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philbrown

Odd Assumed Temp Derate Situation

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Took a stab at making some simple derate charts for the 777 today. Found an odd thing with the assumed temperature setting.The N1 setting doesn't seem to pay any attention to the current OAT. 30*C derate ALWAYS gives you 103.6% N1.It means that when the temperature is 0*C, the normal TO N1 is 98.4 as given by the FMC. Then derating the aircraft to 30*C increases the N1 to 103.6%. Derating the aircraft actually gives you MORE thrust than performing a normal takeoff.I've tested with some other FMC equipped aircraft, and derates for set temperatures will differ depending on the OAT.

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I don't see a problem in the constant N1 limit for a given assumed temp, but normally with decreasing OAT, the Takeoff N1 limit would increase up to a maximum value.I would say the problem is the N1 limit at 0 deg C. 98.4% N1 seems very low. Do you have any fixed derates selected (TO-1 or TO-2) as well? Looks like the thrust limit calculations may be wrong.It's possible that the assumed temperature limit is ignoring fixed derate selections and just looking at temperature for full TO rating. That way you could have lower N1 limit at 0 deg (@ TO-2) than at 30 deg (@ TO).Either way, something is wrong in the N1 thrust limit calculations. Interesting problem, I'll take a look at the various addons I have to see how they behave, compared to the 777.Kevin

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On reflection I now agree entirely with the first poster. I checked my other add-ons with GE and CFM engines and they all behave in a similar fashion, which is different to the PSS 777.On all these add-ons, PSS included, N1 limit increases slowly with OAT, then decreases from a point somewhere above 15 deg C (30 deg C gave the highest N1 limit in most cases I tried). For assumed temperature to work as a derate, it must give a lower N1 than the existing OAT, so simply selecting the limit calculated at the assumed temperature will not always work (this is what PSS have assumed (and me too originally)).In all the other add-ons I tried (PMDG744, LDS767, PMDG737, Wilco737) derating with assumed temperature always decreased the N1. PSS777 simply takes the N1 limit for the assumed temperature, which is often a thrust increase.Kevin

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