Aircraft Pressurization Beginner’s Guide

It’s easy to take flying for granted. We hop on-board a comfy airliner and fly high in the stratosphere without giving breathing a second thought. The aircraft’s pressurization system makes it possible. Here’s how the magic works…

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Balloons on scale - Aircraft Pressurization - AerosavvyHypothetical experiment: If you place a scale in a vacuum chamber and compare the weight of a filled balloon with an empty one, you’ll see that air has mass.

Earth’s atmosphere is about 300 miles thick. At sea level, our bodies are subjected to about 14.7 pounds of pressure from this tall column of air. I’ll bet you don’t even notice! For animals roaming the earth’s surface, a 14.7 psi atmosphere provides the perfect amount of oxygen.

As we climb in altitude, the amount of air pressure acting on us decreases rapidly. You notice the decrease when your ears pop while driving up a mountain or riding a fast elevator. Although the atmosphere is 300 miles thick, most of the air molecules are squashed down to within a few thousand feet of the earth’s surface.

Denver is fine. Going higher spells trouble.

As we climb higher, air molecules are spread farther apart. When we breathe, our lungs take in less air, and less oxygen. Folks living in Denver, Colorado (5600 ft) are quite happy breathing the lower, 12 psi atmosphere. Climbing to a higher altitude, though, and the pressure drops really fast.

At 18,000 feet, the atmospheric pressure is down to 7.3 psi, about half the sea-level pressure. There just isn’t enough oxygen in a breath of air to adequately supply the brain. At this pressure, a healthy adult has only 20-30 minutes of useful consciousness.

NDG_quote-smallAirliners fly between 30,000 and 43,000 feet. At those altitudes the atmosphere provides less than 4 psi of pressure. If you tried breathing at that altitude, your useful consciousness would be less than a minute (followed soon after by death).

To survive high altitudes, occupants of an aircraft need help breathing. The solution is to pump air into the airplane so the interior pressure is high enough to keep the humans happy.

Why bother with pressurization? Why not fly down low?

Airplanes can certainly fly below 10,000 feet where the atmospheric pressure is a comfy 10 psi or higher, but it has some drawbacks:

  • It’s tough to cross a 14,000 foot mountain range at 10,000 ft.
  • Most bad weather is at lower altitudes.
  • Turbofan engines are very inefficient down low.
  • Aircraft ground speeds are slower at lower altitudes.

If you want a fast, smooth ride in a fuel efficient airplane that can fly over a mountain range, we need to pressurize!

How does a pressurization system work?

The airplane body (fuselage) is a long tube capable of withstanding a fair amount of differential air pressure; think of it like a big plastic soda bottle. In theory, we could seal the bottle so, as the airplane climbs, the interior air pressure would stay the same. We can’t do that because it’s hard to perfectly seal a huge airplane fuselage. Even if we could, the passengers would quickly use up the available oxygen. And just imagine the smell inside a perfectly sealed tube on a long flight! Clearly, a big sealed soda bottle won’t work for us without some modification.

Aircraft Pressurization - Soda Bottle - Aerosavvy
A fuselage is a bit like a soda bottle with a hole in the back.

To solve the problems, pressurization systems constantly pump fresh, outside air into the fuselage. To control the interior pressure, and allow old, stinky air to exit, there is a motorized door called an outflow valve located near the tail of the aircraft. It’s about the size of a briefcase and located on the side or bottom of the fuselage. Larger aircraft often have two outflow valves. The valves are automatically controlled by the aircraft’s pressurization system. If higher pressure is needed inside the cabin, the door closes. To reduce cabin pressure, the door slowly opens, allowing more air to escape. It’s one of the simplest systems on an aircraft.

Aircraft Pressurization - B767-Outflow Valve - Aerosavvy
The outflow valve on a Boeing 767-300F

One of the benefits of a pressurization system is the constant flow of clean, fresh air moving through the aircraft. The air inside the airplane is completely changed every two or three minutes making it far cleaner than the air in your home or office.

Pressurization systems are designed to keep the interior cabin pressure between 12 and 11 psi at cruise altitude. On a typical flight, as the aircraft climbs to 36,000 feet, the interior of the plane “climbs” to between 6000-8000 feet.

Altitude graph - Aircraft Pressurization - Aerosavvy
Exterior and interior altitude profile on a typical flight.

Why not keep the cabin at 14.7 psi to simulate sea-level pressure and maximize comfort? The aircraft must be designed to withstand differential pressure, that’s the difference between the air pressure inside and outside the aircraft. Exceeding the differential pressure limit is what makes a balloon pop when it’s over inflated. The greater the differential pressure, the stronger (and heavier) the airplane must be built. It’s possible to build an aircraft that can withstand sea-level pressure during cruise, but it would require a significant increase in strength and weight. A 12 psi cabin is a good trade-off.

That’s just nasty!

Outflow Valve Trivia:

If you look at pictures of airliners taken prior to 1990, you might see brown stains around the outflow valve. The stains are from tobacco smoke. Airlines were thrilled when the industry banned smoking. Tar and nicotine gummed up valves, instruments, and sensors causing thousands of dollars a year in damage. Tobacco is really nasty stuff.

Protecting the Fuselage from Pressurization Problems

Two types of mechanical devices are installed on the fuselage to protect the pressurized section of the aircraft against excessive pressure differential.

Positive Pressure Relief Valves

Every pressurized aircraft has a maximum pressure differential limit. Exceeding this limit (pumping too much air pressure into the fuselage) can cause damage – even blow out doors and windows. To protect the aircraft from over pressurizing, positive pressure relief valves are installed. The devices (sometimes called butterfly valves) are spring-loaded to vent excess air pressure when cabin pressure exceeds the maximum limit.

Positive Pressure Relief Valve - B757 - AeroSavvy
Boeing 757 positive pressure relief valve. Excessive air pressure in the fuselage forces the spring-loaded doors to open, venting excess pressure outside.

Negative Pressure Differential Relief Doors

Negative pressure differential means the pressure outside the cabin is greater than the pressure inside the cabin. This situation could occur during a rapid descent. Negative pressure is bad because it pushes inward on doors and windows. These components are not designed for this type of force.

Again, spring-loaded devices are used to protect the fuselage from damage. Air pressure of less than 1.0 psi against the outside of the doors causes them to open inward against the spring load, venting air into the fuselage to equalize the pressure.

Pressurization - Negative pressure differential relief doors - AeroSavvy
Negative pressure differential relief doors on a Boeing 757. Excess pressure outside the fuselage forces the doors to open inward venting air inside the fuselage.

Where does pressurized air come from?

Boeing Stratocruiser by SDASM

Electric Compressors
Old piston powered airliners, like the Boeing Stratocruiser, used electric air compressors to pump fresh, outside air into the cabin. This system worked well, but the compressors added a lot of weight to the aircraft.

Boeing 707 by ClipperArctic CC BY-SA 2.0

Early jetliners, like the Douglas DC-8 and Boeing 707 used bleed air from the engines to spin turbocompressors. The turbocompressors then pumped fresh outside air into the cabin.

MD-88 by Lvco99 CC BY-NC-SA 2.0

Engine Bleed Air
Most modern airliners use bleed air from the compressor section of the engines to pressurize the cabin. This very hot air must be cooled to a comfortable temperature before it’s directed into the cabin.

Boeing 787 by Tim Wang CC BY-SA 2.0

Electric Compressors (Again!)
The new Boeing 787 Dreamliner brings back the electric compressor. The 787’s electrical system powers compressors, just like on the old Stratocruiser. Advances in technology make this system far more efficient than it’s predecessor from the 1950’s.

What is bleed air?

A jet engine has three main sections: compressor, combustion, and turbine/exhaust. The compressor is at the front of the engine. A series of spinning blades draws in fresh, outside air. As the air is compressed, it becomes very hot. Remember high school physics? As a gas is compressed, its temperature rises. The hot, compressed air then enters the combustion chamber where it is mixed with fuel and burned. The expanded gasses continue through turbine blades which power the compressor blades before exiting the engine producing thrust.

Turbofan Operation by K. Aainsqatsi – CC BY-SA 3.0

Bleed air is fresh, clean, hot air taken from the compressor section of the engine before it is mixed with fuel or exhaust gasses. Common uses for hot bleed air are wing and engine ice protection, cabin pressurization, engine starter motors, and air driven hydraulic pumps.

How do pilots control the pressurization?

Boeing 757 and 767 pressurization panel - AeroSavvy
Pressurization controls on a 757 & 767

It’s really, really easy. The cabin altitude control panel on the 757 and 767 is super simple. During preflight checks, pilots turn the “LDG ALT” knob to display the altitude of the landing airport. That’s it! We don’t touch it for the remainder of the flight. The automatic mode takes care of the outflow valve for us.

The remaining indicators and knobs are for redundancy in case of a malfunction. There are two separate automatic modes. A manual mode allows us to adjust the position of the outflow valve should both auto systems fail. Pressurization systems work great and rarely cause any trouble.

Effects Of Flying In A Pressurized Cabin

Muga-Glass-of-red-wineThe air inside an aircraft cabin is very low in humidity. On a long flight it’s important to drink plenty of water to stay hydrated. When the flight attendant offers you a bottle of water, drink it. You may not notice that you’re dehydrated.

Alcohol consumption: Dehydration increases the effects of alcohol on your body. To make matters worse, alcohol increases dehydration; it’s a double-whammy. If you choose to drink alcohol on a flight, be sure to drink plenty of water and have something to eat while enjoying your cocktail. Don’t be that guy. Drink extra-responsibly when flying.

Does this food taste bland? Yes! There’s a good chance your in-flight meal really does taste bland. The aircraft cabin’s low humidity and lower air pressure reduce your sense of taste and smell by as much as 30% according to a Lufthansa commissioned study. Airline food kitchens often add extra spices and flavoring to meals to compensate for your crippled taste buds!
Special thanks to my Twitter friend (and fellow blogger@Jen_Niffer for tipping me off to the Lufthansa study!

Further Reading About Pressurization:

What happens if there is a problem with the pressurization system?
Your Oxygen Mask vs My Oxygen Mask