When flying on commercial aircraft, one of the last things on your mind is probably the air pressure inside the cabin. You’re trying to find your seat, wrestle the armrest from the person next to you, or get some work done before the meeting scheduled shortly after you land. However, that pressurized air is actually what makes modern commercial flight possible. Read on to find out how and the role that pressure sensors and switches play in the process.
Why is an airplane cabin pressurized?
As today’s commercial jets ascend to their cruising altitude of between 30,000 and 40,000 feet, the amount of oxygen in the air continually decreases. If you were to fly at this altitude without today’s technology, you would quickly begin to feel light-headed and may faint as your brain isn’t receiving enough oxygen to function properly. This is because, at 30,000 feet, there is only 4.4 lbs of atmospheric pressure compared to the 14.7 lbs found at sea level.
To combat this problem, aircraft cabins are pressurized to a pressure equal to low altitude breathable atmospheric pressure in order for a safe and comfortable environment and breathable air for the passengers and crew.
The following chart indicates aircraft cabin pressure at elevations above sea level.
| Altitude (ft) | Pressure (psi) |
| Sea level | 14.7 |
| 2,000 | 13.7 |
| 4,000 | 12.7 |
| 6,000 | 11.8 |
| 8,000 | 10.9 |
| 10,000 | 10.1 |
| 12,000 | 9.4 |
| 14,000 | 8.6 |
| 16,000 | 8.0 |
| 18,000 | 7.3 |
| 20.000 | 6.8 |
| 22,000 | 6.2 |
| 24,000 | 5.7 |
| 26,000 | 5.2 |
| 28,000 | 4.8 |
| 30,000 | 4.4 |
How high can you fly without oxygen or cabin pressure?
If cabin pressurization isn’t possible or oxygen isn’t available, pilots are limited to 30-minute flights at an altitude of 12,500 feet according to Federal Aviation Regulations. Aircraft flight crew and passengers, on the other hand, must have oxygen for any flights at an altitude of 14,000 feet or higher.
How do aircraft maintain cabin pressure?
To keep cabin pressure equal or close to low altitude or ”breathable” atmospheric pressure when flying at 30,000 feet, modern aircraft use air from the engines, called bleed air, and a series of cooling and monitoring devices to stream air into the cabin at a constant pressure.
- controlled by pressure switches and/or pressure transducers to monitor
- environmental control system includes cabin pressure
This bleed air comes out of the engine at an extremely hot temperature, requiring it to be cooled down before entering the cabin. At this stage, the air is sent from the engines into the belly of the plane where a series of devices are used to cool the air before it enters the cabin through an inflow valve. Once inside the cabin, an outflow valve is used to regulate how much air is allowed to leave the cabin.
The outflow valve utilizes pressure switches to regulate how much air is released. Using the bleed air (air that is bled off of the engine as it’s running and exchanges air within the cabin while maintaining cabin pressure), the control valves regulate the pressure of the bleed air and convert it to breathable air (similar to atmospheric and ground air) and feed it through the cabin.
The bleed air could also generate sea-level pressure, even at a high altitude. However, most aircraft systems do not support differential pressure beyond a certain limit.
Any air that is considered used air is released near the tail end of the aircraft. All control valves are controlled by pressure switches to regulate the systems. They also release pressure as the plane ascends.
Cabin air is not recycled but it is now being filtered as well to include fewer contaminants.
What role do pressure switches play in maintaining cabin pressure?
In the type of cabin pressurization system described above, both the inflow and outflow valves utilize pressure switches to determine how much air is allowed to enter and leave the cabin. When a CCS pressure switch senses a change in pressure within the cabin, it either snaps open or closed to activate or deactivate the control valves. Pressure sensors can be used as a backup to these switches and to monitor the pressure within the cabin.
What is the CCS difference?
Unlike the traditional pressure switches used in this application, CCS pressure switches with the pioneered DUAL-SNAP® action technology use Belleville disc springs rather than constant rate device technology to sense a change in pressure. Belleville disc springs help to lessen premature wear and tear by minimizing the amount of moving parts inside the pressure switch.
Ultimately, CCS offers a more reliable, repeatable, and accurate pressure switch at a lower total cost of ownership than other switches on the market. Meaning, when it comes to maintaining air pressure in an aircraft cabin, you can be sure your passengers will always have a comfortable cabin to enjoy their flight from. For more information, read our blog post with everything you need to know about aircraft pressure sensors and switches, take a look at our sensors and switches for aerospace applications, or contact us directly.