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If you fly low, air is dense so you can get more thrust from your engines, but you get more drag.
On the other hand if you fly higher you have less drag but the output of engine decreases as well.

So what's the optimum altitude to fly at, and how does one determine it?

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Are you asking purely about performance, or are you also interested in procedural reasons to fly at a specific altitude? ATC instructions and VFR/IFR cruising rules would be considerations too. – Pondlife Jan 13 '14 at 15:13
The edits have greatly improved the question and made it answerable. I rescind my downvote. – casey Jan 13 '14 at 18:42
See At what point during a long flight do commercial airliners have the best gas mileage? for a more in-depth answer specific to jets. – Lnafziger Apr 29 '14 at 4:30
up vote 12 down vote accepted

The optimum altitude depends on your aircraft, the engines and the weather. But most important is to decide what you are trying to achieve. If you would like to optimize your fuel consumption per distance travelled, the altitude you will fly at will be higher than if you try to optimize your fuel consumption per time unit.

Fuel per distance travelled is usually better at altitude, while fuel flow per time unit is lower at lower altitudes. In other words, if you want to go far, fly high. If you want to keep flying long, fly low.

Winds play an important role as well. If you have a strong headwind high up, you might go further staying low.

There is quite some difference between reciprocating engines and jet / turbine engines here. The former perform better at lower altitudes, while the latter do just fine until much higher.

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Another factor in the decision to fly low or high: turbulence. How important is it to you and your passengers to have a smooth ride? Higher is generally less turbulent but check those Airmets and Pireps!

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While that's a useful point in general, the question's been changed... ;) – Qantas 94 Heavy Jan 14 '14 at 2:20
yes nice point, while its a question in itself, can you explain why turbulence is low at low pressures? is the pressure difference is negligible at low pressures? – shabby Jan 14 '14 at 18:01

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