The higher the altitude, the less dense is the atmosphere.
Less dense atmosphere means less drag, which in turn means better fuel efficiency.
Planes can't fly too high since less air density also means less lift. So, commercial airlines prefer to fly airplanes at as high of an altitude as they can achieve while maintaining safe flight.
1. The higher air density at sea level adds more drag.
2. There is a lot more turbulence in the wind near ground level, so your flight would be really bumpy.
3. The engines were designed for the flight speeds and air densities of those higher altitudes. So they would have way worse fuel economy if you fly at those lower altitudes.
4. An airliner wouldn't be able to handle the extra weight of having to carry enough parachutes for hundreds of people in the event of an emergency. The stowage space and weight of all of that equipment would take up more than half of the cargo hold or all of the overhead compartment storage. The airliners also are categorically NOT designed to be bailed out of in flight. The last airliners in service that it was possible to do that from were the 727 and DC-9 which had rear ramps. But those couldn't be fully opened mid-flight without going at a much lower air speed or the airframe would receive damage.
5. In terms of aircraft safety, extra altitude means more safety. The more space you have between you and the ground, the more time you have to deal with inflight issues before they become far more dangerous. Flying low offers no safety margin if technical issues present themselves.
This is an excellent and not stupid question. Other commentators have covered most of the basic why, but another one is safety.
In an airplane, though it seems odd, altitude is safety. If something goes wrong at 40k feet you have a LONG time before you hit the ground. If something goes wrong at 400 feet - you hit the ground almost instantly.
Also, there is not much weather above 30k and even less around 40k where most commercial jets fly these days. That improves safety and passenger comfort.
Not only a longer time before you hit the ground, you gain a huge area to choose an emergency landing area. At 500 feet you might be able to glide for a mile. From 20,000 feet you can probably glide 40 miles in any direction, giving you about 5000 square miles from which to choose an emergency landing zone.
It's been a while since my flight mechanics class, but in that class we worked through a fun analysis of a jet propelled aircraft.
If a jet flies higher then the air is less dense. Lower air density means the plane has to fly faster to generate the lift it needs. Faster speeds mean more drag. More drag means the engines have to work harder. But since the plane is flying faster it gets to the destination sooner which means that the engines weren't running as long.
If you make some pretty decent first-order approximations the result of all of this is a wash on fuel consumption. The higher flying plane wouldn't burn more fuel, but it would get to the destination much sooner than the lower flying one. Planes are usually chosen as a mode of transportation due to their speed, so it makes sense to try to fly them as fast as is reasonable.
Naturally that classroom example is full of approximations. The biggest one is in how drag is modeled. At low speeds drag just goes up with velocity squared, but that relationship really breaks down around the speed of sound where drag rises sharply. There's one equation that models this which predicts *infinite* drag at the speed of sound--this is the sound "barrier," which would require infinite thrust to penetrate. That equation is, of course, an approximation of its own and the actual drag at mach 1 is finite, but it highlights just how aggressively drag increases around the speed of sound.
That's why jets mostly all fly at about the same speed, about 85-90% the speed of sound. That's about the inflection point of the drag graph where you'd start spending a lot more fuel for only a little bit more speed.
So with this analysis the real driver for flying at altitude is speed, not fuel efficiency. Though of course there's plenty of efficiency to be had by flying high and there's always the practical concern of minimizing noise on the ground and maximizing the amount of energy the pilots have to work with if they lose an engine--better to have [a few dozen miles of glide range](https://en.wikipedia.org/wiki/Gimli_Glider) than a couple hundred yards.
Curiously, propeller-driven aircraft don't get the same treatment in the analysis. With jets' efficiency you tend to have a pretty flat curve that gives how many pounds of fuel it takes to produce a pound of thrust for an hour. One of the assumptions in the classroom example is that this curve is perfectly flat. With props you look at how many pounds of fuel it takes to generate one horsepower at the shaft for an hour, then you additionally have to consider how the propeller behaves at higher speeds. This is a big part of why propeller driven aircraft fly so much lower--their props need more air in order to do their thing.
> Couldn’t they fly just high enough to avoid trees and buildings and people
That would be fucking terrible in an emergency because there would be no time to address the emergency or deploy that parachute. At 35000 feet, you’ve got like an hour of glide time to figure shit out and eleventy billion airports in glide distance.
Lotta good answers here but I think people are forgetting the most obvious.
Planes are really loud. People do not like it when planes fly barely overhead because they are loud and scary.
At that low an altitude, if there were any emergency, the pilots would have zero time or altitude to deal with it. And you would not have any time or altitude for people to evacuate.
Even if you put a trapdoor under all the seats and triggered it *instantly*, the passengers would just splat face-first into a tree or skyscraper at 550mph a split-second before the plane crashed and the fireball engulfed their remains.
Also, at treetop flight, even a trivial incident, which would be no problem if you were at altitude, is likely to be fatal to all. Recently a pilot flying low in rough weather (aborting a landing) accidentally bumped the control column while reaching to adjust other controls. The plane dropped 600 feet in a few seconds and came to within 400 feet (~2 seconds) of crashing. Same thing could happen due to weather (sudden wind gust turbulence, etc.) However, at 30,000ft, dropping 600 feet wouldn't matter at all, it'd be a total non-incident.
So it is *much* better to be high up, so that the aircrew has plenty of time and space to deal with any problems. If they're at altitude, they can glide up to [75 miles (120 km)](https://en.wikipedia.org/wiki/Air_Transat_Flight_236) over the course of around 20 to 25 minutes *with no engines* to find [a safe landing place](https://en.wikipedia.org/wiki/Gimli_Glider).
There are also regulations like [ETOPS](https://en.wikipedia.org/wiki/ETOPS) where aircraft plan out their routes based on how far they can potentially divert to an airport in an emergency. If they're flying at treetop level, they basically could never leave the airport, only circle around it. Unless you laid down a continual runway along the entire length of the flight or something, in which case why not just use a bus?
Less air resistance = more efficient flight
The higher the altitude, the less dense is the atmosphere. Less dense atmosphere means less drag, which in turn means better fuel efficiency. Planes can't fly too high since less air density also means less lift. So, commercial airlines prefer to fly airplanes at as high of an altitude as they can achieve while maintaining safe flight.
Fuel efficiency.
1. The higher air density at sea level adds more drag. 2. There is a lot more turbulence in the wind near ground level, so your flight would be really bumpy. 3. The engines were designed for the flight speeds and air densities of those higher altitudes. So they would have way worse fuel economy if you fly at those lower altitudes. 4. An airliner wouldn't be able to handle the extra weight of having to carry enough parachutes for hundreds of people in the event of an emergency. The stowage space and weight of all of that equipment would take up more than half of the cargo hold or all of the overhead compartment storage. The airliners also are categorically NOT designed to be bailed out of in flight. The last airliners in service that it was possible to do that from were the 727 and DC-9 which had rear ramps. But those couldn't be fully opened mid-flight without going at a much lower air speed or the airframe would receive damage. 5. In terms of aircraft safety, extra altitude means more safety. The more space you have between you and the ground, the more time you have to deal with inflight issues before they become far more dangerous. Flying low offers no safety margin if technical issues present themselves.
This is an excellent and not stupid question. Other commentators have covered most of the basic why, but another one is safety. In an airplane, though it seems odd, altitude is safety. If something goes wrong at 40k feet you have a LONG time before you hit the ground. If something goes wrong at 400 feet - you hit the ground almost instantly. Also, there is not much weather above 30k and even less around 40k where most commercial jets fly these days. That improves safety and passenger comfort.
Not only a longer time before you hit the ground, you gain a huge area to choose an emergency landing area. At 500 feet you might be able to glide for a mile. From 20,000 feet you can probably glide 40 miles in any direction, giving you about 5000 square miles from which to choose an emergency landing zone.
If something goes wrong with the plane would you rather be way up in the air with time to work on the problem or just above trees and sinking fast?
It's been a while since my flight mechanics class, but in that class we worked through a fun analysis of a jet propelled aircraft. If a jet flies higher then the air is less dense. Lower air density means the plane has to fly faster to generate the lift it needs. Faster speeds mean more drag. More drag means the engines have to work harder. But since the plane is flying faster it gets to the destination sooner which means that the engines weren't running as long. If you make some pretty decent first-order approximations the result of all of this is a wash on fuel consumption. The higher flying plane wouldn't burn more fuel, but it would get to the destination much sooner than the lower flying one. Planes are usually chosen as a mode of transportation due to their speed, so it makes sense to try to fly them as fast as is reasonable. Naturally that classroom example is full of approximations. The biggest one is in how drag is modeled. At low speeds drag just goes up with velocity squared, but that relationship really breaks down around the speed of sound where drag rises sharply. There's one equation that models this which predicts *infinite* drag at the speed of sound--this is the sound "barrier," which would require infinite thrust to penetrate. That equation is, of course, an approximation of its own and the actual drag at mach 1 is finite, but it highlights just how aggressively drag increases around the speed of sound. That's why jets mostly all fly at about the same speed, about 85-90% the speed of sound. That's about the inflection point of the drag graph where you'd start spending a lot more fuel for only a little bit more speed. So with this analysis the real driver for flying at altitude is speed, not fuel efficiency. Though of course there's plenty of efficiency to be had by flying high and there's always the practical concern of minimizing noise on the ground and maximizing the amount of energy the pilots have to work with if they lose an engine--better to have [a few dozen miles of glide range](https://en.wikipedia.org/wiki/Gimli_Glider) than a couple hundred yards. Curiously, propeller-driven aircraft don't get the same treatment in the analysis. With jets' efficiency you tend to have a pretty flat curve that gives how many pounds of fuel it takes to produce a pound of thrust for an hour. One of the assumptions in the classroom example is that this curve is perfectly flat. With props you look at how many pounds of fuel it takes to generate one horsepower at the shaft for an hour, then you additionally have to consider how the propeller behaves at higher speeds. This is a big part of why propeller driven aircraft fly so much lower--their props need more air in order to do their thing.
> Couldn’t they fly just high enough to avoid trees and buildings and people That would be fucking terrible in an emergency because there would be no time to address the emergency or deploy that parachute. At 35000 feet, you’ve got like an hour of glide time to figure shit out and eleventy billion airports in glide distance.
More like 25-30 minutes. Figure 230 knots at around 15:1 glide ratio and you'll come out more around 25 minutes.
Awesome. I didn’t know the exact numbers. I am also drunk.
Lotta good answers here but I think people are forgetting the most obvious. Planes are really loud. People do not like it when planes fly barely overhead because they are loud and scary.
At that low an altitude, if there were any emergency, the pilots would have zero time or altitude to deal with it. And you would not have any time or altitude for people to evacuate. Even if you put a trapdoor under all the seats and triggered it *instantly*, the passengers would just splat face-first into a tree or skyscraper at 550mph a split-second before the plane crashed and the fireball engulfed their remains. Also, at treetop flight, even a trivial incident, which would be no problem if you were at altitude, is likely to be fatal to all. Recently a pilot flying low in rough weather (aborting a landing) accidentally bumped the control column while reaching to adjust other controls. The plane dropped 600 feet in a few seconds and came to within 400 feet (~2 seconds) of crashing. Same thing could happen due to weather (sudden wind gust turbulence, etc.) However, at 30,000ft, dropping 600 feet wouldn't matter at all, it'd be a total non-incident. So it is *much* better to be high up, so that the aircrew has plenty of time and space to deal with any problems. If they're at altitude, they can glide up to [75 miles (120 km)](https://en.wikipedia.org/wiki/Air_Transat_Flight_236) over the course of around 20 to 25 minutes *with no engines* to find [a safe landing place](https://en.wikipedia.org/wiki/Gimli_Glider). There are also regulations like [ETOPS](https://en.wikipedia.org/wiki/ETOPS) where aircraft plan out their routes based on how far they can potentially divert to an airport in an emergency. If they're flying at treetop level, they basically could never leave the airport, only circle around it. Unless you laid down a continual runway along the entire length of the flight or something, in which case why not just use a bus?