The G1000nxi and VNAV

[Boeing_Driver 76]

I was getting some CTDs and decided to remove everything from my Community folder. That solved the problem, and I’ve been slowly adding scenery back in, bit by bit.

The strange thing is, I seem to have lost a small function of the G1000. It used to give vertical guidance from the FAF down to the runway but now it doesn’t. VNAV works perfectly, right up to that point, then it holds me at the last step down. Can anyone think of why this would be happening? When I was getting the CTDs, I could rely on the unit to give a vertical path down to the threshold.

[ark 1,295]

1 hour ago, Boeing_Driver said:

It used to give vertical guidance from the FAF down to the runway but now it doesn’t.

Some thoughts:

Do you have the runway touchdown elevation set as the final target VNAV altitude?

One thing to realize is VNAV provides a 3 deg descent. So it will level off if necessary until it reaches the 3 deg "TOD" point for the next descent segment.

And if the approach has a glide slope or glide path, then I would expect you would arm the approach before reaching the FAF and follow that down.

Al

Edited December 5, 2021 by ark

[Boeing_Driver 76]

So typically,  prior to top of descent,  I would "dial the dirt", that being field elevation. In the past, this would give vertical guidance all the way to the runway. Now it stops at the very last crossing restriction.

Also of note, that the most recent edition of the G1000 seems have cured the AP of having to engage Appoach mode on an RNAV procedure, and instead, use VNAV all the way to the runway. However, I'm not evening seeing the magenta diamond come down, regardless of what mode I've selected.

[ark 1,295]

26 minutes ago, Boeing_Driver said:

Also of note, that the most recent edition of the G1000 seems have cured the AP of having to engage Approach mode on an RNAV procedure, and instead, use VNAV all the way to the runway.

Just to be clear, by the most recent edition of the G1000 I assume you are talking about the G1000NXi. The G1000NXi seems to work as expected for me. But I have not tried using VNAV all the way down to the runway.

Al

Edited December 5, 2021 by ark

[jrw4 626]

You might want to go into the content manager and check to see if you have the latest version of the G1000NXi. The previous version had precisely the malfunction that you described in the original post.

Quote

Issues Resolved

• Fixed issue where RNAV was using incorrect units for GPS altitude causing wrong vertical guidance

You should be in version 0.9.2

Hope this helps.

[Stearmandriver 2,316]

It is actually normal operation of the real box, to require the use of Approach mode to follow an RNAV glidepath from the FAP to the runway.  I don't know if VNAV would actually follow that guidance or not in the real NXi, but Approach mode is technically required for regulatory compliance. 

Basically, on an approach with approved vertical guidance like an LPV, arm Approach prior to the FAP.

I know; different than what we're used to while doing RNAVs/RNPs in the Boeing, but my wife teaches in G1000 equipped aircraft, and she's the only reason I know anything about this haha.

[ark 1,295]

If you have selected an approach that has vertical guidance I don't see why you would not use that vertical guidance.

If on the other hand you are doing an approach without vertical guidance, than perhaps you could make use of VNAV in support of implementing a Continuous Descent Final Approach (CDFA) which these days is recommended over the old "Dive and Drive" technique.

Al

Edited December 6, 2021 by ark

[Bert Pieke 7,585]

5 hours ago, Boeing_Driver said:

Also of note, that the most recent edition of the G1000 seems have cured the AP of having to engage Appoach mode on an RNAV procedure, and instead, use VNAV all the way to the runway.

I also believe that VNAV is used to get you down to approach altitude, and APR is used to guide you down to the runway.

APR is a more sensitive mode than VNAV/NAV... similar to ILS.

[David Roch 1,982]

2 hours ago, Bert Pieke said:

I also believe that VNAV is used to get you down to approach altitude, and APR is used to guide you down to the runway.

APR is a more sensitive mode than VNAV/NAV... similar to ILS.

Yes, that's it. 😉

[Boeing_Driver 76]

So here is what I'm observing (with the most current version of the G1000NXi);

After activating the approach, dialing in field elevation and selecting VNAV, the plane descends along the proper path, meeting all the crossing restrictions of the approach. However, it won't go below the last crossing restriction of the approach, even after I select APPR mode at the FAF. I'm observing that a white diamond (which is presumably the GP) hangs at the top of the band and never comes down on the altitude tape.

As a side note, when I select APPR mode, the autopilot goes into Roll mode, which is obviously incorrect behavior.

As I recall, when one approaches the FAF, a magenta diamond (not a white one) is supposed to drop and having selected Approach mode, the plane will descend with vertical guidance. I'm no longer seeing this.

Edited December 6, 2021 by Boeing_Driver

[jrw4 626]

I tried this yesterday and saw the same thing .So I flew the approach as a "dive and drive" to the MDA of the LNAV approach and by then the diamond showed up again. Something not quite right here. BTW, I did not use VNAV for the descent to the FAF, but rather flew the approach using FLC from the IAF. I activated the approach using APP a mile or two before the FAF.

There may still be something strange going on, at least for some approaches.

[ark 1,295]

53 minutes ago, Boeing_Driver said:

As I recall, when one approaches the FAF, a magenta diamond (not a white one) is supposed to drop and having selected Approach mode, the plane will descend with vertical guidance. I'm no longer seeing this.

Regarding the white diamond, in the G1000NXi Pilot Guide under Vertical Deviation it says:

"A hollow gray diamond represents a preview of the glidepath indicator. This is shown while inbound to the FAF waypoint, but before the FAF waypoint is the next active waypoint. The preview is also shown when the AFCS is coupled to the Vertical Path Tracking Mode (VPTH) while inbound to the FAF waypoint, until the AFCS captures the glidepath; refer to the AFCS section for more information about Vertical Path Tracking Mode.

  The hollow gray preview diamond changes to a solid magenta diamond to indicate the glidepath indicator is active".

Al

 

[Boeing_Driver 76]

3 minutes ago, ark said:

Regarding the white diamond, in the G1000NXi Pilot Guide under Vertical Deviation it says:

"A hollow gray diamond represents a preview of the glidepath indicator. This is shown while inbound to the FAF waypoint, but before the FAF waypoint is the next active waypoint. The preview is also shown when the AFCS is coupled to the Vertical Path Tracking Mode (VPTH) while inbound to the FAF waypoint, until the AFCS captures the glidepath; refer to the AFCS section for more information about Vertical Path Tracking Mode.

 

  The hollow gray preview diamond changes to a solid magenta diamond to indicate the glidepath indicator is active".

Al

 

 

Thanks, that's good to know. However, I would like to know why mine isn't turning to magenta?

[ark 1,295]

16 minutes ago, Boeing_Driver said:

Thanks, that's good to know. However, I would like to know why mine isn't turning to magenta?

Well, I don't know what's happening on your system,  but I think you have to have APPR mode armed. I usually arm APPR mode as soon as the FAF becomes the next waypoint, or a bit earlier. 

The AFCS system will revert to Roll and Pitch mode when the mode your are in no longer "works" for some reason -- that is, some constraint for the current mode is no longer valid. If the AP is on, the AP has to be in some mode, and Roll and Pitch is the "safe" fallback mode. VNAV does have constraints.

Here's one reference for the G1000NXi:  http://static.garmin.com/pumac/190-02177-00_A.pdf   The quote I posted above came from page 52 of this document.

 

Al

Edited December 6, 2021 by ark

[Boeing_Driver 76]

@arkYes, I arm APPR mode prior to the FAF. But it shouldn't go into roll mode. That means something has gone wrong, as you've said. It's as if the sim isn't detecting a GP for the approach.