The PFD selected altitude is set at 1900 with a current attitude of 4000.  The VNV Target is 1964 which is also ALT for SCENN (faf) in the MFD flight plan. 

The C172 was at 4000  as it crossed FAVDU, the waypoint 8.0 NM prior to the SCENN (faf).

The plane was in GPS & VNAV and VPTH is shown in white on the PFD.  

The Active Flight Plan / Active VNV profile shows SCENN 1964ft BOD 0.27 

The AP never began the descent and overflew the faf at 4,000.

 I know that white text in the G1000 systems means reference only - NOT used for guidance.  But I have no idea why the KTW ILS 17 procedure would generate invalid VNAV guidance.

What do I misunderstand here?

Hard to tell what the autopilot should have done - at any rate you'd want to be in vloc and using APR mode at the point of your screenshot.

Well before you reached the FAF was there ever a purple diamond on the left side of the altitude scale?  It might even be a carrot - I almost never use vnav.  It looks like v path was armed correctly - very well could be a bug.

Check in the Content Manager you have the latest version of the G1000 Nxi there was an update to overcome a VNAV and Glideslope capture bug a few days ago. (There have been two updates since SU9 - so you may have missed one!).

Edited May 2, 20224 yr by mfahey
Typo Fix

I just finished a flight and it worked perfectly.  The only thing I could suggest is that you may have failed to engage the Approach Mode (GS).

After VNAV has finished it's work getting you to the proper altitude it quits and hands it over to the Glide Slope, which takes it in from there.  VPTH will be displayed in Green and GP in white while it is active. When GP takes over, VPH goes away and GP turns Green.

Just my take.

Roy

Edited May 2, 20224 yr by Roy Warren

I just checked the chart for Tacoma Narrows (KTIW) and it says LNAV only.  No VNAV.  See the *.

Roy

21 minutes ago, Roy Warren said:

I just checked the chart for Tacoma Narrows (KTIW) and it says LNAV only.  No VNAV.  See the *.

That is only on the RNAV(GPS) RWY 17 chart.  I was using the ILS or LOC RWY 17 chart.  I just flew it again and did see GS as armed. 

Based on these comments - I may have a fundamental misunderstanding here.  

I fly the "pseudo arrival" using GPS with a custom flight plan (VPBKD @ 4000 and FAVNU @ 3500). Once I pass FAVNU and am on the final approach heading of 167° I allow VNAV to take me down to 1900 at SCENN (faf) as shown on the ILS 17 Jepperson.  

Once I hear ITIW on NAV1 I switch the CDI to NAV1 and the nav mode to APR.  At that point I get a normal ILS approach. 

But odd things happen at times - the last flight VNAV worked perfectly all the way to SCENN and then the LOC turned green and I saw GS on the display.  The AP followed the glide slope down to the 500' announcement, then leveled out and refused to go lower.  My target altitude was set to 300'.

On another recent approach VNAV was working perfectly and was right on the slope from FAVNU to SCENN.  Six NM from SCENN (faf) I activated vectors to final.  At that point the ITIW signal was not being received.  But, the AP switched the CDI to LOC and then tried to fly the Procedure turn that takes the plane a long way from the airport with no hope of using ITIW as a Nav signal source.

Why does vectors to final change the nav source to LOC?  

 

Have you checked that you have the most current G1000NXi build from the content manager, as someone else mentioned?

A few things:

VNAV guidance ends at the FAF. You cannot go further down on VNAV once you reach the FAF, you must engage APR to get GS (ILS) or GP (RNAV) vertical guidance after that.

CDI auto-switching (nav to nav transfer) happens when your current active leg is the leg to the FAF, a LOC or ILS approach is loaded, and you are within 7NM of the FAF fix. Since you activated VTF, your active leg switched to the FAF, and because you were within the right distance, the CDI auto switched to green needles. It is not possible for the AP to follow anything other than the radio guidance in green needles, though, so it could not have been trying to navigate through the procedure turn. More likely the AP reverted to ROL because you were not receiving the localizer signal yet and you ended up in a held bank.

In general I would advise against using VTF unless you are in the one specific scenario in which it is needed: ATC is giving you vectors right to the FAF. If you're already on the approach, there is no reason to use VTF at all, and you may be changing the vertical path by doing so (VTF erases considering all previous waypoints other than the FAF in the approach).

-Matt

Far be it for me to claim to be an expert on the G1000, but consider also that there's a difference between "coupled" VNAV and "advisory" VNAV. With the former the automatics will actually decend the plane, the latter is just a reference. If you're trying to capture a glide path, you use approach mode, not VNAV. So, with that in mind, does the G1000 even have coupled VNAV?

I discovered a problem yesterday with LNAV/VNAV approaches with the G1000NXI. When I press the approach button on the Autopilot, the CDI swirches from GPS to LOC1, and since it is a GPS approach, there is no LOC signal, so the AP switches into ROL mode.   This happened everytime at three different airports. I reported this to working title on their Discord channel. THis happened in the last two days, so either a bug in one of the updates or a problem with SU9. 

4 hours ago, andreh said:

So, with that in mind, does the G1000 even have coupled VNAV?

Very much so, a relatively fully featured descent-only coupled VNAV.