Sam Spade writes:

In the 727 we didn't have any FMC to keep happy. We actually had to
figure it all out by ourselves.

What little experimentation I've done seems to indicate that the FMC
is very often correct when it says a particular descent cannot be
achieved. Of course, it assumes certain constraints that you can
ignore if you fly by hand (unless, I suppose, you're stuck with an
Airbus).

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