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RNAV approaches - GS and minima

It seems this is because you don’t have a proper signal with a good intergrity.

I tried this in the GTN simulator, with a proper 3D fix including SBAS, it shows LNAV+V

If I do the same in the simulator without a 3D fix including SBAS, it shows only LNAV

If you also have this during flight, be sure to check that the EGNOS provider is enabled. Without EGNOS enabled you don’t have SBAS, which results in the same behavour as you describe.

You can find this setting under Home → System → GPS Status → SBAS

Last Edited by Jesse at 29 Dec 22:28
JP-Avionics
EHMZ

@NCYankee I will get back with which SW version the GTN650 has.

I can understand why the GTN won’t show LNAV+V or L/VNAV when there is bad satellite signal reception. LOI – Loss of Integrity

But how come LPV is shown during LOI? That is totally inconsistent.

ESTL

Your main software version is 5.13, which is the latest release for main software.

For LPV only SBAS (In this case EGNOS) enablement is being checked.

ESGP without any fix and with EGNOS disabled shows LNAV:

ESGP without any fix, but EGNOS enabled shows LPV:

I am unsure what the actual reason for this, so for LPV they don’t require a fix, only SBAS enablement
For LNAV+V they require an 3D differential fix (including SBAS), else it will show LNAV only.

JP-Avionics
EHMZ

I have asked Garmin about this.

They indicate that that this is due to an LPV being an official coded and approved approach, which is always expected to be available, and is only downgraded when the horizontal and/or horizontal limits are not met 60 seconds prior to FAF.

They also indicated that the +V is not part of the official procedure and is only be expected during good GPS reception, hence the removal of it from the menu anytime you don’t have good GPS fix.

JP-Avionics
EHMZ

Thanks. It is clear for LPV and LNAV+V.

What about LNAV/VNAV? Does it also behave like LPV?
It is also, sort of, officially coded.
Or does it behave like LNAV+V and good signal reception is needed for it to show?

ESTL

Really interesting especially as both LPV and +V require EGNOS…

I wonder what is behind this…

I did ask previously how an airport monitors the LPV status.

Administrator
Shoreham EGKA, United Kingdom

Jesse wrote:

They also indicated that the +V is not part of the official procedure and is only be expected during good GPS reception, hence the removal of it from the menu anytime you don’t have good GPS fix.

Is this a CYA thing on Garmin’s part? Most LNAV approaches will be flown using the CDFA technique and in that case surely an advisory glideslope which is a bit off due to a less-than-good GPS fix will still be better than what the pilot can achieve by cross-referencing altitudes with distances? (The same question for not getting +V on a non-SBAS unit.)

ESKC (Uppsala/Sundbro), Sweden

Peter wrote:

I did ask previously how an airport monitors the LPV status.

I don’t think they do (unless they have a GBAS installation).

Eurocontrol Augur does monitor RAIM and issue NOTAMs, so it’s quite possible that they do it also for LPV, or maybe EGNOS does it…

LSZK, Switzerland

Peter wrote:

Really interesting especially as both LPV and +V require EGNOS…

I wonder what is behind this…

I did ask previously how an airport monitors the LPV status.

I seriously doubt that an airport monitors LPV status, although there is no reason they could not. At least in the US, they would not be the source of a NOTAM indicating a WAAS outage

The TSO C146c and the referenced portions of RTCA DO-229D determine the operational characteristics of the GTN series. SBAS vertical guidance is only provided when a suitable VPL value is determined to be at or below the VAL (Vertical Alarm Limit) for the type of approach. The SBAS system provides the VPL data. There are only two types of approaches that are considered as SBAS, LPV and LP. LNAV/VNAV is not an SBAS approach type, but the SBAS vertical guidance may be used in lieu of the Baro/VNAV vertical guidance. Advisory vertical guidance (+V) may be provided on LNAV or LP procedure types. The VAL for LPV (DH >=250 feet), LNAV/VNAV, and +V is 50 meters. The VAL for LPV (DH<250 feet) is 35 meters.

If operating outside of an SBAS service volume or there is an SBAS outage, then SBAS vertical guidance is not available. If the VPL exceeds the VAL, then the approach will downgrade to LNAV and there will not be any advisory vertical guidance. On the GTN, one can get a good indication if they should expect vertical guidance by examining the dynamic value of VFOM (Vertical Figure of Merit) which is displayed on the GPS status page. This value is in feet and in most cases can be related to the value of VPL. A VPL of 50 corresponds roughly to a VFOM of 60 feet. A VPL of 35 corresponds roughly to a VFOM of 40 feet. Another advance warning of a likely downgrade to LNAV is the color of the annunciation for LPV, LNAV/VNAV, LP, LP+V, or LNAV+V. If it is green, then the current value of VPL is satisfactory for vertical guidance. If it is amber (yellow), then the current value of VPL is below VAL and a downgrade is likely. The downgrade, if it occurs will be at 1 minute to the FAF.

KUZA, United States

Today I was back at the aircraft to look at RNP approaches again. This time with the aircraft out on the apron… No “Loss of Integrity” LOI indication that messes things up.

First I got really weird results. The approaches into ESGP only showed LNAV even though there are LPV available.

It turned out that someone had thought it was a good idea to turn off WAAS, EGNOS and MSAS.

After turning them back on.

LPV looked like it should.

Then it was time to check what EKRK would look like. I expected LNAV+V. But it only showed LNAV.

And for EDDE I expected LNAV/VNAV but it showed LNAV.

This is what the GPS status page looked like.

Is it so that I would actually get LNAV+V when flying into EKRK but that I need to be closer to the EKRK airport for the LNAV+V to show?

ESTL
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