Originally Posted by
100 above minimums
1) In the VNAV descent forecast page, it says: "Enter the average ISA deviation for descent in °C (+/–XX°C) or °F (+/–XX°F)". What is meant by that? How does one calculate the average ISA DEV for the entire descent? At my outfit, the normal is that we'd put the destination's ISA dev and some others opt for putting in the average of ISA DEV at cruise and ISA DEV at destination.
The average ISA deviation has no effect on descent profile unless you operate in extreme latitudes.
Average ISA cannot be determined unless you can verify ISA at multiple altitudes during your descent.
If you have ISA deviation at say every 10,000' then you can average the TOD and intermediate ones and find their average.
As mentioned above and by some proviso posters, the average ISA in DESC page in FMC has little to no impact unless you operate in atmospheres of extremities.
Think of -10C and +15C average during descent,
not at landing but during descent.
If you do not operate in these conditions, there is no need to use it.