View Full Version : Assumed Temp Thrust Reduction - yes, again!

24th Mar 2010, 19:24

This had been thrashed to death no doubt many times so sorry for inducing nausea in those who couldn't care less. But I just don't get it...!

I can repeat it parrot fashion all day but when asked to explain the detail my description ends up describing the opposite. By the way please look away now if, like me, you get a reasonable grasp of something only for some numpty to come along and chuck a spanner in the works!

I'm thinking like this. Our engine is factory rated under ISA+15c conditions to, say, 20,000lbs so that under ISA+15c it will produce 20,000 when asked to deliver 100% - all it's got.

Then I muse about the same setup (a/c, runway, pax, wind, etc..) but 5deg cooler. Here our engine(s) will still develop the 20,000lbs (same as before - that's the factory limit) so theoretically same wear and tear forces beacause the work done is the same in both cases. So far so good? If we go for 5deg warmer then under full throttle the engine will deliver its all but the output or product of all this will be less, say 18,500lbs, and therefore less wear and tear as the work done is less. All above assuming commanded full throttle.

Why then would we want an engine to "think" that it's got its work cut out by fooling it to believe that the temperature is hotter and that it'd "better work its little legs off for us" - so presumably will be giving all its got in preparation for these adverse conditions. Surely only in the case of a genuinely hot day would the engine develop less thrust because of external factors. But in the case of the temperature being less and the engine giving all it's got (beacuse it thinks we're under inferior conditions) then this defeats the principle and we end up with the limiting arbitrary output for the actual (real) temperature....?

Excuse me whilst I puke....:yuk:

24th Mar 2010, 20:03
I think you're getting things a little bit mixed up?

The higher the flex, the more the de-rate so the engine is producing less thrust, the engine is fooled into producing what it thinks is the max performance for a theoretical higher temperature which doesn't exist, therefore with the ambient temperature being lower the thrust is derated from maximum for the ambient temperature.

Does that make sense??

24th Mar 2010, 20:25
Evening Half Cuban,

I see what you're saying but I have this image in my mind of a case where we have a genuinley hotter day and where every ounce of performance is needed as opposed to engine life considerations. Are we saying that the engine will develop a "user defined" thrust if told that the temperature is hotter - or - will the engine still deliver 100% of all it can given the requirements for full performance. I am remebreing that the hotter ambient temperature will affect a/c aerodynamic as well as engine output performance.

Put another way, lets say we tell the engine its hotter out there when its actually cooler - as is the case with assumed temp reductions. I must be wrong in thinking that the engine will still try to achieve 100% of the 20,000lbs, although it knows it won't beacuse we just told it the temp was hotter, but we're in for a surprise when it actually does because it's cooler??

I get the bit about flex and logically thinking any throttled-back settings ought to produce a corresponding throttled-back performance. I just don't get the bit about assuming a higher temperature when the engine would try to offset that as much as it could.

24th Mar 2010, 20:35

Using your example of full thrust = 20,000 lbs in ISA. If your performance calculation says you could do it at 70 degs with full thrust of 15,000 lbs say, then you limit the thrust to 15,000 lbs on your ISA day.

24th Mar 2010, 20:47

You're saying (or i think you are) that the performance calculations work back to the worst case scenario where beyond that performance is insufficient. Once the worse case scenario is known (you say 70deg giving us 15,000lbs) then you could very well use that 70 deg knowing that the conditions are in reality more favourable?

24th Mar 2010, 21:33
Hi Statorvane,


Flight Detent
25th Mar 2010, 03:22
1/ check out the airplane performance at the higher temperature (full thrust).

2/ though the temperature is actually somewhat lower, we set that previously 'checked out' thrust, by not actually pushing the thrust levers all the way up, (hence the lower work-load).

3/ we know the performance will be sufficient at that lower thrust setting because we checked it out. (thrust, ASDA etc)

4/ in actual fact, the airplane performance will be better than that 'checked out' performance, because the airframe will actually respond better in the cooler environment.

That's about as simple as I can manage!


FE Hoppy
25th Mar 2010, 10:39
You are misunderstanding what does the wear and tear. It's not the thrust produced. it's the temperature (ITT TGT EGT).

We derate to reduce the ITT. Work done is ITT not thrust. Thrust is density X ITT.

So when I ask for "full take off thrust" at a temperature above the flat rated temperature of your engine (typically isa +15 but not always) then the fadec looks in it's little table to see what N1 would normally be required and sets it.

As the N1 required will be reducing with temperature in this area above flat rated temp then the work done to produce that N1 will be less than the work required to achieve the N1 for the real temperature.

Less N1 will require less work and therefore less ITT.

Due to the density difference between real and assumed temperatures the actual N1 for assumed temperature will always be lower than the N1 required if the real temp = the assumed temp used.

25th Mar 2010, 11:14
Exactly. StatorVane think of it like this:

Engine is Max rated to ISA+15.

If the temperature is COLDER than this, the engine thinks - I'd better spin a bit slower, because if I spin at 100% with this cold temperature, I will produce so much thrust I may break something!

If the temperature is HOTTER than this, the engine thinks - I'd better spin a bit slower, because if I spin at 100% sucking in this hot air, by the time it is compressed & fuel is added it will be SO hot I will damage the turbines in the hot section! I know this will result in reduced thrust at these temps, but the pilots know that as well, and take the reduced thrust I am providing into their calculations.

If we use REDUCED THRUST we tell the engine it is really hot, so the engine spins slower, thinking it is protecting it's innards (when it is actually just providing a larger buffer before the limit, reducing wear & tear). We know that the engine is providing less thrust - and use the appropriate temperature table to take that into account. (Incidentally, we use the table for the thrust provided at that hot temperature, with the engine spinning a bit slower, however because the engine is taking in denser (colder) air than the table assumes we actually get a little bit of "free" extra thrust.)

oz in dxb
26th Mar 2010, 18:06
Guys, some of you are mixing flex/assumed temperature method of thrust reduction with derate. They are different ways to reduce your thrust.

Assumed temp/flex means that you reduce the thrust by assuming that you have a higher temperature. An advancement of the thrust levers after T/O is OK as your T/O speeds are taken care of.

Derate means that you operate the engine at a fixed percentage of thrust reduction. It's like bolting on a smaller engine. You can also use flex/assumed temperature on a derate.

Problems occur if you want extra thrust after T/O as your speeds have been calculated on the derate. You may not have sufficient rudder authority as the derated speeds are lower than using flex/assumed by itself.


27th Mar 2010, 00:15
All understood guys. Many thanks! :ok: