Pressurization: How and Why

For a given amount of engine power, the higher you fly, the faster your true airspeed. If your engine is very powerful, and especially if it’s turbocharged or a turbine, your airplane can fly to well above 15,000 feet — getting spectacular cruise speeds. High altitude capability gives you many more options for avoiding adverse weather or taking advantage of strong tailwinds… Too bad you might pass out and die from lack of breathable air.

Regulations vary from country to country, and even within a country depending on the type of operation under way (U.S. private pilots need supplemental oxygen, by regulation, when over 12,500 feet for more than 30 minutes or any time above 14,000 feet. Commercial flights require O2 at altitudes as low as 10,000 feet). The “quick fix” for a high-altitude flyer is to use an oxygen bottle with a mask or a nasal cannula, but this is a somewhat cumbersome and uncomfortable option, especially for passengers. The most elegant solution is to fill the entire cabin with breathable air — to employ cabin pressurization.

Pump it Up
Imagine a balloon. Blow air into the balloon and it’ll inflate — it will become pressurized. The more air you blow into the balloon, the more inflated it will become … that is, until it overstresses the rubber and the balloon bursts.

In Aviation, Everything’s More Complicated
Let’s say you want to keep the balloon inflated to a specific diameter. Blow it up to the desired size, then stop blowing and tie it off. That’s easy enough, so long as you hold a constant altitude. Put the balloon in your car (or airplane) and go up in altitude, though, and the balloon will begin to get bigger. Why? Because the pressure inside the balloon remains constant, but the pressure outside drops. The balloon will continue to grow with an increase in altitude until the pressure differential is so great that the balloon bursts. Even if the balloon doesn’t break, it won’t hold its desired diameter with any change in altitude. We need some way to adjust the pressure with changes in altitude.

Now imagine you can poke an extremely small hole in the balloon, so small that it allows some air to escape, but not so big that it compromises the balloon’s strength. Pretend you have a way of widening or decreasing the hole’s size. Inflate the balloon to its desired size, then open the hole just enough to allow it to escape at the same rate you’re blowing air into the other end. As you go up and down in altitude, adjust the size of the hole to allow more or less air into the balloon, holding its desired diameter — and keep blowing into the open end to keep the air inside fresh … halitosis not withstanding.

That’s all cabin pressurization is: a balloon with a small hole in it, calibrated to allow air to escape at a variable, controlled rate, coupled with a source of inflation constantly blowing air into the balloon.

Pressure Vessel
The pressure vessel is the part of the airplane designed to hold pressurized air in that part of … the “balloon” … you, your passengers, and usually at least part of the cargo reside. Pressurization air comes from:

  • The upper deck air in turbocharged airplanes. “Upper deck” refers to the air that was already boosted by the engine’s turbo compressor. Most upper deck air goes to the cylinders for combustion, but some pre-combustion air is piped into the cabin for pressurization.
  • Bleed air in turboprop or “pure jet” airplanes. Air from the compressor section of the turbine powerplant is plumbed into the cabin.

NOTE: Compression heats air. Pressurized air, by nature, is therefore hot — so hot as to be uncomfortable. That’s why almost all pressurized airplanes are also air conditioned … you need some way of keeping the cabin temperature comfortable while taking advantage of mask-free, high altitude flight. A carbon monoxide warning system is a great idea, too, since pressurization air (after all) comes from the engine compartments and a leaky system might allow exhaust into the cabin.

To work correctly, the pressure vessel has to be virtually airtight (except for the cabin outlet valve). Inflatable door seals are a must. Window and door diameters need to be smaller, with more rounded corners (than unpressurized airplanes) to reduce stress. Door latching mechanisms are heavier, bulkier and more complex. Most windows will not be openable, to preserve pressure vessel integrity. A small scratch on a window or a ding in the windshield may be enough to ground the airplane from pressurized flight.

More Hardware
Your pressurized airplane will have additional hardware:

  • Outflow valve. The outflow valve is that adjustable “hole” in the “balloon.” Usually in the rear of the pressure vessel in the aft cabin, the outflow valve opens and closes automatically to keep the cabin pressurized at the desired level.
  • Cabin pressure regulator. The cabin pressure regulator automatically adjusts the outflow valve to maintain set cabin pressure. Usually, the pilot manually selects a desired “cabin altitude” (air pressure equivalent to a given natural altitude) on a cockpit selector; the cabin pressure regulator references this setting and the barometric pressure inside the cabin to hold cabin altitude.
  • Cabin altitude warning system. Most pressurized airplanes have an annunciator light and/or an audible warning that activates when the cabin altitude exceeds as preset level, usually 10,000 or 14,000 feet.
  • Cabin rate controller. Similarly, the pilot can select the rate at which cabin pressure changes during climb and descent. For crew and passenger comfort, the cabin altitude rate change is usually kept at or below 500 feet per minute — mitigating the pressure changes of a faster airplane rate of climb or descent.
  • Cabin air safety valve. The pressure vessel also has a safety valve to prevent too great a difference between the cabin’s pressure and that of the outside air — the so-called “differential pressure.” Most lightplane pressure vessels are stressed to hold a 3.5 to 4.0 psi differential pressure — usually able to hold a breathable 8,000 to 10,000 foot cabin pressure at altitudes in the low 20,000-foot range.
  • Cabin dump valve. A panel-mounted switch allows the pilot to open the outflow valve to “dump” the cabin pressurization. The dump valve is also often wired through the airplane’s landing gear squat switches, dumping a pressurized cabin on landing so the pressure is equalized before someone opens a window or a door.

Pressurization requires some pre-planning and finesse on the part of the pilot. He/she must manually input the desired cabin altitude before climb and descent (including intermediate level-offs) to avoid “pressure bumps,” or rapid, uncomfortable-on-the-ears changes in cabin altitude. The rate controller is usually left alone unless the flight will be above the altitude where the maximum differential pressure doesn’t allow a constant cabin altitude. The pilot must also remember to equalize the pressure before landing, or crew and passenger ears will “pop” when the dump valve opens on touchdown.

Bring water for yourself and your passengers, also. Pressurization tends to dry out the cabin atmosphere, inviting the discomfort (and possible adverse decision-making effects) of dehydration.

In addition to the task of operating the pressurization system, the pilot must be constantly aware of his/her physiological state, and personal symptoms of hypoxia (insufficient oxygen intake). In the U.S., “altitude chamber” physiological training for civilians is available on many military bases and at the FAA’s Oklahoma City headquarters. Contact your local FAA office for details.

BOTTOM LINE: With so many elements, sensors and moving parts, and with pressure vessel integrity being so vital, it’s no wonder older, pressurized airplanes often require substantial investment to troubleshoot and maintain. Once up to specs, however, cabin pressurization is usually reliable and requires only periodic maintenance investment. Pressurization adds a few steps to takeoff, climb, approach and landing checklists, but the increased workload is manageable; “altitude chamber” physiological training is extremely recommended. Given pilot effort and owner investment, pressurization provides comfort while allowing the higher speeds and weather avoidance options of high-altitude flight.