FUEL PRED CRZ TEMP change

[Longjon]

Any one can help with information on changing the Crz temp on the fuel Pred. Page when in reaching Crz alt .
----- / trop alt is what is seen on reaching Crz and the---- can be changed by reinserting the Crz alt in the PROG. page.
What is the real advantage of doing this and inserting the actual Crz temp.??:

[Longjon]

Is there a real advantage in incerting the actual crz temp in the fuel pred page and updating it as opposed to letting it be ----/ trop

[rudderrudderrat]

Hi Longjon,
Any one can help with information on changing the Crz temp on the fuel Pred. Page when in reaching Crz alt .
FCOM FCTM 070 Cruise FMS use:
"WIND AND TEMPERATURE

When reaching cruise FL, the crew will ensure that the wind and temperatures are correctly entered and the lateral and vertical F-PLN reflect the CFP. Wind entries should be made at waypoints when there is a difference of either ​30 ° or ​30 kt for the wind data and ​5 °C for temperature deviation. These entries should be made for as many levels as possible to reflect the actual wind and temperature profile. This will ensure that the FMS fuel and time predictions are as accurate as possible and provide an accurate OPT FL computation."

You insert the forecast winds and temperature along the route (i.e. in the future), not at your present position.

[Longjon]

-rudderrudderrat
Hi thanks for your reply what you say is true but my question is entering the Crz temp in the fuel pred page this shows -----/tropause when the Crz flight level is reached and if we want to change this you have to go to PROG page and re insert the FL then when you go back to the fuel pred page the Crz temp can be inserted now what is the real benefit of doing this ??

[rudderrudderrat]

now what is the real benefit of doing this ??
None at all. The FMGC will use forecast wind and temperature down route and sensed conditions in the present position.

[heggo48]

I believe it has a benifit.....first of all it brings your Mach No. predicted by FMGC closer to your actual Mach number which reflects on you time and fuel predictions(Mach No. is directly affected by temp.)..... if you don't update it in the fuel pred page, the FMGC wil use the pre-inserted temp. in the wind/temp. page which might be inaccurate....you can always update that one instead....so its a matter of fine tuning your predictions in flight when you find a deviation from the forecast figures....
secondly and more importantly, updating your temp. in cruise affects both your OPTIMUM and your REC MAX altitude, which can have a great efffect on your flight with respect to ATC restrictions, routing and fuel savings(check your QRH tables for MAX/OPT alt.).

[Natstrackalpha]

So who is running the show then OP? the FMGC or the ADIRS?

Not being picky - I can understand the . . computor trying to sort out millions of calculations because the climb, cruise, or descent winds are different to what we banged into the MCDU but the end result is that it will figure it out. If you like, it has less distance to go in its thinking process and will not have to think so much if the FMGC values (input by the pilot) are close to the ambient temps/winds/pressures.

But rudderrudderrat has a point too in that it does not have to do it this way, the thing is, while it is concentraing on making calculations dervied from garbage that might have been input in, then it cannot apply so much attention to refinement. In some situations future predictions can be affected.

The same it also true of managed speed(s) if you fly at a greater spd than the one in the FMGC then all the calcs will be for that spd - if you then go back to managed and FMGC spd (brought about by your inputs of WAT at INIT) then it`s estimates will be out, as the FMGC is now calculating your increased selected spd - or higher input CRZ spd.

Anyway, don`t listen to me, am only scoring 67% at best, these days!