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Author Topic: Assumed Temperature/Take-Off Performance  (Read 323 times)

DogsEarsUp

  • Join date: Feb 2021
  • Posts: 8
Assumed Temperature/Take-Off Performance
« on: Mon, 8 Feb 2021 20:30 »
I know that there have been previous threads on this but I wanted to try and check my understanding of the current situation is correct?

I'm running PSX single screen on a 27" iMac, with a second screen that I use for displaying Charts, Flight Plans and Maps (and the manual, of course). It runs beautifully!

At present, the only way to properly calculate an assumed temperature for the FMC is by using TOPCAT (Windows only) and/or possibly BACARS (Windows only & BA specific).

The calculation can alternatively be done with the relevant tables, but these are airline/aircraft/airfield specific, and owned and published by the respective airline, and are not available anyway.

The video by Peter (Britjet) on "The Take Off Ref page:" seems to advise that a form of assumed temperature plan can be created by selecting an approximate, guesstimate temperature (or even DERATE) and then adjusting it using the WIND/SLOPE LSK on the TAKEOFF REF page - although this doesn't take into account obstacle clearance.

So, in summary, on a Mac, except for the FMC procedure above, Assumed Temperature calculations are not possible at the moment?

Apologies if I have missed the obvious.  :D

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 12310
    • Aerowinx
Re: Assumed Temperature/Take-Off Performance
« Reply #1 on: Mon, 8 Feb 2021 20:49 »
In PSX itself there's no function that would calculate an assumed temperature for you.

If the NG FMC is installed, you could request TAKEOFF REF data from a simulated dispatch system which also provides two THRUST LIM datasets (STD and ALTN) including a suggested assumed temperature. But this simulated dispatch system incorporates random factors and doesn't replace programs like TOPCAT.


Regards,

|-|ardy

DogsEarsUp

  • Join date: Feb 2021
  • Posts: 8
Re: Assumed Temperature/Take-Off Performance
« Reply #2 on: Mon, 8 Feb 2021 21:06 »
Thanks Hardy.

So good option seems to be the NG FMC, but the 'random factors' could possibly mean that I might end up tangling the gear in the approach lights at the other end?

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 12310
    • Aerowinx
Re: Assumed Temperature/Take-Off Performance
« Reply #3 on: Mon, 8 Feb 2021 21:50 »
As long as you use reasonable combinations of takeoff weight, runway length and density altitude, your VNAV will always engage before you overfly the other end of the runway :-)

The randomization will keep the climb path within a small, reasonable tolerance, like in real-world operations. It's not a lottery machine that would spit out absurde data.


|-|ardy

dhob

  • Join date: Sep 2019
  • Posts: 67
Re: Assumed Temperature/Take-Off Performance
« Reply #4 on: Wed, 10 Feb 2021 04:16 »
The NGFMC does know the runway length, and if the field limit weight from the entered takeoff data exceeds the runway distance available, the FMC scratchpad message V-SPEEDS UNAVAILABLE displays. With this in mind, enter Flaps 10 or 20, and select TO 2. If no scratchpad message, then TO 2 is a valid rated thrust selection. An assumed temperature can then be entered. The FMC allows up to 25% off the selected rated thrust. The temperature that corresponds to 25% can be determined by entering a high temperature, 90 for example. The FMC will round the temperature to 25%, usually around 64C. Again if no message, it’s a valid performance solution. If the message appears, then try a lower temperature. The thrust break temperature is usually around 30C for a standard day. Enter values between 30 and 64 until the highest temperature value that doesn’t generate the scratchpad message is found. You now have the lowest thrust available for the selected runway.
If TO2 fails to give a solution, try TO1, then TO.
These are balanced v-speeds of course, and the FMC runway length may not be equal to the ASDA, TORA, or TODA distances. Again this is only a valid field limit weight. Obstacle limit, climb limit, Vmbe limit etc are not known.

DogsEarsUp

  • Join date: Feb 2021
  • Posts: 8
Re: Assumed Temperature/Take-Off Performance
« Reply #5 on: Wed, 10 Feb 2021 06:45 »
Many thanks - I'll try that this morning!