Pressurizing a cabin puts a lot of stress on the fuselage. Most commercial aircraft pressurize to an equivalent altitude of 8000 feet (10.3 psi), and mix exterior air with interior cabin air to keep the cabin fresh. Higher pressures exert too much force on the fuselage and can shorten its lifespan, especially with short-haul aircraft that pressurize and depressurize as many as 14 times a day.
Extra credit: Boeing’s 787 (currently finishing FAA certification) is spec’ed to pressurize down to 6000 feet (11.3 psi) and, because the fuselage is made of a composite material instead of aluminum, the cabin humidity can also be increased to more ‘normal’ levels, allowing for a more comfortable flight and somewhat reducing the need for in-flight hydration.
Extra extra credit: The Space Shuttle maintains air pressure of 14.7 psi (sea level) while the Space Station is pressurized at 14 psi (1,100 feet above sea level). Prior to the Space Shuttle, US space capsules were only pressurized to 4.3 psi, and used a mix of nearly pure oxygen instead of a 30/70 oxygen/nitrogen mix. This was a problem because it made the interior of the capsule highly flammable (widely regarded as a major factor in the Apollo 1 catastrophe where three astronauts died in a cabin fire on the launch pad), and created difficulties during the Apollo-Soyuz space rendezvous, as Soviet craft were at 14 psi.
Both Space Shuttle and ISS space suits are pressurized to 4.3 psi with a pure oxygen mix because a full-pressure suit would have to be much stronger and the pressure would make the arms and legs stick straight out.
Author – Kevin Fox
