from the cockpit

You yawn and chew gum, but your ears still pop on takeoff.

Captain Meryl explains what cabin pressure has to do with it.

As many as one in three passengers suffers from painful pressure in the ears while flying. A brief lesson in how aircraft cabins are pressurized might help you understand this common problem.

Airplanes that routinely fly at altitudes of around 10,000 feet or lower are usually not equipped with the ability to pressurize their cabins. When an unpressurized airplane climbs or descends at a rate of, say, 1,000 feet per minute, so does the cabin. It’s no different than driving at various elevations where the altitude outside your car is the same as inside. You may experience your ears “popping” as they try to keep up with the pressure changes.

Commercial aircraft fly at altitudes far higher than 10,000 feet and have pressurized cabins. In older aircraft, such as the DC-10 and B-727, it was one of the flight engineer’s tasks to keep the cabin at breathable altitudes while the pilots flew the airplane. In more modern aircraft, pressurization systems are completely automated.

Airplanes are not sealed tubes, as some may think. Instead, there is a constant flow of fresh air coming into the cabin, supplied by engine “bleed air.” This is compressed air taken from inside the engines before fuel is injected. The bleed air passes through a pre-cooler before entering the cabin. Although cabin air may seem stuffy at times, especially with a full load of passengers, it is actually highly filtered and has tested cleaner than the air found in many office buildings.

An airplane may have either one or two outflow valves to control the flow of air out of the cabin as bleed air flows in. These valves are also controlled automatically by the pressurization system. Pilots do not manipulate any controls or switches in order to provide proper pressurization. We can always take over manually, however, should a malfunction occur and the need arise.

When your airplane is cruising at, say, 41,000 feet, which is at the high end of cruise altitudes utilized by commercial airliners, your cabin altitude is likely to be around 7,000 to 7,500 hundred feet, depending on the type of aircraft.

An obvious question is, why don’t aircraft just stay pressurized to sea level all the time? Pressurizing an airplane exerts tremendous pressure outward, just like the air inside a balloon. There is a maximum amount of force, known as the “pressure differential,” that an airplane is certified to handle.

The new B-787 Dreamliner, scheduled to go into production this year, will be the first airliner capable of handling a sea-level cabin with the airplane at 40,000 feet or slightly higher. Extensive testing, however, has shown 6,000 feet to be the optimal cabin altitude, with passenger symptoms almost unvarying compared to simulated flights with a sea-level cabin. New, nearly indestructible composite materials used in the aircraft’s structure have allowed this lower cabin pressure for the first time in history.

A 6,000-foot-altitude cabin may not sound like a dramatic change from 7,000, but trust me, it is. The fatigue level should go way down as you fly at
a lower, denser and more tolerable cabin altitude. In addition, the 787 is “smart,” in that it knows how many passengers are on board. Air conditioning that adjusts itself to the correct number of people in the aircraft (entered from the cockpit) will reduce the amount of humidity sucked out of the air, helping passengers stay better hydrated, which also helps reduce fatigue.

While climbing or descending, the automatic pressurization system (or flight engineer) controls the rate of altitude change to a comfortable range, usually around 350 to 450 feet per minute. Years ago, when I was a flight engineer myself, we referred to the task of keeping the cabin altitude at the proper pressure differential relative to the airplane’s altitude as “flying the cabin.”

There’s a lot of science and engineering behind your experience of ears popping during ascent and descent. If you have a cold or suffer from allergies, be sure to check with your doctor for a safe product to help relieve congestion before you fly.

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captain meryl getline (fromthecockpit.com) is a retired B777 pilot. She is the author of The World at My Feet. If you are a nervous flier, check out her Web site: flyingfearless.com. Email Captain Meryl at editor@executivetravelmag.com.