Originally Posted by
rudestuff
From cruise to waypoint A the FMC will generate a path based on idle descent at whatever descent speed you've put in the box. (75 & 76 this is a default 250 it's so we generally overwrite with .78/290 or similar, and VNAV always gives 240 below FL100)
This will give a top of descent point (green circle). If you increase the descent speed (a steeper descent) then the TOD will move closer to the destination.
If you wind down the MCP window and descend, you'll leave the path and descend in VNAV SPD with a bit of power applied and eventually regain the path the throttles will come back and it will descend in IDLE PATH. If you reset the MCP window and do nothing else, it will get to the TOD and descend by itself in IDLE / PATH
That is all assuming you've programmed it with the correct winds: if you have a headwind you will need thrust, if you have a tailwind you will need drag.
If ATC ask you to increase your descent rate you'll need to leave the path (if you stay on the path you'd need to increase your speed!) - basically you need to put it into a pitch speed mode and either dive or use the speedbrakes. FLCH works, V/S works, or force it into VNAV SPD with SPD intervene. Once you get "own rate" again you can set -500fpm and drift back to the path.
Another point to note is deceleration: let's say you're at FL240, flying to ALPHA (220/FL120) with a descent speed of 290 in the box. The VNAV path will allow for an idle descent at 290 to a point several miles before ALPHA, at which point it will slow the rate of descent to allow almost level deceleration to 220. It basically does a dive and drive only slowing down at the last minute. If you've got an unexpected tail wind, you'll definitely need speedbrakes and there will definitely be anxiety on the flight deck. Those who understand VNAV will program a descent restriction of 220/FL130 and trigger the deceleration approximately 3 miles early: those who don't will have selected FLCH already!
As for above and below restrictions: let's say you're routing ALPHA, BRAVO, CHARL then RW14.
With no restrictions, the VNAV path will project backwards from RW14 to the cruise level ignoring all the waypoints. Each waypoint will have an altitude next to it on the LEGS page, in small numbers, showing where the path crosses the waypoint, but it will be an idle descent all the way down.
If you put in hard altitudes, they will show as large numbers, and between each hard alt the will be a different path, some steep, some shallow.
If you put in a conditional alt such as /FL120B then the path might change, but it might not: imagine a rubber band stretched between ALPHA (FL250) and CHARL (FL150) representing an idle descent. If BRAVO is exactly in the middle, it's predicted alt will obviously be FL200.
A /FL220B restriction will make no difference (because it was already going to do that) but an /FL190B will bend the elastic band down, forcing a steeper path ALPHA-BRAVO and a shallower path BRAVO-CHARL.
In a real world scenario, I might be in a descent and told "descend to FL160 level by DELTA" - I'll look at the LEGS page, find DELTA and look at the predicted altitude next to it in small numbers. If it says FL142 I know the path will meet that constraint anyway. If it says FL174 then it won't, so I'll overwrite /FL160B and it will calculate a new path from my position to DELTA, and of course I'll now be 1400' high on that new path - so out come the brakes or up goes the speed!
Thanks for the explanation on VNAV path.
My next question after having read your comment was, yes, VNAV does build up idle path starting from all the way down to TOD but when when I descend at TOD, sometimes FMA still remains in FMC SPEED instead of ARM.
Also, in the example I provided above, I was already in a descent, and FL180 was cancelled by ATC and pressed ALT INTV. Then FMA changed to VNAV SPD. Now we do not know whether FMA will remain inPTH or SPD by pressing SPD INTV and confirm that geometric path is active from TOD as you mentioned, then is not FMC going to build another PATH starting from point B to present position to allow idle or non idle descent path?
In another words, did it change to VNAV PATH because FMC did generate new PATH starting from point B and we were still high above the path so the plane dived in SPD mode?