PMDG 737NGX 0.00.139
TUTORIAL #2
For Simulator Use Only DO NOT DUPLICATE 27FEB15 RTM
-54.2C - (-58.5C) = +4.3C ISA deviation
There’s only one place in the FMC to enter this for cruise, so go back to
the PERF INIT page.
Enter the actual average temperature (-54) into LSK 4L. The
FMC does the calculation and we see that the ISA deviation in
the 3R field changes from 0 to 3C.
As I said before, the way of estimating that I used to show you
how this actually works is not exact. The FMC has the exact
calculation built into it and that along with the rounding up to -54
results in it being +3C instead of +4.3C. Again, this makes a very
small difference and it’s not important on the 737 to have it 100%
exact. You can see this by entering really high numbers like 30 or
40 and seeing how the fuel burn changes very little.
Descent wind entry:
The FMC allows for the entry of descent wind predictions for three
different altitudes of the pilot’s choosing on the DES FORECASTS page.
Ideally you want to split these up so you have predictions spacing out the
range between Top of Descent and where you’d start maneuvering or
getting vectored for the approach where it no longer matters. In our case
this is going to be between FL390 and 9500 feet where we start the
approach.
These entries are often not done until prior to Top of Descent in cruise –
the primary purpose here since descent is usually at idle thrust is not fuel
burn related, but instead to give VNAV the corrections it needs to stay on
path instead of getting high or low due to the winds. Failure to enter these
descent wind forecasts is a primary cause of VNAV appearing to get off
the path.
To figure out which altitudes to use, we can look at the predictions on the
LEGS page to match up winds aloft forecasts to our waypoints.
Press LEGS and then NEXT PAGE until you see the start of the
descent, which should be between DKB and XERUM. On the
ground this is page 4/9.
It wouldn’t make sense to use FL390 as one of our three altitudes
because we already have a prediction for that altitude at DKB, which is
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