GFC 500 GPS aiding

[PT20J]

The GFC 500 requires a GPS position input in order to track VHF navaids (VOR, LOC, ILS, BC).

 

I asked Garmin support if the G3X VFR GPS would supply the necessary GPS position so that I could use the autopilot to track VHF navaids should the GTN fail and Garmin said NO! So I tested it. I turned off the GTN and the autopilot flew an ILS and tracked to and from VOR radials. Next I disconnected the G3X GPS antenna and set up the GFC to track a VOR radial. It worked fine until I turned off the GTN and then the autopilot changed lateral modes from NAV to ROL. With neither G3X nor GTN GPS available neither NAV nor APR could be selected on the GFC. 

So, Garmin was wrong (again) and the GFC is able to use the G3X for GPS aiding. Note that in the event of an actual GPS outage, the autopilot will not track a VHF navaid and you will have to use HDG.

Skip

[jamesm]

  interesting   ... I had originally thought in my case 2 G5's GNC255 (nav/com) and GNC355(GPS/Com) and GFC500 AP, that there was some sort of GPS overlay GFC was following. So while on instrument rating journey a  on going battle, I had chance to see what would happen if I swap Nav Freq on GNC255 (Nav/Com) unit while AP was engaged while on ILS approach  (If I remember correctly the nav source was the GPS GNC 355) and it disconnected.. This was ILS KPWT 20. I was expecting that it wouldn't disconnect, in my setup. so it must have been using some signal from the GAD29B which is converting the GNC255 ARINC 429  to CAN signal which talks to the G5's. so much for my GPS overlay theory.

[Ragsf15e]

16 hours ago, PT20J said:

The GFC 500 requires a GPS position input in order to track VHF navaids (VOR, LOC, ILS, BC).

 

I asked Garmin support if the G3X VFR GPS would supply the necessary GPS position so that I could use the autopilot to track VHF navaids should the GTN fail and Garmin said NO! So I tested it. I turned off the GTN and the autopilot flew an ILS and tracked to and from VOR radials. Next I disconnected the G3X GPS antenna and set up the GFC to track a VOR radial. It worked fine until I turned off the GTN and then the autopilot changed lateral modes from NAV to ROL. With neither G3X nor GTN GPS available neither NAV nor APR could be selected on the GFC. 

So, Garmin was wrong (again) and the GFC is able to use the G3X for GPS aiding. Note that in the event of an actual GPS outage, the autopilot will not track a VHF navaid and you will have to use HDG.

Skip

Did you also have the internal g5 gps off?  Will it work on that one?  Probably if it works on the g3x.

[PT20J]

10 minutes ago, Ragsf15e said:

Did you also have the internal g5 gps off?  Will it work on that one?  Probably if it works on the g3x.

The G5 GPS isn’t active in my installation. But I’m pretty sure it will work since the GFC was originally designed to use the G5.

[carusoam]

14 hours ago, jamesm said:

  interesting   ... I had originally thought in my case 2 G5's GNC255 (nav/com) and GNC355(GPS/Com) and GFC500 AP, that there was some sort of GPS overlay GFC was following. So while on instrument rating journey a  on going battle, I had chance to see what would happen if I swap Nav Freq on GNC255 (Nav/Com) unit while AP was engaged while on ILS approach  (If I remember correctly the nav source was the GPS GNC 355) and it disconnected.. This was ILS KPWT 20. I was expecting that it wouldn't disconnect, in my setup. so it must have been using some signal from the GAD29B which is converting the GNC255 ARINC 429  to CAN signal which talks to the G5's. so much for my GPS overlay theory.

In most cases…

The ILS is a very close to the ground, and can be in an IMC situation…

Swapping a nav frequency during the approach would be…. unusual….?

Confused AP probably gives an aural warning as it drops out… beeep your plane.

 

Depending on how the trim condition is set… this could result in a faster dive or a climb if not manually controlled…

My KAP 150 uses the trim servo and attitude servo differently than the way I do when I fly the plane… cutting the AP out, often leaves the plane un trimmed by a small noticeable amount… 

 

PP thoughts only, not a CFI…

Best regards,

-a-

[jamesm]

1 hour ago, carusoam said:

The ILS is a very close to the ground, and can be in an IMC situation…

you are correct.  however I Had a theory that there was GPS overlay was really what the AP was following on approach. I could see it a GNS or GTN navigators. But since the GNC255 and GNC 355 physical separate units.Some how there is some smarts behind the scenes that I am unaware of.  again I was wrong in my assumptions. but by switching Nav vor/ils frequency it shows me that it was more complex then what I had originally  envisioned.  The Approach was in VMC conditions and had my instructor on board. for the reasons that you had stated i would never even think of trying this in IMC conditions. I have hard enough time focusing on approach step down altitudes even know this fairly simple straight in approach.

[aviatoreb]

So what happens if it looses sufficient signals?  Does the GFC500 just go belly up shut down and refuse to do anything?  Or will it still give you wings level and perhaps heading mode?

[rbp]

How do you activate the G3X GPS? It doesn’t show up in my list of navigation sources

[PT20J]

1 hour ago, aviatoreb said:

So what happens if it looses sufficient signals?  Does the GFC500 just go belly up shut down and refuse to do anything?  Or will it still give you wings level and perhaps heading mode?

The GFC 500 operates normally without GPS in ROL or HDG lateral modes. You just cannot use NAV or APR which means that it will not fly an instrument approach.

[PT20J]

21 minutes ago, rbp said:

How do you activate the G3X GPS? It doesn’t show up in my list of navigation sources

The G3X GPS is always running in the background and the G3X and GFC 500 will revert to it automatically should the external navigator (e.g. GTN) fail. It is a non-WAAS VFR GPS, so it won't do instrument full approaches, but you can still fly a VOR or ILS approach using a VHF navigation receiver. You can force the G3X to use the internal GPS by touching the HSI which brings up a menu and then selecting the FPL Source to Internal. 

Skip

[PT20J]

7 hours ago, jamesm said:

you are correct.  however I Had a theory that there was GPS overlay was really what the AP was following on approach. I could see it a GNS or GTN navigators. But since the GNC255 and GNC 355 physical separate units.Some how there is some smarts behind the scenes that I am unaware of.  again I was wrong in my assumptions. but by switching Nav vor/ils frequency it shows me that it was more complex then what I had originally  envisioned.  The Approach was in VMC conditions and had my instructor on board. for the reasons that you had stated i would never even think of trying this in IMC conditions. I have hard enough time focusing on approach step down altitudes even know this fairly simple straight in approach.

I believe it is required that the navigation source (which is driving the autopilot) match the navaid listed in the approach name. A VOR approach would thus have to use a VOR for navigation unless it was listed as (or GPS). I believe it's in one of the manuals that the GFC 500 will revert to ROL if the navigation source is changed. 

But, good for you to try stuff out! That's the best way to really learn how these systems work. Most instructors only know the normal modes. The Garmin documentation isn't that great, is sometimes incorrect, and it is written to describe the operation each individual unit rather than describe the interactions of all the possible configurations. 

Skip

[Ulysse]

1 hour ago, PT20J said:

non-WAAS VFR GPS

I always wondered what was the difference between a VFR and IFR gps.

[carusoam]

14 minutes ago, Ulysse said:

I always wondered what was the difference between a VFR and IFR gps.

Somewhere in the early 90s…

GPS was initially accurate enough to navigate in cruise flight…

To be approved for IFR navigation it included approaches in its database…

and the ability to update its database on a near monthly basis…

The early BK KLN88 and 89B required the box to be removed from the instrument panel to swap out a memory cartridge monthly… leading to operator errors prior to flying in IMC….  After a while, the operating systems were updated to allow for a more durable upload mechanism…

It was mostly computer technology of the time… where processor speeds were barely keeping up… Ram was minimal… and updates were challenging to do…

This was 2D navigation only… accuracy was about 100 yards… with updates about every second or so…

For 3D navigation accuracy… WAAS is required…. Accuracy is about 300 inches… or better… with updates at a much higher rate…

Oddly… Portable GPSi of the time were using WAAS accuracy, with their 1 second updates… nice, but too slow to keep the clean side up in IMC bumps….

In the early days of GPS… VFR only was a low cost method of getting the new technology…  IFR GPS was ridiculously expensive as a full on GTN is today…

PP thoughts only, not a tech geek historian….

Best regards,

-a-

[donkaye]

4 hours ago, PT20J said:

I believe it is required that the navigation source (which is driving the autopilot) match the navaid listed in the approach name. A VOR approach would thus have to use a VOR for navigation unless it was listed as (or GPS). I believe it's in one of the manuals that the GFC 500 will revert to ROL if the navigation source is changed. 

Per AIM Section 1-2-3 you can fly the final approach segment of a VOR approach with GPS under the following conditions summarized by John Collins, one of the most knowledgeable avionics persons in the Country.

"To fly a VOR approach that does not include (or GPS) in the title using a GPS for navigation, the VOR must be in service, the approach must not be NOTAM out of service, the VOR must be tuned on a working and installed VOR receiver in the aircraft, the course set and the VOR CDI indications must be monitored for course guidance".  This would include a Bearing Pointer as well for the monitored indicator.

Vor approach with Gps.pdf

[PT20J]

Don is correct, of course. I was sloppy in my language. What I meant was that if you have the CDI selected to a VHF navigator, the autopilot must use that as the primary navigation source even though it may have GPS information available. 

Skip

[PT20J]

9 hours ago, Ulysse said:

I always wondered what was the difference between a VFR and IFR gps.

The IFR navigators must be TSO’d.  The technical difference is usually in the monitoring and alarm functions. For instance, a VFR GPS probably won’t have RAIM. 

[donkaye]

1 hour ago, PT20J said:

Don is correct, of course. I was sloppy in my language. What I meant was that if you have the CDI selected to a VHF navigator, the autopilot must use that as the primary navigation source even though it may have GPS information available. 

Skip

For even more clarity from my posting above, if you have an IFR GPS, then it can be used as the primary Navigation source from the FAF to the Missed Approach Point, if the underlying navaid is operational and monitored, even if a Bearing pointer is the monitored source.

[rbp]

According to the IAP database inventory  (https://www.faa.gov/air_traffic/flight_info/aeronav/procedures/ifp_inventory_summary/)  there are 6,462 GPS approaches and 1,352 VOR and VOR/DME approaches. 

[donkaye]

One other thing; in the almost 3 years that I have had my GFC 500, there has never been a time when it failed during an ILS approach.  Even if it had, the approach could have been completed by flying the approach by hand.

[Andy95W]

Why not just practice flying an occasional ILS using HDG and V/S mode?  Should be pretty easy and well within the skill set of every instrument rated pilot.

Edited April 4, 2022 by Andy95W

[PT20J]

7 hours ago, Andy95W said:

Why not just practice flying an occasional ILS using HDG and V/S mode?  Should be pretty easy and well within the skill set of every instrument rated pilot.

 

7 hours ago, donkaye said:

One other thing; in the almost 3 years that I have had my GFC 500, there has never been a time when it failed during an ILS approach.  Even if it had, the approach could have been completed by flying the approach by hand.

I would do as Don suggested. It seems to me that manually adjusting the autopilot in vertical and lateral modes is akin to hand flying, but more difficult due to the inherent time lag between input and execution. 

I believe it is important to keep our hand flying skills sharp as that is always the final fallback when all else fails. This is not easy when the modern automation makes it so easy to punch a few buttons and sit back and watch. Who wants to go back to flying NDB approaches and DME arcs by hand without GPS guidance?

BTW, I’m not suggesting that there is anything wrong with the GFC 500 design. However, it is important to understand how these things work so you know what to expect. I’ll bet many pilots would assume that the autopilot would fly an ILS if there were a GPS outage and would be surprised to find that it will not.

My second point is that you cannot always believe what Garmin support tells you because their knowledge, beyond what is in the manuals, is not deep. There are simply too many products for the support staff to know the deep dark details of each, especially when the software is so frequently updated.

Skip