GTN Xi / GFC 500 VNAV question

[PT20J]

Let’s say you are on a GPS flight plan with the autopilot engaged in NAV ALT at 8,000 ft. Ahead is waypoint A with an altitude constraint of 7,000 and waypoint B with an altitude constraint of 5,000. You are cleared to descend, pilot’s discretion to 5,000. You set the altitude bug to 5,000 and hit VNV and the autopilot captures the vertical track at TOD and descends and crosses A at 7,000. At this point, you are told to cross B at 6,000. If you set the altitude bug to 6,000, will the autopilot stop the descent and switch to ALT at 6,000?

[donkaye]

19 minutes ago, PT20J said:

Let’s say you are on a GPS flight plan with the autopilot engaged in NAV ALT at 8,000 ft. Ahead is waypoint A with an altitude constraint of 7,000 and waypoint B with an altitude constraint of 5,000. You are cleared to descend, pilot’s discretion to 5,000. You set the altitude bug to 5,000 and hit VNV and the autopilot captures the vertical track at TOD and descends and crosses A at 7,000. At this point, you are told to cross B at 6,000. If you set the altitude bug to 6,000, will the autopilot stop the descent and switch to ALT at 6,000?

Yes.  The VNAV will go to ARM, and the VNAV deviation carrot will continue on down.  When ATC gives you a descent, you can then hit VS, adjust the wheel for a large VS and possibly recapture VNAV.  If you do recapture the VNAV slope, the VS light will extinguish and VNAV will continue.

[PT20J]

1 hour ago, donkaye said:

Yes.  The VNAV will go to ARM, and the VNAV deviation carrot will continue on down.  When ATC gives you a descent, you can then hit VS, adjust the wheel for a large VS and possibly recapture VNAV.  If you do recapture the VNAV slope, the VS light will extinguish and VNAV will continue.

Thanks, Don. That's how I thought it worked, but I couldn't find it in the documentation except a statement that the altitude bug has to be set at or below the lowest altitude constraint that kind of implies it. 

[Marc_B]

2 hours ago, donkaye said:

you can then hit VS, adjust the wheel for a large VS and possibly recapture VNAV.

The issue I usually run into is that VNAV to me seems to be a "one shot thing."  Meaning, if you set up all your constraints, capture TOD, descend and everything works well it's great.  Not sure your experience, Don, but I have a hard time recapturing unless it's a brief level off for traffic separation and a quick back down to VNAV slope.  But usually it's a bigger change, like a vector (using HDG so VNAV out) or a longer change in altitude clearance, etc.; so trying to re-set & reengage VNAV, make sure TOD is there and captures, etc seems to have more quirks.  So usually set up VNAV and then with changes move to VS and ALTS from there.

I have a much easier time with VNAV flying VFR when I'm controlling the narrative...  (because I'm a lowly GA pilot not typically flying a full arrival)

Can't remember where i found this but I like it:  VNAV flow = Vertical constraints set, Nav set to GPS, Altitude bug at lowest cleared, VNAV mode engaged.  (is this yours @midlifeflyer??)

 

[Marc_B]

I guess the solution is to use the VNAV vertical flight path as synonymous to a lateral course...meaning use VS as an "intercept" to get back to the decent profile.  @donkaye if you have the VDI showing and you're on the vertical descent and press VNAV (or have VNAV armed) does it immediately re-capture and show VNAV mode in green? (taken of course that other parameters are met i.e. constraints, bug, etc)

That's the one thing with VNAV that I find makes it the most difficult.  VNAV will only capture the decent profile if it's out ahead of you, it won't "turn to intercept" (i.e. descend to intercept) unless you actively command it.  So if you blow through the VNAV profile and forget to arm VNAV or forget to change your altitude bug, it is more of a hurdle than just hit the VNAV button and it will gently accommodate.

[midlifeflyer]

1 hour ago, Marc_B said:

The issue I usually run into is that VNAV to me seems to be a "one shot thing."  Meaning, if you set up all your constraints, capture TOD, descend and everything works well it's great.  Not sure your experience, Don, but I have a hard time recapturing unless it's a brief level off for traffic separation and a quick back down to VNAV slope.  But usually it's a bigger change, like a vector (using HDG so VNAV out) or a longer change in altitude clearance, etc.; so trying to re-set & reengage VNAV, make sure TOD is there and captures, etc seems to have more quirks.  So usually set up VNAV and then with changes move to VS and ALTS from there.

I have a much easier time with VNAV flying VFR when I'm controlling the narrative...  (because I'm a lowly GA pilot not typically flying a full arrival)

Can't remember where i found this but I like it:  VNAV flow = Vertical constraints set, Nav set to GPS, Altitude bug at lowest cleared, VNAV mode engaged.  (is this yours @midlifeflyer??)

 

LOL! Definitely not! I think there are about two mnemonics in all of aviation that are worth anything, so eve the idea I might create one is downright insulting!!

On @pt201J's scenario,, I'd change the selected altitude to 6,000 and, yes the change "should" cause the AP to level off at the new, higher, altitude.  But always be prepared for the AP to not do what's expected. For example, are we already so close to 6,000 that the AP might not handle it properly. The simple alternative is to be prepared to (or proactively) hit the ALT button to capture 6,000, which will terminate VNAV until you re-engage it.

[donkaye]

26 minutes ago, midlifeflyer said:

LOL! Definitely not! I think there are about two mnemonics in all of aviation that are worth anything, so eve the idea I might create one is downright insulting!!

On @pt201J's scenario,, I'd change the selected altitude to 6,000 and, yes the change "should" cause the AP to level off at the new, higher, altitude.  But always be prepared for the AP to not do what's expected. For example, are we already so close to 6,000 that the AP might not handle it properly. The simple alternative is to be prepared to (or proactively) hit the ALT button to capture 6,000, which will terminate VNAV until you re-engage it.

In this video I deliberately failed to Arm VNAV.  I let the VDI descend before I Armed VNAV.  I then tapped the VS button and rolled the VS down until the VDI started up.  The VS annunciated Green and indicated the VS while the VNAV showed White.   On intercept VS went off and VNAV went Green showing VNAV was captured.  I didn't go a good job of showing the VS button going off.  Same for the final annunciations.  However, if you look closely at the end you will see I got the camera up high enough to show the proper Annunciations.

 

[midlifeflyer]

15 minutes ago, donkaye said:

In this video I deliberately failed to Arm VNAV.  I let the VDI descend before I Armed VNAV.  I then tapped the VS button and rolled the VS down until the VDI started up.  The VS annunciated Green and indicated the VS while the VNAV showed White.   On intercept VS went off and VNAV went Green showing VNAV was captured.  I didn't go a good job of showing the VS button going off.  Same for the final annunciations.  However, if you look closely at the end you will see I got the camera up high enough to show the proper Annunciations.

 

Yep. It will capture VNAV (and for that matter GP) if your vertical speed is high enough to intercept the VDI. I think the manuals talk about it in terms of "capturing from above." When I mentioned "should" level off, I had in mind the note in the manual that 

But I think we may have read @PT20J's question differently.

I saw it as a simple altitude instruction change - the common "amend altitude to" 6,000 after initially clearing down to 5,000 . I don't know what the next instruction might be so my answer doesn't address what comes after the level off.

[donkaye]

One other point may be of interest.  ALTV vs ALTS.  ALTV would be annunciated as a VNAV altitude while ALTS would be annunciated as a Selected altitude.  Thus, if you were on a VNAV profile ALTV would be annunciated beside VNAV.  If you were told to stop descent, ALTV would change to ALTS for selected altitude.

[Marc_B]

8 minutes ago, midlifeflyer said:

But I think we may have read @PT20J's question differently.

I think @donkaye answered it to begin with. Then we drifted into VNAV from above.