Is there a reason why human-powered planes travel at low altitude? Is it a function of air density at lower heights being necessary for lift or is it simply for safety?
 A: Summary: It would seem most likely that the lack of fluid density for lift and for oxygen supply for jet engines and economic constraints that dictate the use of these technologies that are the dominant reasons setting the current commercial flight altitude.

This is more an engineering question: but the basic physics (Earth science) that sets safety and performance limits is the following:


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*The lower the outside pressure, the greater the stress on the aeroplane's structure, when the latter is thought of as a pressure vessel;

*There is a raised cosmic ray and micrometeorite flux density at high altitudes owing to the lack of shielding atmosphere. This is a both a physiological safety hazard for humans as well as a problem for avionic electronics as cosmic rays induce errors / malfunction in the latter;

*The lower the outside pressure, the less oxygen for technologies such as air breathing jet engines as opposed to rockets that carry their own oxidant supply;

*There is also, as you say, less fluid density at height to generate lift; this problem can be overcome by increased ship air speed (speed relative to the atmosphere rest-frame). Indeed, the practical, von Kármán definition of the altitude of boundary of outer space is the altitude at which one needs to reach close to low Earth orbit speed to generate enough lift to hold the ship aloft. For most flight technology, this altitude is empirically found to be about 100km altitude.


Since we clearly have technologies that can overcome all of these factors, i.e. rocket engines with hyperbaric vessel engineering and re-entry technologies, the answer is clearly that economic factors set this cruising altitude. The technologies available at reasonable cost for a high enough number of airfarers to allow a workable business are those of commercial airliners: air breathing jet engines with the kinds of ship hulls that people commonly fly in. If we look at the factors above for these technologies:


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*Factor 1 is not a critical one. The air pressure at cruising altitudes of about 10km to 12km is about a fifth of an atmosphere. Current commercial airliner cabins are pressurized to about three quarters of an atmosphere, so there is not a great deal of difference between the pressure vessel technology that would withstand the full vacuum of space - cabin pressure difference versus the difference felt by a commercial airliner. I understand the economic factor is the increased fuel consumption needed to pressurize the cabin (the pressurized cabin air is bled off the jet engine's bypass flow just behind the main compressor). More modern jetliners have higher cabin pressures and indeed future models are mooted with full, sea-level cabin pressure. So clearly one could fly a higher at comparable-to-current cost if this were the only factor;

*This factor has unknown long term effects. It is clearly possible for spacefarers to live in the International Space Station for a year without immediate ill effects; a year is be more than the vast majority of commercial airline passengers fly in their whole lifetimes. How high routine flight altitudes could get before the higher altitude air flight would beget enough radiation induced sickness to affect the commercial airline business significantly is probably very hard to tell;


So it seems that 3 and 4 are indeed the dominant effects here. Commercial airliners fly at just under the speed of sound, and supersonic / trans-sonic flight would be needed to overcome factors 3 and 4 if commercial flight altitudes were significantly raised. Supersonic technology seems, for the time being, simply too expensive to be commercially workable - we have the Concorde project as evidence of this fact.
A: It depends on a happy balance between a lot of different variables. As you get higher in altitude you need to spend more fuel getting there, as well as colder temperatures and less oxygen. On the other hand, higher altitudes mean less drag, and are less crowded from other aircraft like amateur pilots, animals, and other passenger airlines.
