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I was playing with reduced takeoff thrust in a Boeing 737-800 simulator yesterday and was surprised that I could enter assumed temperatures below the flat rated temperature of the engines. The permissible range was from OAT up to 70°C.

The FCOM (11.40.58 - Flight Management, Navigation - FMC Preflight) says:

Selected Temperature (SEL)

Entry of an assumed temperature calculates a reduced thrust takeoff N1.

Entry can be made in degrees C or degrees F.

Maximum allowable entry is 70 degrees C (158 degrees F). The FMC, however, will limit the N1 to 25% takeoff reduction.

The lower limit is never mentioned. But according to the following chart, the flat rated temperature of a CFM56 engine is 30°C for TO thrust:

CFM56 flat rating (source)

I would expect the thrust to be constant for all assumed temperatures below 30°C, but I saw a reduction in the Takeoff N1 field of the FMC when increasing the temperature in this range:

Reduced Takeoff Thrust

The blue points are what the simulator tells me at a sea level airport in ISA conditions, the red model is calculated with

$$ N1(T) = N1_\mathrm{max} \times \sqrt{\frac{T_0}{T}} $$

where $ T_0 = 288.15 \, \mathrm{K} $ is the OAT (15°C).

Is this a bug in the simulator? Or can the assumed temperature actually be lower?

Related:

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  • $\begingroup$ If an OAT lower than around 30 degrees is added, according to the charts, the thrust is on the flat part of the curve. The simulator should just be giving you "flat rated thrust" at any temperature below 30. More interesting is the N1 curve, which peaks at 30. $\endgroup$
    – Robert DiGiovanni
    Commented Sep 26, 2019 at 8:18
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    $\begingroup$ The performance software for commercial aircraft of a different model I‘ve used in the past a very long time ago did indicate slightly higher thrust with decreasing temperatures below flat rated temperature. This change was not of any significant magnitude though. $\endgroup$
    – Cpt Reynolds
    Commented Sep 26, 2019 at 19:09

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