How Things Work: Cabin Pressure
Why you remain conscious at 30,000 feet.
- By George C. Larson
- Air & Space magazine, January 2002
NASM NEG. # A2168B
(Page 2 of 2)
The structural strength of the airplane determines how much differential pressure the cabin can tolerate—a typical figure is eight pounds per square inch—and the fuselages of new airplane designs are pressurized and depressurized many thousands of times during testing to ensure their integrity. The higher the maximum differential pressure, the closer to sea level the system can maintain the cabin. Federal Aviation Regulations say that without pressurization, pilots begin to need oxygen when they fly above 12,500 feet for more than 30 minutes, and passengers have to use it continuously above 15,000. On airliners that operate at altitudes well above that, regulations require that everyone aboard be supplied with 10 minutes of oxygen in the event the cabin pressure can’t be maintained, which brings us to the dramatic scenario known as explosive decompression.
If the door blew off a jet at altitude, all the air in the cabin would depart very quickly and a momentary thick fog would envelope the cabin as the water vapor in the air condensed instantly. Loose articles would fly around and foam rubber would burst as the tiny air bubbles within it expanded. Within a couple of seconds, oxygen masks would drop down from the overhead panels, and you would have to pull yours toward you and place it over your mouth and nose. The act of donning the mask tugs on a lanyard that starts the flow of life-sustaining oxygen.
If you’ve been following news reports and have heard that more armed air marshals are flying these days and that a stray bullet would cause decompression, you can stop worrying. The airplane already has a huge hole in it called the outflow valve. And air marshals are reputed to be excellent marksmen.