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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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    $\begingroup$ 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. $\endgroup$ – Pondlife Jan 13 '14 at 15:13
  • $\begingroup$ The edits have greatly improved the question and made it answerable. I rescind my downvote. $\endgroup$ – casey Jan 13 '14 at 18:42
  • $\begingroup$ 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. $\endgroup$ – Lnafziger Apr 29 '14 at 4:30
  • $\begingroup$ If you mean your total fuel consumption, the best economy is a function of cruise thrust and cruise altitude, not one without the other. $\endgroup$ – user40476 Jun 2 at 18:41
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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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  • $\begingroup$ Could the downvoter please be so kind to explain why he thinks this answer deserves a -1? I'd be grateful for the opportunity to improve. Thank you. $\endgroup$ – DeltaLima Jun 2 at 20:21
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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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  • $\begingroup$ While that's a useful point in general, the question's been changed... ;) $\endgroup$ – Qantas 94 Heavy Jan 14 '14 at 2:20
  • $\begingroup$ 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? $\endgroup$ – shabby Jan 14 '14 at 18:01
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Years ago I wrote a program to do this for a C182, a C210 and later for a Saratoga. The challenge is that in a piston single, the cruise speeds are low enough so that winds can be quite significant. Because actual winds change from forecast, it is desirable to have the calculations updated in flight. When I did this, GPS was not available. And calculations were done on laptops or on a programmable calculator.

The smallest solution was to use simultaneous differential equations, although I know one person who later wrote software for this is did so with look up tables. The performance of the aircraft (initially fitted from the POH) was reduced to equation form.

At the time gas prices were shockers, and seldom were flights optimized for minimum time, it was always for minimum fuel burn. I used this for pleasure flying only, and for the routes and weather I flew in, the solutions were typically 4 to 12k altitude. If a tailwind, one ran at the lower end of the power range, whether that was 45 or 55%. If a headwind, one was often pushing the higher end of the power range, usually 65 to 80% depending upon the aircraft.

Today, I just guess the power setting I want, and go. If in the summer, I might go below 55%, but cruising in the northeast US in the wintertime, at 12k, at 45% is not a warm proposition.

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