B737 derate & trim
 

This is a question regarding pitch trim settings with different de-rate thrusts. 1 operator uses a basic loadsheet trim setting. For 26k & 24K de-rates this is adjusted with a -ve figure, for 22K it is adjusted with a +ve figure. I was told this is because of the effect of underwing engines. More thrust gives a more nose up pitch and thus needs less nose up trim. I believed this explanation, but can not guarantee its truth.
I start my question with an assumption: you derate 22K, assumed temp or not, so that the thrust setting matches the ATOW. The elevtaor trim is adjusted +ve from the loadsheet. Now you decide to use 24K or 26K and greater assumed temp to also achieve a thrust = ATOW. My assumption is that the thrust derived in all cases is the same, as it matches ATOW. However, in the latter cases the elevator trim is adjusted -ve from the load sheet, the former it is a +ve adjustment. If the thrust given is the same then both trim settings can not be correct. Is my assumption correct, and if it is, what's the answer about the trim?

Hi Rat 5,

I agree with your logic.
The stab trim figure is the best approximation assuming TOGA thrust.

Using OPT for takeoff performance , and doing often derate/assumed temp, I can say it will give you different stab trim in case of derate.
By doing derate you may sometime gain a few% reduction on takeoff, weight/rwy permitting. So you'll have a different N1 setting for 22k or 20k derated, hence a different stab setting.
If you'll get the same N1 (derate and assumed combined) ,then a derate is not helpfull.

We use the FMC-derived trim setting, it even changes with bleed configuration changes as that affects take off N1 as well.