Originally Posted by
Officer Kite
Altitude to descend x 3 gives distance to descend on a -3 deg path
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
That's assuming your GS is the same? When does that happen in the descent? What if you have a 120 kt TW at FL410, it's 80 kts at FL250, 40 kts at FL180, and a headwind at FL150?
The box tries to figure that out but it only has the wind 'cuts' in the box. If mother nature is different, which it almost always is to some degree, the computation is off, so guys blame the box. But they'd never be able to compute an idle descent as well as the VNAV can.
In low traffic areas it wasn't uncommon to go to idle at TOD (typically FL410) and be at idle thrust until 1,000'. It used to be 700' until stabilized rules changed. Now it's about 2,000' due to configuration SOP's.
Minor deviations from the path could be corrected by slight increase in descent speed for going high on the VTI (VNAV path) or a slight speed decrease (better glide profile) if you got slightly low on the VTI (VNAV path).