outside pressure exceeds that inside the cabin (in the event of a rapid descent).
“Submarines are designed to maintain internal pressure greater than the external pressure, whereas aircraft do not function this way. Therefore, the negative relief valve is a more responsive component in the safety system.” This explains why sometimes, upon landing, you hear a loud rush of air. That is when the negative pressure relief valve is activated.
Horning mentioned that in rare cases where an aircraft experiences pressure issues, there are still other safety measures in place. There will be a sensor to alert when the cabin pressure drops to levels equivalent to 12,000 feet (3,658 meters) above sea level, at which point oxygen masks will automatically deploy for passengers and crew to continue breathing.
