B737 OAT input error with correct ATM

[cmmsense]

An incorrect OAT temperature input to the FMC resulted in an insufficient N1 generated by the FMC. The incorrect OAT input was -52C instead of 16C. However, the correct Assumed Temperature of 50C was entered into the FMC as well. How is it then, that the FMC produced an N1 that was grossly insufficient for the take off? I cant seem to find the relationship between the OAT and the Assumed Temperature and N1. I mean, if you input -52C instead of 16C the initial N1 will be quite low since the air is denser. But then you are Assuming to 50C. As the temperature increases the N1 will increase to the Flat rated temp (around 30C) then the N1 will start decreasing again until you get your Assumed Temperature N1. So the question is, how can entering a wrong OAT make such a huge difference on the N1 considering you entered the correct Assumed Temperature in the box?

[Skyjob]

Incorrect: As the temperature increases the N1 will increase to the Flat rated temp (around 30C) then the N1 will start decreasing again until you get your Assumed Temperature N1.
The ATM reduces the N1%, not increase it.
ATM is a method of reducing engine N1% when a suitable performance margin exists.
ATM and FMC N1% prior rely on the correct temperature input in FMC PERF INIT.
The baseline figure used by FMC is the N1% based on OAT, reducing the temperature reduces this N1%, a lot in this case (+16C to -52C).
You then reduce N1% further by enabling the performance reduction using ATM, in this case -52C to +50C, a huge reduction of N1% on top!

[cmmsense]

I should have said as the OAT increases so does N1% until about an OAT of 30C, then the N1% starts to decrease. This fact threw me off. N1% at an OAT of +50C is irrelevant.
I think I got it now. The assumed temp of +50C gives a lower thrust. To achieve this lower thrust at an OAT of -52C the FMC will have to decrease the N1%.
Ok, thanks Skyjob.

[torghabe]

For CFM56 FRT=32C. I might be wrong, does not matter what the temperature (OAT) input to the FMC. Until FRT (32C) N1 increases and reach limit N1 at FRT, if the OAT continue increases, engine power start decreases, but N1=limit N1. What is the assume temperature? This is temperature when actual TOW= max TOW and does not matter what OAT is. What is the derate TO thrust? This is 5 or 15 percent of Full TO thrust reduction (not of N1).

[cmmsense]

Full thrust at an OAT of 50C will give a different N1% than let's say: ATM of 50C at an OAT of -16C...you agree?

[Avenger]

U12:

Adds a comparison of an OAT entry made by the pilot with available OAT information provided from the EEC (NG) or TAT (Classics). If a difference is detected the FMC alert message “OAT DISAGREE -DELETED” and TAKEOFF SPEEDS DELETED will be displayed.

[BluSdUp]

Torghabe
The CFM 56 7B series are all Flatrated at 30 degree Celsius.
Ie, At sealevel STD but Isa pluss 15 ( 30c) the engine will produce 26300lbs for an CFM 56-7B26 for example.
Any hotter than that and the engine produces LESS pwr at full thrust as it gets any warmer.

I always find it fascinating when when the FO ask " we can assume with 31c "

[torghabe]

BluSdUp
I agree. CF6-80..., GEnx... FRT=+32C. My mistake, thanks!