For derated thrust, you enter a higher flex temp than is actually the case, right? Since airplanes like colder air, wouldn't making it think it was hotter cause it to use more thrust, not less? Isn't it harder to get off a hot, humid runway in the summer than a nice cold one in the winter? Or do I need to go back to ground school?

Check out the Captain Jeeves Flex Temp Thread at PPRuNe and see if that makes things any clearer.

That's interesting, but it still seems backwards to me. I was taught that colder air is denser and the airplane requires less thrust for the same amount of lift. So I would want to fool my airplane into thinking it was colder than it actually was if I wanted the engine to do less work. I'm not doubting that this is wrong; I am trying to understand the logic behinda higher flex temp, rather than a lower one. This is what it seems like to me:Airplane: "Yay! Nice cold day. At this weight, altitude, humidity and temperature, I won't have to work as hard to get off the ground."Me: "Sorry, but it is actually forty degrees warmer than you thought it was." Airplane: "Drat! This is going to be harder than I thought. I'll need more engine power."Me: "No, says here if I fool you into thinking it is warmer than it really is you'll use less."Airplane and Me Together: "Huh?"

It would seem the obvious thing to do to just 'firewall' the throttles when hot and high and hope that you'd accelerate enough to get the lift required, but unfortunately that doesn't take into account the fact that you can swamp the engine with warm air on occasion in such circumstances (which could actually damage it in addition to not providing enough thrust, which is why it's a careful consideration).So that might help you get your head around things a little bit. That is to say, the air temperature and density is not simply a consideration for the wings (although that's important), but also for what goes into the engine in regard to how dense it is when it goes in and how hot it is too, i.e. if you try to force a lot of less dense air into a jet, it will not get sufficient to compress things enough for the required combustion and thrust you have commanded with the throttle (cooling problems can occur too). This is also important for high bypass turbofans for another reason, because HB turbofans actually put more air around the outside of the engine than they do through the core (hence the high bypass name, and also why they make that howling wind noise instead of a turbojet scream).Isn't flying jets properly fun? :( Al

Isn't flying jets properly fun? :(

Learn something new every day. Thanks!

You are not "tricking" the airplane... you can't fool it... it needs what (thrust) it needs to fly. Your conversation with the Airplane doesn't exist. It only exists with the Engines. Actually the first Line is correct... But your "sorry" isn't you speaking to the "Airplane"... it's you speaking to the Engines. So swap in the lines:Engines: "Drat! I won't be able to make the thrust I could make if the temp was what I thought it was :( & :( "Me: "Aha! So! I will be able to save wear and tear (and fuel :( ) on you by "my little trick!"Make sense now?

For derated thrust, you enter a higher flex temp than is actually the case, right? Since airplanes like colder air, wouldn't making it think it was hotter cause it to use more thrust, not less? Isn't it harder to get off a hot, humid runway in the summer than a nice cold one in the winter? Or do I need to go back to ground school?

Hi Mendota,here is how it works: above a certain temperature (Tref), engine thrust becomes limited by EGT, so the higher the temp, the lower the max thrust the engine can provide.Hence, using the assumed temperature, I'm basically saying to the engine:"The temperature is this high, please reduce your N1 if you don't want to melt your turbine", even if the temperature is actually much lower. :)Obviously reduced thrust means reduced MTOW, so flex thrust can only be used if the take off weight is less than the MTOW at the actual conditions.Marco

Thank you for the responses. See, I was thinking with my GA brain and wondering what I was missing. Now I know.

For derated thrust, you enter a higher flex temp than is actually the case, right? Since airplanes like colder air, wouldn't making it think it was hotter cause it to use more thrust, not less? Isn't it harder to get off a hot, humid runway in the summer than a nice cold one in the winter? Or do I need to go back to ground school?

Noone in the thread have gotten the derate concept correct. Derating the engines simply reduces the maximum thrust output at full thrust so as to reduce engine wear and fuel burn. So, what you are doing is entering an temperature at which the engine will perform as if it would in real life if it were really that temperature. The reason for this is because today's turbofans are extremely powerful and on most flights it is not at all necessary to use the full dry power of the engines at take off. Its a waste of fuel and its unnecessary wear and tear on the engines. Thusly, if you enter 60c, for example, into the FMC, the engines will perform as it would on a 60c day which translates to less thrust at Vmax, but more than enough to get you down the runway and up into the air. The calculation for this takes into account wind component, runway length, actual temp, takeoff weight, etc.So, imagine that an engine has a stated rating of 20,000 lbs of thrust at 59F at 29.92 (standard temp and baro) at full thrust (not to be confused with max thrust). When you derate the engine you will get, for example, a rating of 17,000 lbs of thrust at full thrust in the same atmospheric conditions. Do you need 20,000 lbs to get off the ground? No. Is 17,000 plenty of thrust to get you into the air? Yes. So, why make the engines work to produce 20,000 lbs of thrust when 17,000 is all you need? The maximum amount of thrust your engines will produce is now less, therefore the engines are considered 'derated'...its not about melting engines or "swamping your engines with warm air". Now, you COULD simply say, why go through the process of derating the engines and simply use less thrust. Well, back in the olden days, the second officer was the flight computer and he/she derated the throttleswhen the captain called for Max Thrust at takeoff. Now-a-days in a two person flight deck, by putting in a derate value into the FMC, you can firewall the throttles and the computers make sure that max thrust is your derate value so no one is looking down at the engine gauges trying to trim the throttles while still needing to call out Vspeeds. The downside is, should you realize half way down the runway that your derated MAX thrust of 88% N1 is not enough and for some reason you require FULL thrust of ~105% N1, you ain't gonna get it! (Imagine a jumbo jet is crossing the runway ahead of you and stops just as you hit V1 and have 1000 feet to go!)You should also note that derate washout occurs ~12000 - FL180. As the air REALLY gets thinner you will need that thrust that you didn't need at sea level to make it to your final cruise altitude so the derate value will go away, and your engines will perform as they regulary do, which is fine since they are at their most efficient at cruise levels anyway. If derate did NOT washout, then your climb and cruise speeds would be slower, or you might not have the thrust to get you to final cruise in the first place.

Noone in the thread have gotten the derate concept correct.

Wow... and here I thought the captain jeeves (who stated he's been driving the Bus for about 10yrs when that reply was posted) gave quite the elegant and concise explanation of what Flex was all about. :(

Obviously reduced thrust means reduced MTOW, so flex thrust can only be used if the take off weight is less than the MTOW at the actual conditions.

No, not true, flex thrust can be used even if you are at MTOW provided you have sufficient runway in front of you and other (balanced field) conditions are satisfied too.

I am trying to understand the logic behinda higher flex temp, rather than a lower one.

I know exactly what you are after and there is probably not a single person on this forum (including myself) who could competently answer this question with all the nuances. A turbine engine is a very complex thermodynamic machine and what you are after would require more sophisticated knowledge of all the processes and engine controls. One very simplistic way of looking at it is that since warmer air is less dense, the resulting pressure in the ignition chamber will also be lower and FADEC (or another type of electronic engine controll) will deliver less fuel keeping air-fuel mixture in proper balance. So by artificially keying in higher temperature you are effectively telling engine controller to deliver less fuel which then results in less thrust.

No, not true, flex thrust can be used even if you are at MTOW provided you have sufficient runway in front of you and other (balanced field) conditions are satisfied too.

I should not have used the acronym MTOW (Maximum Takeoff Weight). What I meant was the "maximum takeoff weight at the actual conditions", or as it should be correctly called, the "Maximum Permissible Takeoff Weight".Flexible thrust can only be used when the actual takeoff weight is lower than the max permissible T/O weight.You may want to take a look at the A319/A320/A321 FCOM, 2.02.14:"DEFINITION OF FLEXIBLE TAKEOFFIn many cases the aircraft takes off with a weight lower than the maximum permissible takeoff weight. When this happens, it can meet the required performance (runway, second segment, obstacle,...) with a decreased thrust that is adapted to the weight: this is called FLEXIBLE TAKEOFF and the thrust is called FLEXIBLE TAKEOFF THRUST.The use of flexible takeoff thrust saves engine life."and"USE OF FLEXIBLE TAKEOFFThe pilot can use flexible takeoff when the actual takeoff weight is lower than the maximum permissible takeoff weight for the actual temperature. The maximum permissible takeoff weight decreases when temperature increases, so it is possible to assume a temperature at which the actual takeoff weight would be the limiting one. This temperature is called FLEXIBLE TEMPERATURE or assumed temperature and is entered in the FADEC via the MCDU PERF TO page in order to get the adapted thrust."and"DETERMINATION OF FLEXIBLE TAKEOFF TEMPERATURE AND SPEEDSBefore determining the flexible temperature, calculate the maximum permissible takeoff weight (see previous section) and ensure that the actual takeoff weight is lower than the determined maximum takeoff weight."Technically speaking, the sentence of my previous post is actually true: if conditions permit an aircraft to take off at MTOW, it's actually taking off at the largest max permissible t/o weight, and hence it does not make sense to use flexible thrust: when the actual T/O weight is equal to the max permissible T/O weight, the assumed temperature coincides with the OAT and therefore there is no thrust reduction.

Derating the engines simply reduces the maximum thrust output at full thrust so as to reduce engine wear and fuel burn.

using reduced thrust actually INCREASES fuel burn. See here:http://www.b737.org.uk/assumedtemp.htm"9. Increases fuel burn.Strange, but true. This is because: 1. Assuming an uninterrupted climb, it will take longer to reach the more economical cruise altitude than a full thrust climb. 2. Engines are less efficient when not at full thrust."

The maximum amount of thrust your engines will produce is now less, therefore the engines are considered 'derated'...its not about melting engines or "swamping your engines with warm air".

At higher temperatures, the engine becomes limited by EGT. From the AIRBUS A319/A320/A321 FCOM, 2.02.10:So when I'm using the assumed temperature, I'm basically saying to the FADEC: "The temperature is this high, limit your N1 (and therefore thrust) or you'll melt the engine", even if the engine of course would not have melted, because the temperature is actually lower.Marco

"So when I'm using the assumed temperature, I'm basically saying to the FADEC: "The temperature is this high, limit your N1 (and therefore thrust) or you'll melt the engine", even if the engine of course would not have melted, because the temperature is actually lower."NO, NO, NO! The actual performance of the engines due to a given temperature, humidity and altitude is not the purpose of derated thrust! What you are saying to the engine is: "I want you to produce ONLY THE AMOUNT of thrust as you would at the derated temperature value and no more, since that is all I need to get this aircraft into the air. So, if it were 60c outside, your thrust would be x, BUT its NOT 60c outside, but I want you only to produce x anyway because that's all the thrust I require today." The engines look at that derate value and responds by saying "okay then, at 60c I would only produce x amount of thrust even though I'm capable of producing much, much more, but if that's all you want, then fine, you get x amount of thrust." As long as you have enough runway, you can derate the aircraft to your heart's content no matter HOW HEAVY you are.That's it. Think of this: If you are a world class runner, you can 'derate' yourself when running against other runners who cannot run as fast. You don't need to run at your fastest speed and tire yourself out or risk injury when running at your 'derated' speed is fast enough to beat everyone else.It has nothing to do with saving the engines from exploding due to the ingestion of hot or cold air!!!! As a matter of fact you never have to derate your engines if you don't want to, whether its -40 or 140 degrees, the engines don't care, they won't melt or explode unless you run then at Full thrust ~105% N1 which will indeed damage the engines. Make sense?

"So when I'm using the assumed temperature, I'm basically saying to the FADEC: "The temperature is this high, limit your N1 (and therefore thrust) or you'll melt the engine", even if the engine of course would not have melted, because the temperature is actually lower."NO, NO, NO! The actual performance of the engines due to a given temperature, humidity and altitude is not the purpose of derated thrust! What you are saying to the engine is: "I want you to produce ONLY THE AMOUNT of thrust as you would at the derated temperature value and no more, since that is all I need to get this aircraft into the air. So, if it were 60c outside, your thrust would be x, BUT its NOT 60c outside, but I want you only to produce x anyway because that's all the thrust I require today." The engines look at that derate value and responds by saying "okay then, at 60c I would only produce x amount of thrust even though I'm capable of producing much, much more, but if that's all you want, then fine, you get x amount of thrust."

That's right. In either case, the final result is the same. I'm just saying the same thing with different words! :)If I set the assumed temperature to 60c, I'm using the thrust I would have available if the temperature would have been 60c. We all agree on this.

It has nothing to do with saving the engines from exploding due to the ingestion of hot or cold air!!!! As a matter of fact you never have to derate your engines if you don't want to, whether its -40 or 140 degrees, the engines don't care, they won't melt or explode

That's right. I never said something different!

As long as you have enough runway, you can derate the aircraft to your heart's content no matter HOW HEAVY you are.

I don't know about derated thrust (derated thrust and assumed temperature are two different things. You can use derated thrust without using assumed temperature). If we're talking about thrust reduction using assumed temperature (Airbus calls it FLEXIBLE TAKEOFF THRUST), it can only be used if the aircraft weight is lower than its maximum permissible takeoff weight at the given conditions. Re-read the Airbus FCOM parts in my previous reply.Marco

. (sorry for double post).