TLR Aerodata FLEX Thrust

[barrow]

Ground delay issued during taxi out, So pull over, shut em both down. Takeoff numbers were input at gate just prior to push, temp from ATIS 23 deg C Planned Flex 35 deg C. After approx 50 minutes we now get " Start em up, Taxi 24L full length" New atis report is now 25 deg C.

My question:

Do AeroData servers predicate the performance and flex temp on the OAT that is input into the FMS before the perf numbers are requested (23 c) ?
We also set ECS, TO-1,2,3, Anti Ice config, CLR, FOB and passenger count into the FMS prior to sending for perf numbers, so do trim, flap, flex temp, %mac, GW and speeds all come back predicated on OAT 23 C. If I now set OAT 25 C in thrust page, are the original perf numbers still valid? Specifically Flex temp?

[FE Hoppy]

Don't know the system you refer to or AC type??

However, 23 flex 35C is NOT THE SAME as 25 flex 35C.

1) In order to find the N1 value appropriate to the flex temp you need to correct for the difference between the OAT and the Flex temp. If the OAT has changed the N1 is no longer valid.

2) Lift depends on rho. rho changes with temperature.

[barrow]

ERJ 170-200, GE CF34-8E Turbo Fan, HoneyWell Primus Epic load 25.6 Avionics

[Sidestick_n_Rudder]

AFAIK, performance calcualtion assumes OAT=Tflex and respective air density. Therefore your numbers should be the same whether it's 23, or 25 degs outside.

[FE Hoppy]

The performance should still work but the N1 will be about 0.3% higher at 25 than at 23.

The Ejet thrust page will calculate the N1 based on OAT and Flex temp inputs but as we don't use dispatchers to do our take off calculation I can't comment on the ACARS system in use or the legality of using the original calculation.

[FlightDetent]

Flex 35 is a flex 35 no matter the OAT as long as it is below 35 itself. Same for the speeds.

I think FE Hoppy is trying to explain, that to achieve the THRUST of Flex 35, somebody needs to work out the N1/EPR for the actual conditions. In case a manual input of OAT is required then the value becomes spinningly relevant.

[Sidestick_n_Rudder]

O.K. I know nothing about E-Jet N1 calculation. However, the v-speeds and FLEX temp should remain valid at any OAT, up to OAT=Tflex

[barrow]

Between the back and forth you Gents have answered my question,
The new input temp 25 c, is needed to get the corrected N1 ??.? or whatever the correction is in order to comply with all performance requirements needed at a flex 35 c. So there should be no need to resend for numbers wrt the increase in OAT, Just input it, and let the box figure out the new settings, Yes?

[FE Hoppy]

Note of caution.

In the case of an increasing OAT like this, generally the original calculation should be OK so long as OAT<TFlex.

But it is not always guaranteed because of the constraints used in the calculation.

In the case of a decreasing OAT there are too many other variables to say it's ok. For example Max TFlex reduces with OAT, Min V1 and Min VR increase with reducing OAT. These must be confirmed before the calculation is declared valid.

So Just using the original calculation so long as OAT< TFlex is not the sort of policy a prudent airman should adopt.

[CallmeJB]

Our FLEX numbers are valid at any number higher than planned OAT (up to the Flex number), and down to 10 degrees C below the pOAT.

Our FMC uses sensed OAT to set TO thrust; we don't manually put OAT into the FMC.