Proper methods for descent with/without VNAV?

[Tord]

I´m very sure I´m doing something wrong and would appreciate some advice from you with good knowledge.

Scenario 1. Descent when only following a VNAV path according to a STAR. 
Let´s assume that I´m nearing an airport and have added a STAR and Approach in the FMC. The path has a couple of AT OR ABOVE, AT OR BELOW and AT altitude restrictions with final approach altitude for the ILS at 2500 ft.

a) I set 2500 as selected altitude prior to TOD. Actually I´d set the altitude to match any STAR altitude restrictions during the descent, but let´s be lazy here. NAV and VNAV are the active modes. (I´m hoping I use the correct names as much as possible). ATS is on and showing SPEED at this time.

b) When reaching the TOD I do no button pushing. The autopilot will initiate the descent, ATS goes to DESCENT if I remember correctly. The VNAV PATH will be followed and the various restrictions will be met. At a suitable point I press APPR which sets LOC and GS as selected modes, and they activate upon catching the LOC and GS respectively.

c) During the descent I manage speed to achieve whatever speed is needed during the approach/arrival.

 

Scenario 2 Descent initially following a VNAV path according to a STAR, but ATC comes with altitude instructions.

a) Setup and initialization as above. While inbound to a waypoint with an altitude restriction of AT OR ABOVE FL180 I get the instruction to descend to FL110.

b) What I´ve done here is to set FL110 as selected altitude, then press FLCH. However, and I haven´t done a lot of trials, if we reach FL180 the AP seems to level off and not proceed to FL110? I then start mashing buttons trying to select VS or FLCH but the descent becomes very messy and I have no real control of what I should select. I guess my question here is, how do I transition from VNAV to "manual" AP modes?

c) What should I di to stay in VNAV for this scenario? Do I need to DELETE any altitude restrictions on LEGS that are above the ATC instructed atitude? If I do that I´m guessing that VNAV path will calculate the descent for whatever the next lower alt restriction in LEGS is, and my selected altitude of FL110 will mean that the VNAV descent will stop at FL110?

Scenario 3. Non-VNAV method. What is the best practise of using AP modes if I don´t want to use VNAV/VPATH?

[Graeme_77]

Scenario 2, part B and C : That’s VNav doing what it’s supposed to do. Either turn it off, or clear the constraint. Use VS or FLC.

Scenario 3. Easiest to use FLC at high altitudes, then VS to manage the descent profile accurately at lower altitude.

Remember VNav on the 650 is a “modifier”, not a mode like it is on a Boeing. So with VNAV turned on you get VFLC and VVS. Without it you have FLC and VS. The only difference is VFLC and VVS will obey any magenta altitude constraints shown on the PFD, as well as the blue altitude constraint.

Hope that makes sense?

[Tord]

Ah, VNav as a modifier makes what I see happening make a lot more sense! As you say I was thinking of it being VNav OR FLC, but in fact I should think of it as FLC OR VS with VNav enabled OR Disabled.
Thanks!

[Graeme_77]

Exactly, and you can see that in the FMAs! VFLC / FLC / VVS / VS. Even VALT indicates ALT with VNav so it will capture VPATH.