RNAV LNAV/VNAV , Garmin GNX375 , GFC500 Autopilot
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RNAV LNAV/VNAV , Garmin GNX375 , GFC500 Autopilot
Can someone provide the definitive answer to the below questions?
When carrying out an approach with the above listed equipment, the autopilot has never been able to fly the LNAV/VNAV approach. There has never been an issue with LNAV only approaches.
1. Why won’t the autopilot capture and fly the LNAV/VNAV approach?
From what I can glean, the LNAV approach with +V (advisory glide path) is based on a database model.
2. Why won’t the LNAV/VNAV approach provide guidance to the autopilot?
When carrying out an approach with the above listed equipment, the autopilot has never been able to fly the LNAV/VNAV approach. There has never been an issue with LNAV only approaches.
1. Why won’t the autopilot capture and fly the LNAV/VNAV approach?
From what I can glean, the LNAV approach with +V (advisory glide path) is based on a database model.
2. Why won’t the LNAV/VNAV approach provide guidance to the autopilot?
Can someone provide the definitive answer to the below questions?
When carrying out an approach with the above listed equipment, the autopilot has never been able to fly the LNAV/VNAV approach. There has never been an issue with LNAV only approaches.
1. Why won’t the autopilot capture and fly the LNAV/VNAV approach?
From what I can glean, the LNAV approach with +V (advisory glide path) is based on a database model.
2. Why won’t the LNAV/VNAV approach provide guidance to the autopilot?
When carrying out an approach with the above listed equipment, the autopilot has never been able to fly the LNAV/VNAV approach. There has never been an issue with LNAV only approaches.
1. Why won’t the autopilot capture and fly the LNAV/VNAV approach?
From what I can glean, the LNAV approach with +V (advisory glide path) is based on a database model.
2. Why won’t the LNAV/VNAV approach provide guidance to the autopilot?
It is common for navigation/autopilot interfaces to be limited by regulations to approved-only functions.
Last edited by Mach E Avelli; 11th Jun 2023 at 11:02.
Paul,
1. Because unless you have a certified Baro-VNAV system, you aren't flying an LNAV/VNAV approach. As has been indicated already, you are flying an LNAV +V approach.
I'll try and explain simply why you are getting advisory and not guidance. So procedures are coded in ARINC424, which includes altitudes at certain fixes. The Garmin boxes do know what altitudes to display as they fly the approach. The problem is that, without a certified BaroVNAV system, the Garmin only knows its height reference to the WGS 84 ellipsoid and not AMSL. The difference between these two datums varies across the country but generally the WGS84 ellipsoid is ~70m below AMSL in the SW of Aus and ~70m above in the NE . If you want to nerd out you can read more about this here, which explains the rough idea as it applies to GDA20 https://www.icsm.gov.au/upgrades-aus...ference-system
So in a nutshell, the Garmin navigator (on its own) doesn't use AMSL as its reference datum for altimetry. The difference between WGS84 ellipsod and AMSL can be calculated, and I am aware that Garmin use a model to do these calculations on the fly. This eliminates this error, however this functionality is not part of the altimetry certification of the navigator and so can only be used as advisory. This is why most people are not observing ~150ft errors when using these systems.
The introduction of an SBAS system will also remove the altimetry errors, and as SBAS a certified system you will be able to fly an LNAV/VNAV as guidance (and coupled). Further the WGS84 ellipsod - AMSL difference can be hard coded for the threshold you are flying the approach to and a geometric 3d vertical path can be generated. These types of approaches, will be known as LPV.
Alpha
1. Because unless you have a certified Baro-VNAV system, you aren't flying an LNAV/VNAV approach. As has been indicated already, you are flying an LNAV +V approach.
I'll try and explain simply why you are getting advisory and not guidance. So procedures are coded in ARINC424, which includes altitudes at certain fixes. The Garmin boxes do know what altitudes to display as they fly the approach. The problem is that, without a certified BaroVNAV system, the Garmin only knows its height reference to the WGS 84 ellipsoid and not AMSL. The difference between these two datums varies across the country but generally the WGS84 ellipsoid is ~70m below AMSL in the SW of Aus and ~70m above in the NE . If you want to nerd out you can read more about this here, which explains the rough idea as it applies to GDA20 https://www.icsm.gov.au/upgrades-aus...ference-system
So in a nutshell, the Garmin navigator (on its own) doesn't use AMSL as its reference datum for altimetry. The difference between WGS84 ellipsod and AMSL can be calculated, and I am aware that Garmin use a model to do these calculations on the fly. This eliminates this error, however this functionality is not part of the altimetry certification of the navigator and so can only be used as advisory. This is why most people are not observing ~150ft errors when using these systems.
The introduction of an SBAS system will also remove the altimetry errors, and as SBAS a certified system you will be able to fly an LNAV/VNAV as guidance (and coupled). Further the WGS84 ellipsod - AMSL difference can be hard coded for the threshold you are flying the approach to and a geometric 3d vertical path can be generated. These types of approaches, will be known as LPV.
Alpha
The following 8 users liked this post by alphacentauri:
Thanks Alphacentauri, your info makes sense.
However Aus AIP lump LNAV and LNAV/VNAV on the same plate and I suspect the Garmin database only has one option when loading approaches. Loading an LNAV (only) approach in the OP system will allow an advisory glidepath to be coupled. If you load an approach that is annotated with both LNAV and LNAV/VNAV, the system complains ..."advisory only" and does not even show the GP, let alone allow you to couple up to it.
Bring on LPV! And why not NOW ..using a known system rather than a "we can do it better than anyone else" system ....but take 5 years to design it. Hopefully it won't need an update to receivers to use it...Remember DMEA anyone?
However Aus AIP lump LNAV and LNAV/VNAV on the same plate and I suspect the Garmin database only has one option when loading approaches. Loading an LNAV (only) approach in the OP system will allow an advisory glidepath to be coupled. If you load an approach that is annotated with both LNAV and LNAV/VNAV, the system complains ..."advisory only" and does not even show the GP, let alone allow you to couple up to it.
Bring on LPV! And why not NOW ..using a known system rather than a "we can do it better than anyone else" system ....but take 5 years to design it. Hopefully it won't need an update to receivers to use it...Remember DMEA anyone?
Thanks Alphacentauri, your info makes sense.
However Aus AIP lump LNAV and LNAV/VNAV on the same plate and I suspect the Garmin database only has one option when loading approaches. Loading an LNAV (only) approach in the OP system will allow an advisory glidepath to be coupled. If you load an approach that is annotated with both LNAV and LNAV/VNAV, the system complains ..."advisory only" and does not even show the GP, let alone allow you to couple up to it.
Bring on LPV! And why not NOW ..using a known system rather than a "we can do it better than anyone else" system ....but take 5 years to design it. Hopefully it won't need an update to receivers to use it...Remember DMEA anyone?
However Aus AIP lump LNAV and LNAV/VNAV on the same plate and I suspect the Garmin database only has one option when loading approaches. Loading an LNAV (only) approach in the OP system will allow an advisory glidepath to be coupled. If you load an approach that is annotated with both LNAV and LNAV/VNAV, the system complains ..."advisory only" and does not even show the GP, let alone allow you to couple up to it.
Bring on LPV! And why not NOW ..using a known system rather than a "we can do it better than anyone else" system ....but take 5 years to design it. Hopefully it won't need an update to receivers to use it...Remember DMEA anyone?
Agent86,
Its not the lines of minima that determine the approach. You are simply loading an RNP (RNAV GNSS) approach.
What gets displayed is a function of the box and the what it reads from the coding. My experience with LNAV/VNAV is that it is shown as LNAV +V, you get an advisory GP but you cannot couple to it.
Alpha
Its not the lines of minima that determine the approach. You are simply loading an RNP (RNAV GNSS) approach.
What gets displayed is a function of the box and the what it reads from the coding. My experience with LNAV/VNAV is that it is shown as LNAV +V, you get an advisory GP but you cannot couple to it.
Alpha
Originally Posted by AerocatS2A
LPV is space-based (SBAS); in the US, using the WAAS.
AlphaCentauri
If a SSM in the south west of WA had a height of 246m (AHD71) and the derived ellipsoidal height of 216m (N=30m), would that not mean the ellipsoid is above amsl?
If a SSM in the south west of WA had a height of 246m (AHD71) and the derived ellipsoidal height of 216m (N=30m), would that not mean the ellipsoid is above amsl?
The GNX 375 is the same GPS navigator as the GPS 175. The GPS 175 is LPV capable and I fly coupled LPV approaches in my experimental aircraft (G3X Touch with GPS 175 and GSA 28 servos). If GPS integrity does not support LPV I am automatically degraded to LNAV and the vertical deviation scale is removed from the PFD.
The Pilots Guide (Garmin doc 190-02488-01 Rev. B) describes LNAV/VNAV, LNAV+V, LP, LNAV, LPV, and LP+V RNAV approaches.
The AFM Supplement (Garmin doc 190-02207-A3 Rev. 3) is tailored by the installer for the capabilities of the aircraft. Section 4.5 "Coupling the Autopilot during approaches" provides a description of several different configuration options. (Garmin restricts distribution of the installation manual and the AFM supplement but they should be provided to the equipment installer. Unoffical copies can be found.)
AFM Section 2.12 "Autopilot Coupling" includes this configuration option that may be "checked" by the installer - "Lateral coupling only for GPS approaches. Coupling to the vertical path for GPS approaches is not authorized."
I found that Garmin's documentation on individual systems is very good. However, I found the description of how the GPS 175 interacted with the G3X Touch system to be inadequate. I ended up creating my own AFM supplement when I installed the GPS 175 in my aircraft.
The Pilots Guide (Garmin doc 190-02488-01 Rev. B) describes LNAV/VNAV, LNAV+V, LP, LNAV, LPV, and LP+V RNAV approaches.
The AFM Supplement (Garmin doc 190-02207-A3 Rev. 3) is tailored by the installer for the capabilities of the aircraft. Section 4.5 "Coupling the Autopilot during approaches" provides a description of several different configuration options. (Garmin restricts distribution of the installation manual and the AFM supplement but they should be provided to the equipment installer. Unoffical copies can be found.)
AFM Section 2.12 "Autopilot Coupling" includes this configuration option that may be "checked" by the installer - "Lateral coupling only for GPS approaches. Coupling to the vertical path for GPS approaches is not authorized."
I found that Garmin's documentation on individual systems is very good. However, I found the description of how the GPS 175 interacted with the G3X Touch system to be inadequate. I ended up creating my own AFM supplement when I installed the GPS 175 in my aircraft.
Last edited by EXDAC; 17th Jun 2023 at 19:20. Reason: Correct list of RNAV types described in Pilot Guide. Revised AFM comments.
