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RedditWE HUMANS NEED AIR TO LIVE, so we do best around sea level. Airplanes are at their best up high, where the air is thin and smooth. And therein lies the rub: We invented a machine that thrives where we don’t. This became obvious as soon as engine power increased to a point at which aviators could reach altitudes where they lost consciousness.
At first, fliers coped by filling tanks with pressurized oxygen and inhaling the gas through rubber tubes; later, form-fitting face masks made oxygen delivery more reliable. In many high-flying light airplanes and military aircraft, oxygen systems and face masks are still used to keep the pilot alive and conscious.
In 1937, the U.S. Army Air Corps began research flights in a modified Lockheed Electra; the XC-35 was the first airplane built with a pressurized cabin. The fuselage was designed with a circular cross-section to eliminate stress points when the fuselage expanded under pressure. Openings were sealed to prevent air from escaping. Windows were reduced in size and strengthened, and the cabin inside became a pressure capsule—like a big aluminum can—that held five people. In 1937, the XC-35 earned the Air Corps the Collier Trophy for most significant development of the year.
Two years later, Boeing submitted a design to the Air Corps for a long-range bomber, the B-29 Superfortress, which would have pressurized compartments for the crew. And in 1940, Boeing’s 307 Stratoliner began flying passengers in pressurized comfort at 20,000 feet. Today all airliners are pressurized, and although the details vary among them, the basic elements of cabin pressurization systems are almost universal.
Air is pressurized by the engines. Turbofan engines compress intake air with a series of vaned rotors right behind the fan. At each stage of compression, the air gets hotter, and at the point where the heat and pressure are highest, some air is diverted. Some of the hot, high-pressure air, called bleed air, is sent to de-ice wings and other surfaces, some goes to systems operated by air pressure, and some starts its journey to the cabin.
The cabin-bound air has to be cooled first in an intercooler, a device like a car radiator that sheds the heat to the ambient air scooped aboard for that purpose. From there the air travels into the airplane’s belly, where air packs cool it further using air cycle refrigeration. An air cycle cooler is perhaps the simplest air conditioner ever invented, because it doesn’t need a refrigerant as an intermediate fluid to dump heat. The air packs compress the incoming air to heat it before sending it to another intercooler to dump the heat to the outside. The air then expands through an expansion turbine, which cools it the way blowing with your lips pursed results in a cool flow of air. (Test the principle by blowing with your mouth wide open to see how warm the air would be if it weren’t compressed and then allowed to expand.)
Now the air is ready to mix with air from the cabin in a mixer, or manifold, that adds the new air to the recirculating cabin air, which is moved by fans. To maintain a comfortable temperature for the passengers, automatic systems regulate the mixture of heat from the engines and cold from the air packs. To maintain the pressure in the cabin equal to that at low altitude, even while the airplane is at 30,000 feet, the incoming air is held within the cabin by opening and closing an outflow valve, which releases the incoming air at a rate regulated by pressure sensors. Think of a pressurized cabin as a balloon that has a leak but is being inflated continuously.
On the ground, the airplane is unpressurized and the outflow valve is wide open. During preflight, the pilot sets the cruise altitude on a cabin pressure controller. As soon as the weight is off the main wheels at takeoff, the outflow valve begins to close and the cabin starts to pressurize. The airplane may be climbing at thousands of feet per minute, but inside the cabin, the rate of “climb” is approximately what you might experience driving up a hill. It might take an average airliner about 20 minutes to reach a cruise altitude of, say, 35,000 feet, at which point the pressurization system might maintain the cabin at the pressure you’d experience at 7,000 feet: about 11 pounds per square inch. Your ears may pop, but the effect is mild because the climb rate is only 350 feet per minute. When the airplane descends, the pilot sets the system controller to the altitude of the destination airport, and the process works in reverse.
Comments
when the cabin door closes on 14.7 # pressure [assuming sea level takeoff]and the objective is to maintain pressure at say the 8,000 ft. level [10.9psi] is pressure first extracted until that altitude is attained and then pressure maintained through the bleed system? thanks for clearing this up, george
Posted by George Sites on May 30,2008 | 11:35AM
To answer George's question, the climb through 8000 feet only takes a few minutes, and the cabin pressure controller allows the cabin pressure to follow the decrease in outside pressure until the target pressure is reached, then maintains that pressure throughout the remainder of the flight until the descent through 8000, then allows cabin pressure to again follow ambient outside pressure to touchdown. At least that's how I understood it when banging around in back of C-141B's as a USAF flight nurse. I remember my ears popping on the way up to a particular point, and again on the way down after a particular point.
Posted by Ken Hodges on July 11,2008 | 06:54PM
My wife are missing 2 bones in her head (parietal)and I want to Know if presure in the cabin will affect her. I will apreciate this information. Editors reply: We aren't qualified to answer. We suggest you ask a neurosurgeon.
Posted by Thomas on September 22,2008 | 09:40PM
George, Aircraft are not designed to ever have a negative pressure differential (more pressure on the outside than on the inside). If the cabin altitude is set at 8,000ft before takeoff, the air inside the fuselage will gradually escape through the aircraft's outflow valves as the aircraft climbs. For example, if the aircraft is climbing at 4,000 feet per minute, the cabin altitude might only be increasing at 500 feet per minute. Once the cabin altitude reaches its set value (8000 ft in this example) the outflow valves will modulate to maintain the cabin altitude. If an 8000 ft cabin altitude can be maintained with a 5 psi differential (as would be the case if the aircraft were flying at ~25,000ft) and the aircraft climbs to a higher altitude (35,000 as an example), the aircraft will have to increase the differential pressure (~8 psi).
Posted by Titan on November 9,2008 | 02:07PM
I would like to know the pressure inside the luggage part of a plane. Is it the same as in the passenger area? And what about pressure inside the freight area? This is specific information I need for transport of sensitive medical products.
Posted by Hans Middelbeek on February 24,2009 | 07:41AM
I am searching for a chart or graph plotting aircraft altitude versus cabin altitude at various pressure differentials. For example, what would be the aircraft cabin altitude flying at 15,000 feet with a differential of 2.5 psi?
Posted by Frank E. Adams on June 8,2009 | 06:06AM