denopa wrote:
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.
I have a setup with GTN750 with VNAV enabled, and G500TXi PFD. The autopilot doesn’t talk with the G500 in terms of any altitude constraints (the GAD43e doesn’t support a connection to it). This is actually what I would suggest as a mental model when you try to understand the behaviour of the GTN, i.e. to consider the autopilot as completely separate from the navigation box, or to disregard it altogether. If anything, the AP is driven by the navigator and not vice versa.
One thing I noticed on my last flight is that as soon as you’re just 10 ft below the “At” target altitude and descending, you’ll get a flashing warning message on the GTN “Cannot reach vertical waypoint”. Which makes sense, you shouldn’t be below the target altitude before the waypoint if you’re following your VNAV profile.
To your question, if you consider the GTN as a separate entity, then the active leg determines the constraints (or rather the next waypoint in sequence with an altitude constraint). Since you didn’t have a lower constraint before your 8000ft at DET, it is valid and active; but you should have seen the flashing warning message mentioned above when flying towards DET with that leg active and below your target altitude. If you didn’t, that might be a limitation of the simulator implementation.
denopa wrote:
… the 9000 constraint becomes active and displays on the GTN map page…
That’s because your active leg is now “From present position to EGMC”, completely ignoring the prior leg with the prior (lower) altitude constraint at DET. “Present position” doesn’t have an altitude constraint, so you don’t have any climb in your navigator, and 9000 has become a valid constraint. Again, if flying below, you should get the flashing warning about inability to reach the target.
denopa wrote:
Edit: it’s keeping 3 degrees, I was going a bit fast in the sim.
When you’re within a certain time from your top of descent, you get a button with which to modify the angle on the VNAV utility page. I think it’s around 12 minutes before TOD. Interestingly, if choosing a steeper angle, that obviously pushes back your TOD and makes the button inactive again. If you choose something too steep you can’t easily get back your shallower descent angle, because once the button becomes active again you may be past the TOD point that would result from the shallower angle.
I wonder if there is a different mode for turboprops? On those, one could do a climb in VNAV a lot better i.e. without running out of power.
No. For altitudes higher than current altitude, the autopilot does nothing based on VNAV (does not climb, does not intercept). Not surprising given there is no vertical deviation information displayed on the PFD. It works great in the descent.
Have you tried climbing to the target altitude. Will it intercept it?
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”.
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.
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.
Denopa,
Could it be because you’ve used an airport as a waypoint?
Try using NDB (SND) at EGMC?
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.
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.
