TO performance 737NG wind/temp change ?

[president]

@ Goldenrivett,

I am not using FLEX but if itīs the same as assumed temperatures on a Boeing, I think you are dead wrong.

We are departing light weight in -10 deg C with assumed temperature 65 deg. You are telling me that the temperature can increase 75 deg without having to re-calculate?

The whole concept of assuming a temperature comes from the fact that the engine is flat rated to typically isa +15 deg (around 30 deg C OAT at sl.). Up to that temperature the engine will produce the thrust rating on the engine, and not more. It's limited by internal pressure, engine temperature limits and fan speed limits. After ISA +15 the thrust drops linearly with temperature. You are now telling the engine that what the actual temperature is, and what temperature you want it to assume it is. If you assume the temperature is OAT 45 deg C, the thrust calculated will be equivalent of the full rated TO thrust at 45 deg, but at a much higher N1 setting. In other words, to produce the same thrust (that you want at 0 deg) at 45 deg, the rotor would have to spin quicker. We are on the declining line on the flat rated engine thrust versus temperature now, so we are not N1 rotor limited here, but engine temperature limited.

Imagine you calculated a take off at 0 deg OAT and an assumed temp of 45 deg, your N1 value would be way off if the actual temperature increased to an actual 45 deg C. Iīve done the calculation for you on the EFB right now. On a given take off at 30 deg C it came out with 22K assumed 38 deg and an N1 of 91,3 %. If the temperature goes up to actual 38 deg C, it gives me full 22K thrust but an N1 of 92,5. Say that you took off at 38 deg with 91,3 (like I think you suggest), you would end of with a too low thrust setting. Also from this is given that 91,3 % N1 at 30 deg OAT gives you the same thrust at 92,5 pct N1 gives you at 38 deg OAT.

We do though have whatīs called the call the "true airspeed effect" giving you better performance that calculated. We tell the engines that we have a temperature of 45 deg C (thin air) air, but the actual temperature is 0 deg C (thick air). But this does NOT mean you can accept temperature increases up to your assumed temperature limit.