LNAV+V coupled approaches appear to fly lower than I expect

[mikelupo]

Do LNAV+V approach advisory altitudes only apply at the fixes?

Granted the LNAV+V should really be treated as a lateral guidance only with vertical "advisory", but the AP when coupled, with VNAV, should not descend below the advisory altitudes as depicted on the approach chart (according to what I recall about the 430W).

Since my new HDX install I have test flown (twice) the same RNAV approach (LNAV+V) at KLCI Runway 26 in VFR weather. Both times, the autopilot flew lower than my previously installed TruTrak system.
If you look at the attached photo, the PAPIs are all red. Even at ZUROT they were red.

Before the HDX retrofit, I had a TruTrak EFIS (integrated auto pilot) and was slaved to my 430 WAAS.
I have the same navigator, yet all new Autopilot (Dynon HDX, Dual ADAHRS, ARINC249, etc).
The AP on that did not go lower than the prescribed altitudes. I always saw two red/two white.

After the first time, I surmized that I must have set the altimeter incorrectly. So the second time (days later) I did it again and double checked my altimeter setting.
What I didn't do was verify my altitudes at ZUROT(2900+) or REZDI (2200+) or greater. So that's still a data point I will gather on my next attempt.
I will also try a different RNAV. I'll try an LPV next. Runway 8 at KLCI also has that, do I don't have to venture far.

In case anyone asks, yes, I flew an ILS approach and that was spot on.

Shared experiences are welcome!

Some facts:
430 was in VLOC mode and was indicating LNAV+V.
The 430, not being in GPS mode, was a procedural error.
I'm still learning and hence, why doing this in VFR with a safety is really important.
I had flown for 45-ish minutes at a few different altitudes. So I don't think there was any error that had to be calculated by the HDX. (reading the VNAV section of the user guide). I also do not believe the servo is slipping.
Dynon AP was coupled in VNAV mode.
The Dynon Altimeter agrees with the G5.
Pitot/Static certification was done in Sept, 2024. Technician said that the leakdown test was among the best he's encountered. that said, I have no reason to believe the static system here is at play.
I can confirm that the alt static source is switched off.

 

[Marc_J._Zeitlin]

Do LNAV+V approach advisory altitudes only apply at the fixes?

I believe the answer is yes, as with LOC approaches, as long as you don't drop below the altitude for the next fix.

Since my new HDX install I have test flown (twice) the same RNAV approach (LNAV+V) at KLCI Runway 26 in VFR weather. Both times, the autopilot flew lower than my previously installed TruTrak system.

This is the most interesting point to me. The 430W is what's providing the vertical guidance (showing LNAV+V) - NOT the Skyview (or TruTrak). Either AP is just following the glide path provided to it by the 430W, so theoretically SHOULD do exactly the same thing.

What I didn't do was verify my altitudes at ZUROT(2900+) or REZDI (2200+) or greater.

And this would provide important info. If the crossing altitudes were correct and the CDI shows that you're on the vertical guidance then the AP is doing the correct/legal thing. Now, after crossing REZDI, you could legally drop immediately (if the plane was capable of doing so) to 1420 ft., but of course the 430W isn't going to do that - it's going to continue on the GS and the AP will keep the needle centered on the provided GS.

I'll be interested to see what you find out.

 

[skysailor]

This approach does not offer vertical navigation. The only published minimums are LNAV. The LNAV+V you are getting is exactly that...you are getting the LNAV offered by the approach but the vertical is advisory only. Furthermore, it is only offered to the final approach fix. After the FAF you are in "Last Leg Extended" mode as depicted on your map. There is no point with a published altitude for the system to navigate to vertically. The only point after the FAF is the MAP (missed approach point) which is at the runway threshold somewhere between 1420 and 1460 MSL (straight in Cat A to circling Cat D) but the system has no way to know which. If you had set the Minimums bug you would have already heard the "Minimums!" announcement and you are expected to be able to see and navigate to the runway or go around if neither of these is true. LNAV+V does not offer TERPS terrain and obstacle clearance below the published fix altitudes. In your case, you should have leveled at the MDA (likely 1420 MSL for Cat A straight in) and flown that until arriving at a point to descend to the runway. A qualified flight instructor would have you set the minimums bug prior to commencing the approach and set the altitude bug to 4400 (missed approach altiitude) when the system transitioned from altitude hold to VNAV (ALT -> VNAV becomes VNAV on the flight mode annunciator at the top of the PFD accompanied by "Autopilot Mode" audio). The system is doing what it is designed to do.

 

[prepperpilot]

If your in vnav. It will take you all the way to the ground. Works that way in in 787 also.

 

[Marc_J._Zeitlin]

This approach does not offer vertical navigation...The system is doing what it is designed to do.

I agree with everything you said.

I think the OP was mostly questioning why (if it is) the Dynon AP seems to him to be doing something different than the previous TT AP. I'm not convinced that it is, but _IF_, for some reason, the TT AP kept him on or above the GS on the approach (two red, two white, or two white) on the PAPI while the Dynon AP is showing two red, then there's a difference in performance Don't know how that would be, though, as the APs do not make these decisions on their own.

Since the 430W is driving (at least until the FAF), I'd expect everything to be the same up until then. Now, KLCI's PAPI on runway 26 has a 4 degree PAPI, NOT a standard 3 degree, so if the approach is 3 degrees (which the plate seems to indicate, since there's no indication of the GS being anything else) one would EXPECT to see four reds on a 3 degree approach with a 4 degree PAPI. Not only that, but the 1420 ft. minimum on the 3 degree GS occurs far away (2.5 NM) from the runway - from REZDI to the runway (5 NM) with only a 780 ft. altitude loss is a 1.5 deg. GS, so again, if one continues the 3 deg. GS after passing REZDI, one will definitely be below the PAPI and all lights will be red. Then one levels off at 1420 ft. and 2.5 NM from the runway, as you say, and you'll be below the PAPI for 0.7 NM until you get to the 4 deg. GS at 1.8 NM from the runway.

So, maybe what I suspect is that the PAPI wasn't 4 red with the previous TT AP, and maybe the OP is just noticing that it is now, since 4 reds are completely consistent with a correct approach on the extended GS down to 1420 ft. Just a guess.

 

[ve0kog]

on your picture you are 1290 feet 2NM from the runway with 757 feet to lose. if my math is right that works out to slightly above the 3° descent profile from the FAF. i would expect that from the LNAV+V guidance

 

[mikelupo]

For those watching this thread, Thank you all for your comments. I've not gone out again as the Wx here has been less than VFR along the approach paths and altitudes. When the weather changes, I'll do a LPV for one, and then I'll fly the LNAV+V again, this time I'll take note of my altitudes as I cross the GPS waypoints. I am trying to get back to KLCH for family so it might not be till I get back from there.

 

[ve0kog]

@mikelupo if you could import your HDX user data file to flysto.net. it will plot the flight path overlaid on the approach and mark the fixes automatically.
then compare the TT and Dynon AP paths. just another idea..

 

[mikelupo]

@Marc_J._Zeitlin, I will do just that! Next time
I’m down at the airport I’ll dump that data and upload it. I’ll share here for everyone’s edification