I must say I preferred the comment from shukran , but now Uplink , you've gone and ruined it…
Originally Posted by
Uplinker
almost
“Challenge accepted”
Firstly, mothergoose1 , you don't say if you are on a Type Rating course, already flying the aircraft or on some kind of simulation platform? The good thing about your post is that the confusion is evident!
Lets start with the aircraft status, and phase of flight…
In climb, with VNAV engaged, the AFDS will command a level off at any FMC hard or 'below' altitude restriction. Pushing the MCP Altitude Switch clears one (the next) FMC restraint only.
In descent, with VNAV engaged, the principle is the same. The AFDS will command a level off for any FMC hard restriction, or attempt to maintain an IDLE descent crossing at or above an 'above' FMC restriction - depending on whether the optimum (idle descent) profile is above or below that waypoint.
With LNAV engaged in the descent, and accurate forecast winds in the FMC, VNAV PATH will probably do a good job, always respecting the hard, 'above' and 'below, descent constraints. However, you may see the scratch pad message 'Drag Required' for higher than commanded speed (too little wind, or excess tailwind) or the auto-thrust responding to lower speed.
When on descent with a vector (HDG SEL), the AFDS will still attempt to follow the VNAV path. However, two situations to consider: you are either on an 'inside track' (with track shortening), or an outside track. Pilot awareness and experience should help you determine which is the case.
The recommendation at this point is to use the appropriate AFDS mode, which may not be VNAV: FLCH SPD if you are high on profile, V/S if you are tending low. However, remaining in VNAV should still protect you for altitude constraints. Pushing the MCP Altitude switch clears one FMC restriction at a time.
The system can appear over-complicated at times, and this is where basic judgment says to use the most appropriate mode for the situation.
A suggestion, that always worked for me:
- always use VNAV in climb, (FLCH SPD will cause the pressurisation to level off, VNAV will continue the pressurisation schedule).
- use VNAV in descent, until you are prevented from following the optimum (pre-planned FMC) profile.
- use FLCH SPD to increase rod.
- use V/S to reduce rod.
Good luck!