Hey. 2 quick questions regarding descent planning in the B737.
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.
2) During descent, once vectored off our planned route, we normally do:
"If displaying the arrival procedure is not desired, perform a “DIRECT TO” or “INTERCEPT COURSE TO” the FAF, OM, or appropriate fix, to simplify the navigation display. This provides:
- a display of distance remaining to the FAF, OM, or appropriate fix"..
However, once we do this, the VNAV profile changes rapidly and might show that we're too high (normally). Why is that? Does the VNAV descent profile consider that we're flying directly to the FI, therefore we're high? Would appreciate if you could highlight the reference to the explanation please.
Many thanks.