If the calculation via the Assumed Temp method gives you a target N1 of 98.3% and the fixed derate gives you a target N1 of 98.3%, they give EXACTLY the same thrust and EXACTLY the same margin!

However, assumptions and procedures differ, so your margins will vary. For example:

Your company procedure is to use standard derates. You go into the book and find that at Derate 1 for your conditions, N1 is 98.3% and the max TOGW allowed is 300.0. Derate 2 has an N1 of 96.8% and a max TOGW of 275.0.

If your TOGW is 275.0, you can use Derate 2 with the minimum legal margin, or Derate 1 with more margin. If your TOGW is 275.1, you must use Derate 1, and your margins will be significantly higher than if your TOGW was 300.0.

Now say your company procedure is to use a manual or computer-assisted Assumed Temp Reduced thrust. If the margins used by the company are the same as those used in the max TOGW calculation of the derate tables, and your TOGW is 275.0, it will give you an N1 of 96.8%. If your TOGW is 275.1, it might give you 96.9%.

If the company uses other margins, all bets are off.

De-Rate vs Assumed Temperature
 

Hi Fredairstair. I am not able to compare the "De-Rate" method I used to use against the "Assumed Temperature" method as I have never used the latter. Reading other posts I think I would rather use the old fashioned "De-Rate" method I used, in as much that having determined the thrust required, for the conditions and runway length available, to safely take-off at a given BRW it was simply a setting of the "De-rated" thrust determined with the thrust levers. If for any reason whatsoever extra thrust was felt required, max thrust (up to flat rated thrust up to 29C) could be applied by simply advancing the thrust levers. No presetting of FADEC or engine components came into it. We had a maximum of three levels of "De-Rate" and it could be used at all our ports with the exception of 07/25 at Sydney and at Gatwick. We also had a "Bump Thrust" available, if required, to get us out at weights above those to which we would be limited with normal Max thrust.

I am sure there are some variations between respective methods, however they are all aimed at achieving as long between engine overhauls as is possible.

On the B744 that I fly we have:

TO, TO1 and TO2 available with TO1 being a 10% derate and TO2 a 20% derate.

Additionally assumed temperature can be used to give a flex thrust up to 25% less than the rating selected.

So, for example, we could achieve an 80% thrust takeoff using any of the ratings. But the V1 and Vmcg that we use would be different for each rating, because if we manually push the thrust levers fully forward we will get the thrust rating that we set.

We cannot use a flex thrust on a contaminated runway.

The company computer automatically calculates the "optimum" rating and assumed temp, unless we override it by demanding a particular TO rating.

That seems to be pretty much the same as in the 737s. We have three fixed thrust settings, all with their own VMCg/a limitation and own performance data (for the 700/300 its 22k, 20k as derate 1, 18k as derate 2) and on top we can use assumed temperature to furter reduce the available thrust.

The catch is that with a fixed derate you have to be careful about your minimum control speeds, simply advancing the thrustlevers to full certified thrust can lead to very unpleasant surprises if you are below the relevant minimum control speed.

A notebook tool calculates what thrust setting, assumed temperature are used and if we used improved climb performance, however we can of course override some settings if we deem that necessary.

Still, it is weird to use 3500m of a 4000m runway until you rotate and our ATCO friends certainly don't like that either.

The other "catch" is that tables for the fixed derates are based on the assumption that you SHALL NOT push the throttles any further if you lose an engine at V1! That is a REAL significant difference between fixed derates and assumed temp reduced thrust!