Flat rate temperature

[Noknoipobin]

Eng flat rate temp is 30 degrees (isa+15). If temp is below 30 and I use actual or flex temp below this ,could I get more thrust .? (I saw some GWC got gaining weight up to 5 tons) how?

[barit1]

Below the flat-rate OAT, engine thrust ideally remains constant. But the airplane performance improves with decreasing temp, because the wing is operating in cooler air. Put in other terms, the TAS (inertial speed) associated with V1, Vr etc. is lower than at the flat-rate point. Thus you reach a given V-speed sooner after brake release.

Thus for a given runway, your performance-limited TOGW can be higher, even with the same thrust.

BTW, what is your logic in using T-flex BELOW flat-rate point?

[Noknoipobin]

Thank you very much.According to my GWC we get a few tons of weight below flat rate temperature .

If OAT is 20 *c,flat rate temp is 30*c, t- flex at 30 *c

Do I get the maximum take off weight? (same as 20*c )

[barit1]

Say your performance-limited weight on r/w 28L is 100T at 30C.

But at 20C allowable TOGW will probably be higher, ASSUMING you're not at a structural (airplane or runway) limit. This is because to reach V1, Vr etc. you don't have to accelerate to the same TAS or GS. (The wing is flying in cooler air).

Now if your actual TOGW is 90T at 20C, and you flex to an assumed temp of 38C (I'm guessing here), you'll be flying a profile equivalent to flat rated thrust at 30C and 100T.

Both the reduced GW and the lower OAT contribute to the thrust reduction from the 30C/100T condition.

[Noknoipobin]

If OAT is below the flat rate temperature ,is it difference when I select the assumed temp between oat and flat rate temp or I has the same thrust? (same MTOW) thank you.

[barit1]

For a given ambient pressure altitude, then selecting (e.g.) 38C Flex will yield the same thrust, regardless of actual TOGW or OAT.

And Flex does not affect TOGW; however TOGW affects the max Flex you can legally use.