DFC100 and VNAV

Now that we are closing in on DFC100 certification, a lot of questions are popping up about VNAV.  What is it exactly?  How does it work in R9-DFC100?  and a lot of specific scenario questions.

First a general statement - as soon as we are through the final FAA TIA flight test, we will post the DFC100 Pilot Guide for all to read.  There is always some risk that something might have to change as a result of the final FAA flight test and we don't want to publish a pre-cert version that may have to change a week later.

That being said, this forum thread can be used to answer any specific questions that may come up.

Let me start with a VNAV question that came up in a recent DFC100 webinar.  The question was along the lines of "Are step down fixes available in VNAV?"   The short answer is yes.  As way of more information, think of the type of approach being conducted when VNAV is engaged.   In both cases, it is assumed the desired approach is part of the active FMS flight plan.

For a precision approach (LPV or ILS), if the autopilot was in VNAV mode prior to starting the approach, any intermediate step down fixes between the IAF and FAF will be achieved and departing the FAF, the autopilot will automatically toggle out of VNAV mode and into ALT hold mode with GS armed.  If the vertical position of the aircraft was such that glide slope can be immediately picked up, the autopilot will immediately toggle to GS mode and this may be so fast that ALT may not have been noticed.

For a non-precision approach (LNAV, LOC, etc), if the autopilot was in VNAV mode prior to starting the approach, any intermediate step down fixes between the IAF and FAF AND inside the FAF will be achieved.  The important point to understand in this case is that when the active FMS flight plan leg terminates with the MAP, the autopilot will automatically toggle out of VNAV and into ALT hold mode.  Some approaches have multiple step down fixes between the FAF and MAP and all but the last one will be followed by VNAV mode.

I hope this addressed the specific question.

Please use this thread for any more VNAV questions for a R9-DFC100 equipped system.

Steve Jacobson
VP Product Management
Avidyne Corporation


Edited by BigDaddyJake - 07 Nov 2010 at 2:05pm

If you want to do step downs on an LNAV+V, just leave the AP in VNAV.  If you want to do the +V pseudo-glideslope, then put the AP in GS.  

 

The most compelling reason to use VNAV enroute is that the TOD will be visible on the map and the FMS will start the descent for you upon reaching that point.  As for mode management, I would recommend the following sequence.  During preflight, either select an approach at the destination or enter an altitude constraint on the destination leg (I suggest using the 1000AGL default).  On climbout, use VS/ALT or IAS/ALT to reach the cruise altitude.  Then, engage VNAV.  Manage enroute altitude changes with VS/ALT, but then always go back to VNAV after leveling off.  Because of the defined downpath constraint, VNAV will compute the TOD and start the descent at the correct time.  If you start out with a constraint on the destination and then select an approach as you get close, VNAV will automatically adjust TOD for the first constraint in the approach.

One other VNAV question: If the last waypoint in the FMS is an airport, I understand the VNAV will plan 10 get to 1000 feet above the airport at a range of 5 miles. Does that vertical constraint (1000 feet) have to be manually entered into teh FMS or will the VNAV function automatically assume it? 

Does VNAV consider or otherwise honor the selected altitude as set by the altitude bug?

Brian is correct.  The Alt Bug is honored in almost all of the vertical axis modes.  (It is not honored during ILS glide slope operations)