Airplanes are pressurised to maintain a comfortable living environment for human beings. Ideally an airplane would be pressurised to ground level pressure, but this is not practical as the fuselage of a plane would have to be incredibly strong (and hence very heavy and expensive to fly) to withstand the outward force caused by the high pressure inside.
Therefore modern commercial jets compromise and are pressurised to an altitude of 5000-8000ft. This explains the phenomenon of 'ears popping' on take-off and landing as the air trapped in the ear is effected by the change in pressure from ground level to this effective altitude.
A typical commercial jet (most standard flights) cruises at around 28-35,000ft (up to 6.6 miles of altitude). The main exception is Concorde which was designed to fly at a higher altitude (and hence lower wind resistance) at around 45,000ft. Although many jets could fly at higher altitudes, they are usually certified to an altitude giving a wide safety margin. For example the new generation Boeing 737 is certified to 41,000ft (7.8 miles).
Many military jet aircraft are able to fly considerably higher. Often, the plane itself is not pressurised and instead the pilot wears a pressure suit that provides him with a pressurised environment. Originally, very high altitude military planes were used for surveillance ï¿½ the current versions of the famous U2 spy plane, originally designed in the 1950s can cruise at up to 90,000ft (17 miles). The Stealth Bomber cruises at up to 50,000ft (8.3 miles) and many other combat planes can now also attain significant altitudes.
Answered by: Jules Seeley, M.S., Physics graduate; Strategy Consultant, London