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I am reading from my gas turbine engine class notes that apparently, one indication of compressor stall (besides the banging sounds, torque gauge low, N1 low, ITT gauge up, etc) is the fuel levels starting to drop.

since the compressor stall results in a reduction of airflow to the turbines, more fuel will be required to maintain the current thrust, increasing the fuel consumption increases burner and turbine temp

Ok, so (the last paragraph was from the class notes) yes I would agree that due to the lack of enough airflow turbines and burner sections might increase in temperature.

However, why is there more fuel being released to the combustion chamber whenever there is less air? shouldn't the fuel control unit or another system reduce the fuel due to the reduction/lack of air to keep the ratio of air:fuel ratio for a proper mixture?

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    $\begingroup$ in the title and first paragraph of the question, you are talking about a decrease in fuel flow, but then in the second and third, you are talking about an increase in fuel flow. So I'm not really sure what you are trying to ask about. $\endgroup$
    – Daniel K
    Jun 18, 2021 at 20:24
  • $\begingroup$ that's exactly what I mean. title and first paragraph apparently are facts that's what happens (fuel composition goes up, fuel reading or gauge goes down). however, I don't understand why that is the case when I would suppose that the engine lowers the release/comsuption of fuel to keep a good air:fuel ratio. $\endgroup$
    – YamchaAviator
    Jun 18, 2021 at 20:30
  • $\begingroup$ Maybe you think that fuel is measured to achieve the correct stoichiometric ratio (as usually done in piston engines), but this is not true for gas turbines. They run much leaner so that part of the oxygen is only working mass (as is all of the nitrogen) and much more fuel could be added before the mixture becomes too rich. This is avoided, however, to keep the turbine entry temperature below its limits. $\endgroup$
    – Peter Kämpf
    Jun 19, 2021 at 3:49
  • $\begingroup$ Re "increasing the fuel consumption" and "one indication of compressor stall ... is the fuel levels starting to drop."-- So you are suggesting that the fuel consumption increases so much that the pilot is able to notice that the fuel tank gauges are moving toward empty at a higher-than-usual rate? That seems rather implausible. There are really two questions here-- I would suggest editing to focus entirely on " why is there more fuel being released to the combustion chamber whenever there is less air?" and forget about the fuel tank gauges. $\endgroup$
    – quiet flyer
    Jun 19, 2021 at 14:55
  • $\begingroup$ (Ctd) Once you have answer to the first, if you then want to ask whether rapidly-lowering fuel tank gauges really are a practical, useful indication of a compressor stall, you could ask that as a separate question. $\endgroup$
    – quiet flyer
    Jun 19, 2021 at 14:56

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I might depend on what your fuel control unit is designed to do, and maintaining the optimal fuel/air ration may not be it. For example, some Full Authority Digital Engine Controls will look at power lever position and meter fuel to achieve the pilot desired power output, therefore adding fuel when airflow decreases.

This would be a good question to ask your instructor if your course is engine specific. I will also add that a lower than normal fuel gauge quantity indication will be one of the very last indications of a compressor stall that you will notice!

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  • $\begingroup$ awesome Michael. That last sentence "adding fuel when airflow decreases" ... meaning adding more fuel to what was already being added? or is it adding the same amount of fuel and now with a lack of enough air it looks like is consuming a lot? or how does it make sense to throw more fuel to a tiny bit of air and expect moe thrust? I guess that is where my confusion is. $\endgroup$
    – YamchaAviator
    Jun 18, 2021 at 22:49
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    $\begingroup$ I just learned today that the FADEC I will be flying will add fuel when bleed air is turned on in order to maintain pilot demanded power output. So, with a decrease in airflow that would naturally cause the fuel air ratio to change. But it kinda has to be that way, because if it reduced the fuel flow further to maintain the ratio the power output loss would be even more dramatic than if it did nothing at all, right? $\endgroup$
    – Michael Hall
    Jun 18, 2021 at 22:54
  • $\begingroup$ ok yes, I am starting to see the light. I think it works similar to a car, when your air filter is dirty or basically not enough air coming into the engine your car does consume more fuel, the question is why? I guess I have to look at the chemical reaction of it? $\endgroup$
    – YamchaAviator
    Jun 18, 2021 at 22:58
  • $\begingroup$ Well, you touched on it already, if the mixture is rich more fuel will be used for a given power output, right? $\endgroup$
    – Michael Hall
    Jun 18, 2021 at 23:00
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    $\begingroup$ If something is constant, (in this case power) and you change one variable, (in this case reduced air) then something else must change in order to maintain the constant. (in this case fuel) Otherwise if you reduced airflow without changing fuel flow the power output would decrease. In a car, as your filter gets dirty, you don't just drive slower, you push the gas pedal down more to drive the same speed. $\endgroup$
    – Michael Hall
    Jun 18, 2021 at 23:29

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