Using assumed temperature for reduced thrust

[AnthonyGA]

I've looked at some of the older threads that mention derates and assumed temperature reductions in thrust, but I'm still puzzled about one thing: why is takeoff thrust reduced by specifying an assumed temperature? Why not just enter a parameter that says "reduce thrust by x%"? Or maybe one that says "calculate thrust for a runway roll of xxx feet"?

I assume that specifying reduction of takeoff thrust in terms of an assumed temperature somehow saves labor for the pilots, but how? Is it because of some sort of calculation that pilots normally do anyway, or what?

[411A]

In simple terms...

The assumed temperature method of reducing takeoff thrust insures that the proper thrust setting is appropriate for the takeoff weight (mass, for our European friends).
This method is normally used with runway specific takeoff charts, commonly referred to as Runway Analysis Charts.
The chart is entered with the actual aircraft takeoff weight, and a reduced thrust setting (EPR) is selected that corresponds to a higher ambient temperature than what is actually present at the airport, yet still obtains sufficient runway and climb performance, for the actual aircraft takeoff weight.

This method reduces engine wear and at the same time, makes sure that the reduced thrust selected is still sufficient for the actual takeoff weight, on that specific runway.

[AnthonyGA]

Thanks! (Plus this to get 10 characters.)

[SNS3Guppy]

Anthony,

The higher the ambient temperature, the more takeoff performance suffers. On a warmer day, the air is less dense, and one needs to move faster to achieve the same amount of lift; one requires more runway.

If we can takeoff using performance calculated for a hotter day, then we know we can certainly take off from the same runway on a cold day. By running the numbers with a warmer temperature and determining the reduced thrust we could use if it were that warm, we can plan on a built-in additional performance factor for the conditions today (cooler). It's a method of assuring that we have the necessary performance, not only to take off, but to meet our climb requirements after takeoff, for various phases of flight.

[john_tullamarine]

411A and SNS3Guppy, both experienced heavy iron pilots, have the story.

However, a couple of additional points may make it clearer for AnthonyGA.

(a) when using derate, the effect is similar to bolting on a different (lower rated thrust capability) engine. The pilot's performance charts which are used on the line are changed along with the rating ie we just swap books, in essence.

(b) for routine operations at a given rating, the usual presentation for runway performance data is a table of data usually presented against OAT with columns relating to various wind conditions.

(c) the pilot generally has no quantitative indication of thrust output for any set of conditions

(d) the philosophy behind the performace calculations requires a bit more than setting the thrust to achieve a given runway length (although that is part of the overall set of calculations)

(e) OAT is an input to the setting of thrust.

(f) we COULD achieve the reduced thrust takeoff engine setting using any of

(i) assumed temperature (which is just a case of entering a "false" temperature in accordance with specified procedures to "fool" the engine into setting a lower than capable thrust on the day

(ii) setting the engine parameters (eg N1, EPR, fuel flow, whatever) according to a schedule which specifies the parameter values for a given percentage reduction

(iii) figuring the reduction for the runway length by reference to computers, tables, whatever and then checking that this is within the permitted boundaries of thrust reduction


(g) however, the easiest way to do it is to use a "false" OAT entry as that is data which the pilot has in front of him and is the quickest and easiest technique of the three.