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It so occurs to me that commercial airliners cruise at altitudes of over 30,000 feet. But why do they fly so high if there are many dangers such as cabin de-pressurization when they can simply fly lower?

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  • $\begingroup$ Fuel efficiency would be a basic way of saying it, somebody can probably give you a much better explanation than that. $\endgroup$
    – Ron Beyer
    Commented Jul 25, 2018 at 0:39
  • $\begingroup$ The main reason is air density decreases with altitude, which results in less drag, therefore better fuel efficiency. Both piston and jet powered aircraft are more efficient at high altitude. The altitudes used are the optimum for the current engine and airframe designs. $\endgroup$
    – Mike Sowsun
    Commented Jul 25, 2018 at 13:04

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Aircraft fly high because it puts them above the weather. This is at least the historical reason why aviation has desired higher flying aircraft over the years and subsequently driven design that way. When it comes to weather altitude gives you diversion options, avoidance possibilities and in some cases the ability to simply fly over a system instead of through it.

Jet engines are more efficient at high altitudes which makes it beneficial to fly them up there (you can find the full explanation of that in this Q/A) but piston planes can fly quite high as well jets have prevailed for lots of reasons and that is a different discussion all together.

Altitude, in some cases also provides a safety margin for things like loss of engine power. With sufficient altitude you can have an outcome like this instead of this.

As for cabin depressurization that is not really a huge issue. Aircraft are equipped with oxygen masks to keep everyone safe in the event of a depressurization and all aircraft are certified to be able to do a decent from cruising altitude to 10,000ft. (non supplemental Oxygen altitude) in a specified time. Some newer systems will even bring the aircraft down automatically in the event of a depressurization.

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  • $\begingroup$ I wonder what those automated systems would do if the depressurization happens over the Rockies or Sierra Nevada, where there's quite a bit of terrain above 10,000 ft MSL. $\endgroup$
    – jamesqf
    Commented Jul 25, 2018 at 4:54
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    $\begingroup$ @jamesqf there are systems for avoiding that as well there is a very interesting discussion on it here seems like in some parts of the world they have escape routes and in some places they simply avoid areas with large amounts of terrain above 10K $\endgroup$
    – Dave
    Commented Jul 25, 2018 at 5:03
  • $\begingroup$ One more reason for altitude is noise abatement. A 737 at 5000ft is quite audible for a long way around, the same 737 at 25000ft can't be heard from the ground. $\endgroup$
    – jwenting
    Commented Jul 25, 2018 at 9:11
  • $\begingroup$ I would put engine and airframe efficiency at the very first. Please correct me, if following is not right: With jet engines you produce more thrust, which allows to fly higher and therefore to fly faster through the thinner air, which in turn increases the range. The wings and the their angle are following this, made for beeing efficient at high altitude around 900 km/h. The modern jet planes are designed for being this. Flying on low altitudes would increase fuel burn, decrease range and reduce speed. Weather and silence on ground are a bonus. $\endgroup$
    – Peter
    Commented Jul 25, 2018 at 11:39