Question about Flex Temp

[Midland63]

Was watching a Cap Joe vid about this and I get the general principle about "fiddling" the parameters to "trick" the FADEC (yes?) into setting a particular thrust but setting a high temp to get a lower thrust seems counter-intuitive to me because I thought jet engines produced higher thrust in colder temps (air more dense). Therefore I would have thought the logic would be the computer says to itself "It's cold outside today so we'll produce bags of thrust so we can reduce it a bit."

Any pro's looking in who could explain in simple terms?

(I believe this is Airbus but Boeings the same in principle with different terminology.)

Thx
M63

[Midland63]

Thanks for your reply Tango.

I get that engines don't perform so well on hot days so if you tell them it's 55C, why don't they react by saying "Crumbs it seems to be hot out there today, we better give it full welly to get off this runway"?
See what I mean?

[Midland63]

Gulp! That still doesn't me. Sorry!

Am I overthinking this and the flex temp is just a co-efficient reflecting the environment which happens to be a temperature by convention (perhaps because that was how they first looked at it when the technology was being developed)? It could equally be assumed runway length or inside leg measurement etc?

A bit like how the co-efficient of lift is really just the last figure to make the equation work as empirically demonstrated by the airfoil's performance at different AoA, flap settings etc. once you've entered the square of the speed, clean wing area and air density ?

Thx
M63

[Mad (Flt) Scientist]

I think a point you are missing is that FADECs do not adjust the thrust to achieve a target level of aircraft performance - the engine doesn't really care about that.

Engines are designed to give the commanded thrust, not to modulate the thrust according to some "what the pilot needs" process.

It might be easiest to first consider that you can flex with a NON-FADEC engine too, and work through that more simple system example. then the FADEC case is really just the same.

So on a non-FADEC engine and aircraft, it's a cold day and you have bags of performance "in hand" - the runway is much longer than you need, there are no obstacles etc. So you don't need all thr thrust you have today. But how much can you safely reduce it by?

Well, you look at your performance charts and work out "if it was this (hotter) temperature then i would just be able to takeoff safely". That then tells you how much less thrust you can safely use.

Now you look up your manual thrust setting table, and find on that hot day you'd set say 92% N1 instead of the 95% you can use today. So you "flex" to the temperature that just makes the takeoff safe, and set the corresponding thrust, and away you go.

The only difference in the FADEC case is that you don't have a manual to set the n1 - the FADEC calculates it. So you need to tell the FADEC "hey, let's pretend its hotter today" by giving it the flex temperatuire, and everything else works the same.

[Midland63]

Think I've got it now.

You're not "tricking" the engines, you're giving them a command: "Set the thrust which is the maximum you'd be able to give if the temp was 55C". That will be less than they're able to give in today's actual temperature of 15C.

Right?

[Mad (Flt) Scientist]

Think I've got it now.

You're not "tricking" the engines, you're giving them a command: "Set the thrust which is the maximum you'd be able to give if the temp was 55C". That will be less than they're able to give in today's actual temperature of 15C.

Right?

Correct. You've got it.

[Midland63]

Thanks for the replies. That's cleared up something that's puzzled me ever since I first read about the concept 30+ years ago in an article about BA's introduction of 757s!

When setting the derated thrust for take off, is there a detent or a button to press or does the pilot just have to manually advance the TLs until the the guages show the selected figure? And if the latter, does PF or PM do that?

[Midland63]

Thanks Tango.

I knew about setting 40% initially to allow the engines to stabilise (and "equalise") but I wasn't sure whether the TOGA buttons (on Boeings) commanded the pre-arranged derated thrust or were there for "Yikes! We suddenly need everything we've got!" and you've answered that, Thanks.

Is there any such thing as a pre-arranged derated go-around thrust or is that always full power (if doing it via the TOGA buttons rather than maually advancing the TL's). Seem to remember a thread on here suggesting full power at GA was not always appropriate.

M63

[Midland63]

Thanks, didn't know that. (Each new factoid I learn about this prompts another two questions but I'll spare you!)

M63

[Denti]

To be honest, ATM was available on the non-FADEC 737 classic in exactly the same way, so we did set it in the FMC, pressed the TOGA buttons and the rest was done by magic electric computing. We still used tables, but that was just to get to the correct assumed temperature, not the required thrust setting, back then we didn't have an EFB yet. In theory it allowed for derates as well, however we were not certified for that, unlike on the NG.

In effect the same as for the Boeing is true on the Airbus, put the assumed temperature in the FMCG and the autothrust works it all out. As for setting take off thrust that is slightly different, but in effect the same thing. Set 50% N1, wait for it to stabilise and then move the thrust levers to the FLX/MCT , at thrust reduction pull them back to CLB and they stay there until flare (except if one wants to use manual thrust, which is of course available). For go around simply push them forward to the most forward detent TOGA, which is the same that is used for an unreduced take off. You get a full go around thrust though, soft go-around is only available in new aircraft and i believe in very new A320NEOs as well. One can simulate that however, by going into the TOGA detend, wait until the TOGA mode is activated and them pull pack into CLB.

[MarkerInbound]

In even more primitive aircraft the Flight Engineer would take our weight and look up what the maximum temperature was that we could depart our assigned runway at that weight. They would then look up what "full" thrust was at that temperature. We would set the EPR bugs at that assumed temp thrust setting. We should meet all required performance numbers doing that. We always had the option of introducing Mr. Thrust Lever to Mr. Radar Screen if things started coming unglued.

[Impress to inflate]

On a side note, a good friend of mine flies for Cargolux, he said the Capt's wear a T-shirt that says "Capt Joe is my Co-Pilot"