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I'll be using Airbus terminology here because that's what I know, but I guess the concepts are universally applicable.

The way I understand flex temperature is: It's a user interface for pilots to request a derating of engine power to reduce wear on the engine. Simple enough, makes sense.

But in understanding why this is being input as a temperature, I have often encountered confusing explanations. Many videos and forum posts will say something like this:

Flex temp will fool the FADEC to think the outside air is hotter than it actually is.

That cannot be correct, right? If the FADEC actually thought the air was less dense, then if anything it would increase engine power, not derate it.

What makes more sense to me is this:

By entering flex temp, you're intructing the FADEC to produce only as much thrust as TOGA would produce at that increased temperature.

Simple. We're not fooling the FADEC, it knows full well how much thrust it is producing and that's good.

Using a temperature value as the input (instead of some percentage of engine power) also makes sense because pilots need to be aware of this parameter anyway when considering altitude, runway length, and aircraft weigth to verify a takeoff is possible.

If I'm correct, why is this "myth" of fooling the FADEC so prevalent? I find this notion very confusing and counter-productive when learning about this. Does the FADEC even actually deal with outside air temperature in its logic? Why and how? Or does flex temp really just translate to a thrust value (possibly together with altitude)?

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  • $\begingroup$ M'not an expert but what about the fact that a higher outside temperature would result in higher compressor outlet temperature, and thus higher turbiine inlet temperature? $\endgroup$ – Abdullah Jun 21 at 14:38
  • $\begingroup$ @Abdullah You're saying the FADEC will measure thrust by looking at EGT? Hm, I don't know. It would be interesting to learn how the FADEC can measure actual thrust. Can it even? $\endgroup$ – trehn Jun 21 at 14:48
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    $\begingroup$ The engine must not be allowed to overheat, that's what I'm saying. A higher simulated outside temp = higher simulated turbine inlet temp, right? then we have to throttle back $\endgroup$ – Abdullah Jun 21 at 15:42
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There are at least two misconceptions in your question:

  1. The way I understand flex temperature is: It's a user interface for pilots to request a derating of engine power to reduce wear on the engine.

    This is not quite correct. A derating would imply that the engine thrust needs to remain at the derated level after an engine failure. A FLEX or assumed temperature method takeoff allows setting higher thrust in case of an engine failure, so it's not a derating. See also What is the difference between Assumed Temperature and Derate takeoff thrust?

  2. If the FADEC actually thought the air was less dense, then if anything it would increase engine power, not derate it.

    This is not correct when the engine is temperature limited. The hotter the air, the hotter the EGT at the same fuel flow. Therefore, the FADEC has to reduce fuel flow to the engine for higher air temperatures, which reduces thrust.

Consider the following slide from this Airbus presentation:

Flexible thrust

You can see that the engine is flat rated (see What is a flat rated engine?) below a temperature Tref, which means the engine is not limited by EGT and can provide the same thrust regardless of temperature (see e.g. this answer for other limitations). If the temperature is higher than Tref, the FADEC has to reduce fuel flow to keep the EGT below the limit. This results in less maximum available thrust.

By entering a FLEX temperature, the FADEC will reduce fuel flow to the engine as if the actual temperature would be that high. We can argue about the semantics of whether or not this is actually fooling the FADEC, but the fact is that the FADEC will reduce the fuel flow accordingly. This will then actually give you a lower EGT than the maximum because the actual air temperature is colder and therefore reduces engine wear and tear.

The FCOM contains some additional information about what the FADEC does:

The FADEC system performs the following functions:

Control of gas generator

  • control of fuel flow
  • acceleration and deceleration schedules
  • variable bleed valve and variable stator vane schedules
  • control of turbine clearance
  • idle setting

[...]

Power management

  • automatic control of engine thrust rating
  • computation of thrust parameter limits
  • [...]

(Airbus A320 FCOM - Power Plant - FADEC)

When the thrust lever is in the FLX/MCT detent:

  • On the ground

    The engine runs at the flex takeoff thrust rating if the crew has selected a flex takeoff temperature on the MCDU that is higher than the current Total Air Temperature (TAT). Otherwise the engine produces Maximum Continuous Thrust (MCT).

(Airbus A320 FCOM - Power Plant - Thrust Control System)

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  • $\begingroup$ Thank you! I was unaware that an EGT limit is what causes the thrust reduction, not just the physical effects from the reduced air density. $\endgroup$ – trehn Jun 21 at 17:44
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There are other limits to engine operation as well as thrust and EGT temperature, for example maximum rotor speeds.

Thrust is often limited in "hot and high" situations because the LP rotor can't run fast enough to push enough air through the engine to deliver maximum thrust. So "telling the FADEC the air is less dense than it really is" is likely to produce less thrust, not more, which is the desired result.

I agree that "fooling the FADEC" is not a very precise explanation for what is going on, but it makes a sense to give the pilot a control input that is easy to understand, such as "fictitious outside air temperature".

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