Peter, this is a rough description of the GTN and GTN Xi vertical guidance capabilities. I did not include all details, if you want more, it is best to get the free Pilot Guide and download the free trainer, both from Garmin.
The GTN series of navigators have Baro-VNAV capability, but only up to the FAF. This assumes the GTN has an altitude input adjusted for the local barometric pressure. On my Bonanza, the Baro corrected altitude comes from the G500TXi. The GTN series supports two types of VNAV, the old fashioned one supported by many earlier GPS systems that allows a single point, with a target altitude, VS target or angle target and calculates a vertical path based on the selections and there is a displayable VS required, time to top of descent, and vertical descent rate. This system does not command the autopilot, it just provides the pilot with when to begin a descent at a given rate to get to the desired altitude at the target location.
The more powerful VNAV capability provides the flight plan to include an altitude for each point in the flight plan. It only supports descents. STARs and IAP with altitudes are pre-loaded into the database, and each point in the flight plan can have an altitude assigned. The output of VNAV can control a new digital autopilot and automate all the descents to match what is in the flight plan. It also displays a Vertical CDI on a compatible system (in my Bonanza it is the G500TXi PFD) that is distinct from the vertical CDI indicator for an ILS or +V or LPV. So one can manually fly the vertical path with a vertical CDI descent guidance on the PFD. In my Bonanza I have a legacy Stec 60-2, but the legacy autopilots do not support VNAV coupling, just ILS or +V or /LPV coupling. The GFC500/600 have a VNAV mode that couples to the GTN VNAV and will automate the descent. Each waypoint in the flight plan determines the target altitude for that waypoint and the VNAV mode generates a TOD that arrives at the waypoint at the programmed altitude. The GFC 500/600 will follow the guidance as displayed on the PFD if VNAV is enabled. Baro-VNAV uses the Baro corrected altimeter input, so it respects MSL altitudes, whereas GPS vertical guidance uses GPS altitudes. The Baro-VNAV in the GTN series is not approved for the final approach segment, although in concept it could be if Garmin had it approved. That would enable approaches using LNAV/VNAV minimums on approaches that are outside an SBAS service volume. The GTN uses either GPS altitude for +V or SBAS corrected GPS altitude for LPV or LNAV/VNAV. The G500TXi shows all these types of vertical guidance with their own annunciation and VDI indicator. LPV and LNAV/VNAV and +V show as a magenta diamond for the GP indicator based on GPS. ILS shows the GS as a green diamond for the VDI. VNAV show a magenta > symbol as the GP indication. The GTN Xi added the +V guidance for straight in VOR/NDB approaches. Both the GTN and GTN Xi support a visual approach function intended only to be used in VFR conditions as an aid to lining up with the correct runway and provide a vertical path to the runway. Not all runways will support the visual approach.
Ibra wrote:
“Synthetic ILS”
I have a tough time with using this terminology as what is being described is either advisory vertical guidance or VNAV using Baro-VNAV, neither of which are synthetic ILS other than there is a VDI provided for a vertical path.
I have a tough time with using this terminology
Yes it’s all “advisory”, everything with magenta “<“ or “<>” that goes with +V, GNSS-VNAV, BARO-VNAV, en-route VNAV….the only “official” are L/VNAV (BARO & SBAS) and LPV(SBAS)
I also have a tough time with what Peter call “synthetic ILS”, I prefer if we use it’s technical term from A320 FMS: “FLS Beam” (or “FMS Landing System” with F-LOC + F-GS that appear when you load VOR IAP in Greece)
@NCYankee, would this be correct?
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GTN can theortically be used to program the FAF and the runway threshold user waypoints with correct altitudes AMSL. After that GTN (+ for example GFC + G500TXi) could be used in a mode (confusingly) called “Enroute VNAV” to fly that FAT with fairly accurate vertical guidance.
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Have I made too many mistakes in that oversimplification?
I have the “old” GTN version and it does most of this, I don’t think it’s limited to the Nxi? Unless I’m missing something. I have the visual approach feature, VNAV, and but probably not vertical guidance to non-GPS NPA (but I would not know because I don’t think I’ve flown one since my IR test)
arj1 wrote:
GTN can theortically be used to program the FAF and the runway threshold user waypoints with correct altitudes AMSL. After that GTN (+ for example GFC + G500TXi) could be used in a mode (confusingly) called “Enroute VNAV” to fly that FAT with fairly accurate vertical guidance.
That’s correct. “Fairly accurate” is also correct as you can only set target altitudes in 100-feet steps.
I phrase thread titles carefully for a good reason 
Wigglyamp’s post above shows the large number of limitations on using the “enroute VNAV” (clearly the word “enroute” was chosen to make it clear Garmin do not support the use of it for discourage DIY approaches). And how the hell will you know in advance which of these are going to apply as you are flying to XXXX at 150kt, with 30nm to run?
synthetic ILS
The “confusion” probably rests in that
but probably not vertical guidance to non-GPS NPA
Exactly.
And that is the topic here. It is quite relevant in European GA.
Does anyone know if Avidyne have this feature?
We can have another thread on hacks for flying DIY approaches It will need to contain accurate descriptions of how it is set up, etc, like ncyankee’s post above. Not something which jumps around all over the place, off topic after 2 posts, because nobody can make use of that.
Peter wrote:
Does anyone know if Avidyne have this feature?
Allegedly, it was introduced in the recent firmware version, haven’t used it myself yet.
Here is the latest software update information from Avidyne. I don’t see the addition of VOR/NDN +V as an addition, but enroute VNAV seems to be added along with a Visual Approach feature: https://pilotsupport.avidyne.com/kb/article/816-ifd-5xx-4xx-software-update-10-3-1-2-update-instructional-video-for-loading/
Peter wrote:
(I didn’t read the 116 page PDF )
The PDF is bookmarked. Just look at OpSpec C073 (pdf page 71) and OpSpec C085 (pdf page 98). I attached the entire document, but it is easy to locate the relevant sections to the discussion. You can see what the certificated operators are permitted to do.
