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LGIO RNP Z 32 - vertical profile, and how can avionics fly this?

LNAV+V is an approach annunciation associated with the intermediate segment that connects to the FAF. On this procedure, the approach annunciation comes on passing IO716. It provides an SBAS generated advisory vertical glide path and drives the CDI vertical. The comment regarding that the +V is designed to provide a GP above step down fixes only applies on the FAS. Even then, this is a nominal GP as temperature will change the baro altimeter indication, so although the +V does not move in space, the baro altimeter does move with temperature such that on a cold day, the true altitude will be below the indicated altitude, so you might hit the rocks on a very cold day. On a hot day, following the +V GP will cross the step down fix with an altimeter indication below the minimum, even though the true altitude is the same, ISA, cold, or hot.

Unlike the +V or other SBAS generated GP with an LPV or LNAV/VNAV procedure, the GTN has what is termed Descent VNAV and provides a descent Baro-VNAV capability that supports assigning descent constraints or programmable descent in the flight plan, This does not use SBAS for vertical guidance and is not connected to the vertical CDI, but with a Garmin GFC500/600 autopilot can automate the descents using the Baro VNAV capability. This feature along with the Garmin autopilot can automate the descent from YNN to IO717. Although it could possible be used for the FAS, it is not approved for this purpose, so it only descends the aircraft to the FAF. It establishes a TOD such that the constraint altitude is reached at the fix with the constraint. With the combination PFD/GTN/GFC, the Baro VNAV will step down to the FAF crossing each fix at the correct Baro Altimeter value. At the FAF, the Baro VNAV ceases and SBAS vertical guidance takes over for the final approach segment.

If this approach was in the US, since it has an LNAV/VNAV DA, I would expect it to annunciate L/VNAV instead of LNAV+V, as SBAS can be used for vertical guidance to a DA for LNAV/VNAV.

KUZA, United States

Easy solution for someone flying on SBAS GPS: never descend on +V before FAF no matter what happens: it’s FAF not FAP !

Passing the FAF:
- If there is a further step down: fly it on ROD/GS or altitudes & distances and disregard +V advisory glide path
- If there is no step down: fly it as L/VNAV but use LNAV minima while cross-check against altitude/distance

NCYankee wrote:

The comment regarding that the +V is designed to provide a GP above step down fixes only applies on the FAS. Even then, this is a nominal GP as temperature will change the baro altimeter indication, so although the +V does not move in space, the baro altimeter does move with temperature such that on a cold day, the true altitude will be below the indicated altitude, so you might hit the rocks on a very cold day

Talking about GPS SBAS where indeed +V does not move in space for (unlike in Baro-Nav)

Assuming Garmin does check +V against obstacles in true GPS altitude, then flying +V all the way you will be clear of them but you may not have the required “baro MOC” depending on the days?

Does Garmin really check +V against obstacles in true GPS altitude? I had the impression obstacles in many databases are in baro altitude, does Garmin do a similar conversion (or temperature corrections) before checking that +V hits obstacles?

Paris/Essex, France/UK, United Kingdom

I don’t understand how BARO VNAV works. A GPS generating “VNAV” of any sort should be using GPS altitude.

I wonder how much of this one would get with an IFD540 driving a KFC225.

AIUI, the connection options to the KFC225 are:

Laterally: either directly from the IFD via ARINC429, or via the roll steering converter feature of the SN3500 EHSI (and then the 225 is driven laterally via analog).
Vertically: either directly from the IFD via analog, or via the SN3500 (and then the 225 is driven vertically via analog).

I do know the IFD boxes do support +V – some previous threads.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I don’t understand how BARO VNAV works. A GPS generating “VNAV” of any sort should be using GPS altitude.

AFAIK Baro VNAV requires a source of Baro altitude, for example in form of GIA63/64 on G1000 or an ADC connected to, say GTN.

EGTR

arj1 wrote:

on Garmin you can drive a GPS “glideslope” (VNAV for LPV or +V) from a GTN and many autopilots support it.
But GTN could also do “enroute VNAV” where you put the VNAV constraints for the normal route and it should bring you down as needed.

So my question was just to clarify which case you meant in your statement.

The latter. Except that in this case you don’t put in the constraints yourself – they’re included in the procedure.

I wouldn’t call the former “VNAV” at all. It is a glideslope.

Last Edited by Airborne_Again at 22 Sep 15:25
ESKC (Uppsala/Sundbro), Sweden

NCYankee wrote:

This does not use SBAS for vertical guidance and is not connected to the vertical CDI

It is if you use a G5 (or other Garmin EHSI’s, I guess). But I also think it correct that the analog vertical deviation outputs from the GTN are not activated by a VNAV profile.

ESKC (Uppsala/Sundbro), Sweden

Peter wrote:

I don’t understand how BARO VNAV works. A GPS generating “VNAV” of any sort should be using GPS altitude.

My GTN can generate vertical paths for an approach using an ILS GS or a SBAS based GP, and enroute or terminal descents using a Baro-VNAV computer with altitude as an input. Some FMS can use Baro VNAV for approaches as well, but not my GTN.

Baro-VNAV is found mostly in FMS type systems, but now is available in G1000 and GTN installations. It is a computer based vertical path using a barometric altimeter input. The altitude is corrected to MSL by the altimeter setting. An along track position is used by the computer to determine the associated MSL altitude to maintain the GP and an appropriate VDI indication is displayed to the pilot for a final approach segment (if approaches are supported) or to an autopilot using a digital interface. In my aircraft with a GTN750/G500TXi/Stec 60-2, there are three GP type of indications to the left of the altitude tape on my G500TXi, a solid green diamond for ILS GS, a solid magenta diamond for LPV and +V vertical guidance, and a open magenta diamond for VNAV.using the Baro-VNAV. Only the solid diamonds are connected to the analog signals to the autopilot. I would need to replace the Stec 60-2 with an autopilot that supports the baro-VNAV guidance which is used for enroute and approach procedure descents up to the FAF. So I can hand fly the Baro-vnav vertical guidance on my PFD, but not use the autopilot to do these descents.

KUZA, United States

Ibra wrote:

Talking about GPS SBAS where indeed +V does not move in space for (unlike in Baro-Nav)

Assuming Garmin does check +V against obstacles in true GPS altitude, then flying +V all the way you will be clear of them but you may not have the required “baro MOC” depending on the days?

Does Garmin really check +V against obstacles in true GPS altitude? I had the impression obstacles in many databases are in baro altitude, does Garmin do a similar conversion (or temperature corrections) before checking that +V hits obstacles?

The VDA path for +V extends on a straight line from the FAF or a stepdown fix inside the FAF at the crossing altitude to the TCH above the threshold. The VDA choses the highest angle as long as it does not exceed the maximum angle permitted for the category of approach. Obstacles are only protected for from the FAF to the MDA and not below the MDA. The VDA for a given procedure is part of the database entry and is chosen by the procedure designer, not Garmin. The WAAS integrity limit for vertical 99.99999%, is +/- 50 meters. The latest versions of the WAAS TSO allow for advisory vertical guidance when not in an SBAS service volume. This is safe because any altitude is allowed on the FAC for an LNAV procedure down to the MDA which is still always determined by the altimeter indication and not the GPS height.

I agree with your statement regarding that the altimeter indication when following the SBAS VDA +V, may be above or below the charted minimum. Pilots are not authorized to follow the +V below the relevant crossing altitude indication depicted on the chart.

In most GPS, if there is an obstacle database, GPS altitude above the WGS84 datum (aka geometric altitude) is used in the database to specify the obstacle height and it is evaluated against the GPS geometric altitude. GPS altitude is going to be well within the system maximum specifications of +/- 33 meters 95% accuracy for uncorrected GPS and WAAS performance is better than +/- 3 meters. Baro altitude is another matter and varies greatly with temperature and at 10,000 MSL can vary by as much as +/- 1000 feet, so if you don’t want to hit the rocks, if you are using MSL, better have a very large buffer.

KUZA, United States

+V is Garmin stuff:

Why am I receiving a glide path on a GPS LNAV Lateral Navigation (LNAV+V) approach?

The glide path that is generated is based on the information that is used for the Localizer Performance with Vertical guidance (LPV) and Lateral Navigation / Vertical Navigation (LNAV/VNAV) procedures. It uses a specified decent angle that is centered at the MAWP (Missed Approach Waypoint) and at the TCH (Threshold Crossing Height). If this information is included in the electronic database for the approach, the 400W/500W series, G1000 with WAAS, and GTN 600/700 series will generate a glide path.

What are the criteria to “upgrade” an approach from LNAV to LNAV+V?

Most LNAV approaches are eligible for upgrade to LNAV+V. The criteria for being included in the database are:

It must be a GPS-approved approach.
There must be a non-zero vertical angle for the approach.
A valid geoid height for the runway must exist.
The approach does not contain LNAV/VNAV or LPV service levels.
Additionally, the unit must have a SBAS (WAAS, EGNOS, GAGAN or MSAS) signal of sufficient integrity. This means the GPS Status page must show “3D Diff Nav”.

If all these requirements are met, the Garmin navigator will automatically upgrade the approach from LNAV to LNAV+V.

always learning
LO__, Austria

+V is Garmin stuff

Avidyne too – see link above. The data must come from Jeppesen.

To summarise, would it be correct that the +V guidance would be available on any WAAS box (GNS-W, GTN, IFD) from IO716, but VNAV vertical guidance before that will be available only from Garmin and with a Garmin autopilot?

I wonder if @Steveavidyne knows anyone who knows?

Administrator
Shoreham EGKA, United Kingdom
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