Originally Posted by
Banana Joe
If you don't open the speed window, the FMC will gradually increase the speed to attempt to recover the path. If you open the speed window with speed intervention or use LVL CH it will keep that speed regardless of path. VNAV SPD will always comply with altitude constraints. Again, this is what I have experienced. Sometimes what the FCOM says doesn't seem to be incorrect i.e. when I had to discontinue an ILS approach and I deselected the frequency, the outcome was not what was the FCOM says it would be.
In your scenario, I would just report unable to comply with either restriction and ask ATC what if they prefer me on speed or on profile. And open the speed window if I have to descend faster.
What plane is that? The thing to remember is all VNAV systems are similar yet different. A VNAV path is projected backwards from the last constraint at an angle that assumes idle thrust at the FMC decent speed. The only way to ensure meeting a speed/alt constraint is to be on the path doing that speed (and even then with a tail wind you might need drag).
Put simply, in descant with VNAV engaged you're either on the path (PATH) or you're not (VNAV SPD) - ignoring VNAV ALT for the time being (!)
Descend early? Leave the path > VNAV SPD
Descend late? Same thing > VNAV SPD
Open the SPD window in descant? > VNAV SPD
Modify the path? > VNAV SPD
VNAV SPD gives speed on pitch, just like FLCH - either the MCP window speed or the FMC speed.
High on path? You can (a) pull speed brake and 'drop' onto the path, (b) you can open the speed window, increase the speed and 'dive' onto the path (just remember to close it again or you'll go straight through the path!) or (c) you can increase the FMC speed to give a later, steeper descent and effectively bring the path to you.
Keeping the window closed has the advantage of automatically recapturing the path.
VNAV speed will start an automatic deceleration to 240 kts by 10,000' and any other speed restriction you program (clean/6000 is a good one)