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derated vs. assumed temp T/Os

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derated vs. assumed temp T/Os

Old 1st Sep 2004, 15:01
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derated vs. assumed temp T/Os

Gents and Ladies:
I need to know about derated takeoffs, I am very familiar with flex or assumed temp takeoffs but have never flown an aircraft that has a derated thrust takeoff procedure. Can someone shed some light on this topic, i.e. advantages of derated vs. assumed temp, limits, vmca implications, etc. Thanks in advance.

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Old 1st Sep 2004, 17:34
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xtwapilot ,

All of the following assumes that, as you've stated, you're well familiar with Reduced thrust, be it Flex or the Assumed Temperature method.

An engine may be operated at several ratings, for example the engine which I currently operate (RR Trent) is pilot selectable at TO (100%), TO-1 (92%), and TO-2 (80%). (Other operators may opt for different derates). Thrust reduction by Flex or Assumed temperature may be applied to each of the thrust ratings.

One Commercial advantage is that the guarantee from the manufacturer varies with the level of use of each thrust rating, but now to the performance aspect.

As you will be aware, the stated Vmcg and Vmca for each aircraft are provided for the RATED thrust. Reduced thrust can not take advantage of the obviously lower Vmcg/Vmca, it remains linked to that for the Rated thrust, for the RR Trent example, 3 sets of Vmcg/Vmca exist. Following this, 3 sets of minimum V1, Vr, and V2 exist.

If the aircraft were to operate on a runway where accelerate-stop limited at full takeoff thrust, the performance application is to progressively reduce V1 until the accelerate-stop can be accomodated within the ASDA. The lower limit to V1 is Vmcg, and no further advantage could now take place when V1=Vmcg. So, if the next lower engine DERATE were used, V1 is lower, and we can continue the optimisation process of progressively reducing V1 until we can squeeze the ASDR to within the ASDA, if not, look at the next lower rating and so on. In some ways it is a 'crazy' concept, where you cannot takeoff at all at full thrust, but lift an acceptable payload at a lower thrust. Of course, climb performance suffers, but the big advantage is to use it when Field limited due to Accelerate-Stop.

Where the 'big' Derates really shine is on contaminated Icy / Slippery runways where lateral control on the slick surface is severely degraded. Our company policy is to use TO-2 (80%) here, which allows for dramatically improved Vmcg and lower V1s, allowing lower, but commercially viable loads to be carried whereas operation would be impossible at higher thrust levels.

Another advantage of DERATED thrust is in beating the rules which allow reduced thrust to be no less than 75% of Rated Thrust. If you have an approved derate to 80%, you can now continue to 'Flex' right down to 60% of total thrust available.

To wind up, if you're facing a very long Dry runway with plenty of performance to spare, Reduced thrust is a very attractive option. If for example you needed 90% of maximum thrust for a particular takeoff, there's absolutely no performance difference in Flexing 10% lower than full thrust, or 2% lower than the 92% rating. There might be a difference with the guarantee though!
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Old 1st Sep 2004, 21:41
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Assumed temp – take-off with less than 100% thrust but you can push the thrust up to 100% if needed.

Derated – take-off with less than 100% and that’s all you have (can not or should not push the thrust up.)

In addition to Old Smokey’s good explanation it could also affect fees at some airports which can be based on max certified takeoff weight. Some operators use fixed derates and certifies their aircraft to a lower MTOW.
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Old 2nd Sep 2004, 03:34
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Thanks a million, having never used derated thrust takeoffs at any of the airlines I flew(at least not on my equipment), things are certainly much clearer now.

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Old 2nd Sep 2004, 13:27
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Assumed temp – take-off with less than 100% thrust but you can push the thrust up to 100% if needed
True, but you can only push the power up to 100% of the rating in USE! Its not a good idea to push it to 100% of the installed engine thrust due to VMCG limitations.

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