Ok, without knowing the VNAV setup, just a quick comment on the latter.
I think a review of what VNAV does internally to determine a descent path might be a good starting point.
Forecast winds entered, if correct, gives VNAV a good chance to determine the groundspeed based on the fact it knows what constant IAS and Mach to fly during descent as planned in the Descent pages of the FMC, or VNAV page 3, subject to FMC setup.
Essentially:
- VNAV draws a line from runway, working its way up past FAF and IAF with known Speed and Height restrictions as coded.
- VNAV attempts to create a idle thrust profile (most fuel efficient) within the constraints given, and as long as they are adhered to will fly it.
- VNAV corrects for pressure altitude correction if forecast QHN is entered in Descent Forecast pages.
- VNAV takes into account higher engine idle settings for Anti-Ice usage for the altitude region entered into Descent Forecast pages.
- VNAV assumes 0 wind on touchdown, tapers it for the wind in the Descent Forecast pages to the lowest (if entered) value/FL, above which tapers it to the middle (if entered) value/FL, then upper (if entered) value/FL, then to last entered cruise wind (which is why during descent you are unable to enter winds in Leg RTE Data).
- The resultant of the above is a T/D point which, in theory, should allow for all variants during descent until touchdown and thus should enable a CDA idle thrust from T/D.
However, forecast models are just that, never accurate to the minutest details.
Thus, during descent, when a direct waypoint is executed, VNAV recalculates the actual profile from present position, using actual winds as new T/D wind then implements the forecast winds again as explained above.
This may result in being either remaining on profile or a vertical displacement from it, becoming high or low on the recalculated VNAV idle descent path, based on the (reduced/increased) track miles remaining and actual height.
The two way the aircraft can correct for this are by either:
- Reducing speed, thus creating a greater descent angle which can be used for longer [VNAV calculates this as most favourable energy management solution to avoid becoming hot & high later on in flight]
- Increasing speed, diving off the excess height and dissipate the excess enegery using other means later [Autopilot, Flight Director commands this]
On the speedtape this anomaly can be observed by the IAS bug being driven lower by VNAV in accordance with energy management, but the aircraft increasing speed by diving it off to regain the recalculated VNAV PTH profile for an IDLE descent then to follow it (advising you when flying at VNAV IAS +10kts of DRAG REQUIRED)...
pilotho, I hope this explains why and how the VNAV recalculates the remaining path and recreates the new idle descent profile and why VNAV acts the way it does.