B737 derate & trim

[RAT 5]

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?

[rudderrudderrat]

Hi Rat 5,

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

[alexban]

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.

[Denti]

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