From here
A really interesting post from @ncyankee.
This is really new because until now the +V (offered by both Garmin and Avidyne) was limited to existing LNAV (i.e. GPS approaches but not LPV) approaches.
It is what airliners have had for years, being able to fly an “ILS” using FMS data synthesised from INS, to some Greek island with just a published VOR approach.
The +V feature with VOR/NDB has been around since the GTN 750/650 Xi series, so is not that new. I actually learned about it from a post here on this forum. It was one of several inducements for me to upgrade from the GTN 750 to the GTN 750 Xi.
Peter wrote:
It is what airliners have had for years, being able to fly an “ILS” using FMS data synthesised from INS, to some Greek island with just a published VOR approach.
The FMS in most air carrier and corporate Jets does provide an advisory glidepath or VNAV, most based on Baro-VNAV as SBAS navigators are still the exception in the air carrier fleet. Most FMS are multi-sensor (DME-DME, VOR, ILS, NDB, INS, and GPS) and select the appropriate source, but DME/DME and DME/DME/INS is not approved for approach use. These are the relevant OpSpecs that air carrier operators are allowed to use and the permission to use VNAV with a NPA is subject to the following OpSpecs. The lateral guidance is +/- 0.3 NM and not to ILS tolerances and INS is not one of the approved methods for lateral guidance.
-OpSpec/MSpec/LOA C052, Straight-in Non-Precision, APV, and Category IPrecision Approach and Landing Minima – All Airports.
-OpSpec/MSpec/LOA C073, Vertical Navigation (VNAV) Instrument ApproachProcedures (IAP) Using Minimum Descent Altitude (MDA) as a DecisionAltitude (DA)/Decision Height (DH)
You can read what is in the OpSpecs in the attached document.
DME/DME and DME/DME/INS is not approved for approach us
Well if one is allowed RNAV1 or TEM specs then even older GTN offer “synthetic ILS” to any airport: you press DCT or OBS to destination fix and set en-route VNAV with altitude = 50ft crossing height over the fix
This “synthetic ILS” is offered by old GTN navigators using “en-route VNAV” to any fix in database in ENR or TERM modes, one can even couple an AP to it using VPATH mode !
However, unlike NXI with loaded straight-in VOR, NDB, LOC this is not an approche to runway threshold (more to ARP, VOR, NDB), it uses +/-1nm or -/+5nm sensitivity by default, it suffers from QNH error due to blue/magenta altitude bugs
This DCT-VNAV or OBS-VNAV is simply an en-route vertical navigation profile that mixes barometric and geometric pipes
Don’t try it at home, getting hurt and all of that, useful for IFR training away from aerodrome, take any fix in database and you have a “synthetic ILS” 
I don’t understand the above.
DME/DME is used by airliners to fix up drift in INS. In more modern ones GPS is used also. This is not relevant to GA; there was never an INS product for GA.
The “glideslope” on GA avionics, both the old +V and anything from the TXi, is purely GPS generated AFAIK. Well the +V certainly is.
The lateral guidance is +/- 0.3 NM and not to ILS tolerances and INS is not one of the approved methods for lateral guidance.
That’s interesting, because it leads to the question of what exactly the airline crew does in the cockpit (I didn’t read the 116 page PDF ). I think the relevant details will be in the AOC manual. For example different airline pilots told me different things about required navaids, identing, notaming INOP, etc. Maybe the minima is just higher than ILS; after all this isn’t autoland.
I don’t understand the above.
Since 2011, all GTN units with no exception do allow “OBS-VNAV” and “DCT-VNAV” features that take you to any VOR station with any arbitrary target altitude (set by the user), these give “Synthetic ILS” on HSI
GTN NXI units, allow LNAV+V to straight-in VOR procedures I
I think @NCYankee know all the nerdy differences between DCT/OBS-VNAV to fixes and straight-in LNAV+V to thresholds
Since 2011, all GTN units with no exception do allow “OBS-VNAV” feature to any VOR station with any arbitrary target altitude that is set by the user, what you get is a “Synthetic ILS” on HSI
Is that applicable to landing on a runway? Assuming the VOR is not at the runway threshold – they rarely are.
That is just enroute VNAV.
It would be good to have input from actual users, in Europe. Also does Avidyne offer this?
Yes it’s enroute (GPS) VNAV
Is that applicable to landing on a runway? Assuming the VOR is not at the runway threshold – they rarely are.
At the end of the day, the NXI does not move VOR station toward runway thresholds?
Internally, NXI takes care of it:
You can do these with en-route VNAV on same VOR station using older GTN
Lot of of button pushing though
Yes it’s enroute (GPS) VNAV
Off topic then.
At the end of the day, the NXI does not move VOR station toward runway thresholds?
I don’t understand. Is there something missing?
But please wait until tomorrow, not another post in 1 minute’s time Others may have input here… let’s give them a chance. Few feel they can contribute to threads like this.
How about the visual approach function of the GTN Xi. Extract below from the pilot’s guide:
Visual Approach
FEATURE REQUIREMENTS
• Valid terrain database
FEATURE LIMITATIONS
• Not all airports in the database support visual approaches
Published data is used to determine the visual approach GPA and threshold crossing
height (TCH) for the selected runway. If no published data is available, the default is
3 degrees GPA and 50 ft TCH.
You may load and activate a visual approach from the following apps.
If unavailable, it reads: “NO VERTICAL GUIDANCE”
Terrain and obstacle obstructions along the approach path determine the availability
of vertical guidance advisories for visual approaches.
• If no known obstructions are within the approach path, vertical guidance is
provided to a maximum distance of 28 nm from the runway.
• If there are known obstructions further than 3 nm, but within the 28 nm
maximum distance from the runway along the approach, vertical guidance
is limited to the approach path after crossing the known obstructions. After
loading the approach, a shortened magenta line shows on the map.
If obstructions are within 3 nm to the runway, along the approach path, advisory
vertical guidance is not available.
Points About Visual Approaches
• Provide advisory horizontal and optional vertical guidance for the selected
runway
• Lateral guidance is always provided for visual approaches
• Helps stabilize the runway approach
• Three methods for loading and activation
• Map • Procedures • Waypoint Info
Upon loading the visual approach,
a pop-up informs when vertical
guidance is available.
If available, the pop-up contains
the glidepath angle (GPA) and
threshold crossing (TCH).
Peter wrote:
DME/DME is used by airliners to fix up drift in INS.
Not just for that. Lots of airliners have no INS and use DME/DME as the primary navigation system.
