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High altitude atmosphere has lower oxygen level. How do aircraft, especially the military ones, prevent engine flameout?

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  • $\begingroup$ See here for an answer. $\endgroup$
    – Peter Kämpf
    Aug 30, 2015 at 12:45
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    $\begingroup$ High altitude atmosphere has lower oxygen level. No, it doesn't. $\endgroup$
    – Simon
    Aug 30, 2015 at 17:51
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    $\begingroup$ The air is more thin higher in the atmosphere, but the oxygen level does not decrease in the atmosphere just the air thinness and pressure. $\endgroup$
    – Ethan
    Aug 31, 2015 at 22:06
  • $\begingroup$ I think what he meant was that the partial pressure of oxygen decreases with altitude, which it does. $\endgroup$
    – Vikki
    May 27, 2018 at 13:36

2 Answers 2

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The oxygen level at high altitude is the same as at sea level, namely 20.95%. What is lower is the oxygen pressure.

To avoid flameout, pilots must avoid to throttle the engine down too much, so a minimum pressure in the combustors can be maintained.

Designers can increase the altitude performance by building longer combustors. The speed of ignition is pressure dependent, and with lower pressure it takes longer for the fuel-air-mixture to ignite. More length allows more mixing and leaves the mixture more time to react. Also, they can add flame holders which leave pockets of burning fuel close to the injection nozzles, so the mixture is heated more rapidly and ignites more reliably. Normally, flame holders create drag and are reduced to the minimum necessary.

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    $\begingroup$ Throttle up or down? I assume down but I have no time in jets. $\endgroup$
    – egid
    Aug 30, 2015 at 22:19
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    $\begingroup$ @egid: I think "throttle" is usually used as a synonym for the verb "limit"? I don't think I've ever seen it as a synonym for "relax"/"increase"/etc. $\endgroup$
    – user541686
    Aug 30, 2015 at 23:34
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    $\begingroup$ @Mehrdad that's true, but in aviation, the throttle is the control for how much power the engine delivers. I've heard it verbed as "throttle back", down, up, etc.; it seems to be more useful with a modifier. Limit in this example doesn't make sense either, fwiw: "pilots must avoid to limit the engine too much". $\endgroup$
    – egid
    Aug 31, 2015 at 1:14
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    $\begingroup$ @egid: Mehrdad's understanding is correct (the context was probably helpful), but I agree that clarity of this sentence can be improved. Edited. $\endgroup$
    – Peter Kämpf
    Aug 31, 2015 at 6:25
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    $\begingroup$ @JanHudec: Isn't the action of moving those power levels called throttling (up or down)? $\endgroup$
    – Peter Kämpf
    Aug 31, 2015 at 8:50
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At high altitude, aircraft engines have a number of built in safeguards to prevent flameout, like,

  1. The engine igniter is on continuously. This continuous ignition is set automatically by the FADEC if flameout conditions are detected.

  2. In case flameout occurs, the FADEC tries to restart the engine automatically (in response to loss of thrust, for example).

  3. The engines also have compressor bleed valves, which serve to prevent compressor stall in case a sudden increase in thrust is demanded from the engine.

Also, as Peter says, at high altitude, the percentage of oxygen remains the same, but the partial pressure is less.

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  • $\begingroup$ Better answer IMHO as it actually answers the question as it was asked. +1ed $\endgroup$
    – Alexus
    Aug 31, 2015 at 22:21

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