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Questions on Garmin GTN VNAV

From v6.50, Garmin has added vertical nav capability to the GTN. They call it “Descent vertical navigation” in their press release, and the manual says:

An altitude constraint is invalid is meeting the constraint requires the aircraft to climb.

Given that I am without an aeroplane currently, I have time to play with the simulator (the PC version, the iPad version still doesn’t have v6.50 and more importantly does not allow to see on a PFD the effect of playing with VNAV). There are several things which are confusing me and which I hope someone will be able to shed some light on.

For the sake of demonstration, I simulated a flight from Biggin Hill (EGBK) to London Heathrow (EGLL) – it’s a simulation, might as well! Routing via DET, EGMC and LAM, then ILS27L via LAM for Heathrow. The vertical constraints of the approach are automatically loaded by the system, and I have added two, manually as indicated by the pen next to them: 8000 feet at DET and 9000 feed at EGMC :

The first surprise is that the constraint at DET appears as valid and active, despite requiring a climb. But that’s a good surprise. The second surprise is: why is the second constraint displayed as invalid (crossed)? In map view the first constraint is displayed, the second one is not.

On the PFD, the VNV constraint is displayed (in magenta below the altitude bug in cyan). In the climb, the desired vertical speed wasn’t displayed, and neither was the glideslope like indicator, but the Vertical Speed Required (VSR) on the GTN map was.

On passing DET at 8000 feet, with the active leg going to DET→EGMC, not only was the constraint of 9000 at EMGC not displayed, the 7000 feet at LAM automatically loaded on the approach was.

But if you then select direct EGMC…

… the 9000 constraint becomes active and displays on the GTN map page…

… and on the PFD.

But if from there you activate the DET→EGMC leg, you loose it again.

At no point during these climbs did the (simulated) autopilot start climbing to follow the VNAV indications. I suppose that’s indeed the expected behaviour.

On the descent, the feature is excellent, the only gotcha so far is that I need to check what VS target it gets (on some legs it had 1700 fpm, and I’m not yet sure what drives it). Edit: it’s keeping 3 degrees, I was going a bit fast in the sim.

Has anyone who’s been playing with the sim or flown the real thing noted similar behaviour?

Last Edited by denopa at 22 Apr 15:15
EGTF, LFTF

Off subject, apologies, but looking at the pic below copied from Denopa’s post above, how do you clean up the screen and get rid of all those lines, most of which are irrelevant to anyone.

If you are VFR all you want to see is the airspace that affects your level and if IFR all you want is what is relevant to that.

As depicted it looks a total mess

Yes completely off topic ;-) You go to menu, map setup, Airspace, Smart Airspace On, Show Airspace below 9000

EGTF, LFTF

Thanks, I have the G500 and G530, unfortunately it appears they dont have a Smart Airspace option or I haven’t found it yet.

I have turned off all the airspace above 6000ft (on the G500) as you don’t really need it but it’s still a cluttered mess.

Apologies again for going way off topic.

The vertical planning is only applicable for descents. Without auto-throttle kind of capability, climbs can result in stalls. Regardless, the feature only applies to descents.

KUZA, United States

VNAV is a great feature and highly intelligent. I can’t look at those details because I am away with only an iPhone, but if no-one else has done before I get back, I’ll have a look.

It is very disappointing that Aspen has no concrete plans for VNAV, and when they do come out with it, it will require a hardware upgrade. A nail in the coffin, I fear.

EGKB Biggin Hill

Denopa,

Could it be because you’ve used an airport as a waypoint?
Try using NDB (SND) at EGMC?

EGTR

NCYankee wrote:

The vertical planning is only applicable for descents.

That’s what the manual says, but not what the simulator does

Without auto-throttle kind of capability, climbs can result in stalls..

And descents in Vno exceedance. Which may be why Garmin’s only released it with autopilots featuring envelope protection

arj1 wrote:

Could it be because you’ve used an airport as a waypoint?

Thanks! Good idea, but that’s not it, just tried.

Quatrelle wrote:

Thanks, I have the G500 and G530, unfortunately it appears they dont have a Smart Airspace option or I haven’t found it yet.

The G500txi does, but on the HSI map you cannot turn off airspace completely, which would be nice – it’s small and busy, but useful especially with the selected ALT range ring, which shows an arc describing where you will reach your selected altitude.

EGTF, LFTF

denopa wrote:

That’s what the manual says, but not what the simulator does

I don’t see it working at all in a climb. I have it in my aircraft, GTN750/G500TXi.You must be above the next altitude for it to work, so once you depart and climb above the first altitude, it would begin to work. When you program a climb on a segment, that segment is invalid, but the first one isn’t invalid, but will not provide any vertical guidance until you are above the altitude.

KUZA, United States

Here’s what I see in the sim when the aircraft is below the target altitude:

- no vertical guidance on the PFD
- no autopilot climb
- the target altitude is, however, displayed on the PFD (below the armed altitude) and on the GTN map
- Vertical speed required is displayed on the GTN

But that’s only for the first waypoint on the FPL, or when doing “direct to”.

EGTF, LFTF
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