GFC-500 is a GPS only Autopilot

[NordicDave]

I'm getting ready for IFR testing and wanted to specifically understand EXACTLY what happens in various failure modes for my complete Garmin panel. G5/G3X/650Xi/GTX-345/GFC-500

The GFC-500 Supplemental flight manual says any failure of GPS, even if navigating by ground based nav goes to PIT & ROL mode. Concerned about this, I wrote Garmin support and got a definitive answer.

Here's my question and their answer:

Dear Garmin,

After seeing a Garmin GFC-500 webinar and reading the AFMS section 3-6 https://static.garmin.com/pumac/190-02291-06_10.pdf
I’m unclear what happens while flying using only ground-based navigation driving the GFC-500 and GPS loses signal.

For context my panel is a G3X, G5 back-up, GTN-650Xi, GTX-345, and GFC-500 in a 182P.

According to the way I’m reading the AFMS; a GFC-500 in Nav mode using a ground based navigation source like a VOR via the GTN-650Xi, and the GPS system is lost, the GFC-500 will STOP taking this navigation guidance and revert to ROL mode where the pilot MUST manually adjust GFC-500 heading manually mimicking the VOR or fly manually.

If this is true, why would the autopilot drop a PERFECTLY good ground based nav source if GPS signal coincidently just happens to also fail?

Also seems alarming while tracking an ILS glide slope with GTN-650Xi and GPS signal is lost, the GFC-500 goes into PIT mode and the pilot must manually fly the missed approach. I can hardly believe this is true, which makes the GFC-500 a GPS only device. Completely illogical and need clarification.

Thank you, and looking forward to clarification.

-David, Garmin Customer


Garmin Reply:

Hi David,

It is correct that the GFC 500 will not remain in LOC/VOR and GS modes in the event of a complete GPS signal loss and the autopilot would revert to ROL and PIT. However, you will still be able to put it into HDG mode, as well as to climb/descend/maintain altitude using IAS, VS and ALT modes.

The reason for this is that Garmin Autopilots use GPS aiding to improve accuracy and smoothness when tracking a ground-based navaid. Signal from ground-based navaid is inherently noisy, leading to spikes in your CDI. To prevent the autopilot from chasing the CDI needle in a reactive manner leading to unnecessary oscillations, Garmin Autopilots use GPS position as a cross-reference to ensure that it can track the needle more smoothly. For some of our other autopilots, such as the GFC 600, extensive tuning and certification work, on a per-airframe basis, is done to allow the autopilot to continue to track ground-based navaid when GPS signal is lost, similar to how autopilots before the GPS era would function, as a backup method. This, however, is not available for the GFC 500 in order to keep the price of this autopilot low. By taking advantage of GPS aiding, we are able to reduce the amount of tuning and testing required for each individual airframe and offer a modern autopilot to a wider range of aircraft types at a lower cost that would otherwise not be possible. If the ability to track ground-based navaid without GPS aiding is desired, the GFC 600 autopilot is also certified for the Cessna 182P.

Hope this helps, and please let me know if you have any additional questions.

Sincerely,
Chun Hei Ng
Aviation Pilot Instructor
Garmin
1200 E. 151st Street | Olathe, KS 66062, USA |
aviation.training@garmin.com

http://welch.com/n46pg/

 

[Martin Pauly]

Your finding is accurate, David. This is well documented in Garmin's AFMS and has been widely discussed in online forums, including here on PoA.

Note that the GFC500 is capable of flying HDG mode, as well as VS and IAS. That can be a big help in the rare (at least in the US) situation that one encounters a GPS outage. You are not limited to PIT and ROL.

Furthermore, it's worth mentioning that Garmin's GFC600 does not have this limitation. While it loses the benefit of GPS-aiding in case of a GPS outage, it can continue to track a localizer, glideslope, or VOR radial.

Regards,
Martin

 

[flyingcheesehead]

They make a sensible explanation... And for the price difference between the GFC500 and GFC600, I'd be OK with using non-nav modes or hand flying in the incredibly rare circumstance of a complete GPS failure.

The GFC500 is both far cheaper and far more capable than anything that came before it except the DFC90, which was (I think) only available for older Cirruses that had the Avidyne flight displays and S-TEC autopilot.

 

[asicer]

Note that the GFC500 is capable of flying HDG mode, as well as VS and IAS. That can be a big help in the rare (at least in the US) situation that one encounters a GPS outage. You are not limited to PIT and ROL.

As I recall, it would have to be a widespread GPS outage. The G5 has an internal GPS that if configured correctly can be used as the necessary cross reference, if I'm not misremembering things.

 

[asicer]

Also, I think "GPS Only" is a misnomer. "GPS Aided" or "GPS Reliant" is more apt. The GFC500 uses GPS position as a reference, not GPS DTK. For example, if a brand new VOR or LOC popped up but the GPS database hadn't been updated yet, I believe the GFC500 could still track it. That plus PIT/ROL/HDG/VS/IAS modes still being available means it's more than "GPS only".

 

[texasclouds]

Oh wow, that would be quite the automation surprise in certain scenarios. However, GPS signal loss has been quite rare in my experience.

 

[Jim K]

While it's an important limitation to be aware of, it's one I happily accept considering the cost (and in my case, lack of approval) of the gfc600, vs the likelihood of a GPS outage. My old autopilot only worked in heading mode anyway lol. It's pretty easy to fly an ILS in heading/VS mode.

 

[midlifeflyer]

Oh wow, that would be quite the automation surprise in certain scenarios. However, GPS signal loss has been quite rare in my experience.

I should not be a surprise. When I work with someone after a new avionics installation, I ask the pilot to read through the AFMS. It's not about a quiz. You want to incorporate any new procedures into your checklists - the FAA has a good video about the importance of doing that. And while there will be things that are familiar, especially if you have flown other autopilots, there will be differences you want to understand. At least read through the emergency and abnormal procedures to highlight for yourself things which may be different than you expect, especially in terms of failure events.

The GFC500 AFMS is only about 40 pages long with lots of white space to its a quick read. And, looking at the Abnormal Procedures section, you might well say, "Well. Duh!" to the information that a red X in the autopilot scoreboard means the AP failed it's prefight test (well, duh!), the "NOTE" in this one should grab your attention even on a casual read through.

 

[NordicDave]

As OP, wanted to share this post as so many are not aware of the limitation. My opinion, Garmin is deliberately not as clear as they could be in promoting the GFC-500's nav sources. In the Garmin GFC-500 webinar I attended they talk about tracking green needles with the this A/P and should have said it will track green needles only if the GPS system is working correctly.

I have this A/P in my plane and it's nothing short of amazing performance. The only criticism; is the company could be more upfront and be more crisp with the rather obscure limitations language in the AFSM section 3.6

Example: Loss of GPS Information; SELECT different lateral or and/or vertical mode (as necessary)

OK, I'll pick a new lateral mode by selecting a ground based nav source, but can't as only heading and pitch modes are available. They should be very clear by stating ground based navigation is not an option.

 

[TCABM]

As OP thought it good to make this post as so many are not aware of the limitation. My opinion, Garmin is deliberately not as clear as they could be in promoting the GFC-500's nav sources. In the Garmin GFC-500 webinar I attended they talk about tracking green needles with the this A/P and should have said it will track green needles only if the GPS system is working correctly.

Considering you can’t hook up a stand alone GFC500 to a King Silver Crown panel, I kind of felt it was obvious a GPS source was required. It’s not billboarded, but the pressers I’ve seen have been clear on the required configurations.

Garmin receives additional GFC 500 autopilot certifications - Garmin Newsroom

OLATHE, Kan./Sept. 13, 2023 – Garmin (NYSE: GRMN) today announced it has received Federal Aviation Administration (FAA) Supplemental Type Certification (STC) for the GFC™ 500 digital autopilot in additional aircraft to include the Cessna 177/177A Cardinal and the Beechcraft T34A/T34B Series. The...

www.garmin.com

I’m fairly certain every digital autopilot requires a GPS for the STC and has limitations in the event a suitable GPS signal is lost.

 

[midlifeflyer]

Example: Loss of GPS Information; SELECT different lateral or and/or vertical mode (as necessary)

OK, I'll pick a new lateral mode by selecting a ground based nav source

I guess the manual requires us to understand what an autopilot “mode” is and that, for example, NAV mode is NAV mode regardless of the source of the NAV information. A different lateral mode than NAV is HDG or ROL.

 

[Hang 4]

Example: Loss of GPS Information; SELECT different lateral or and/or vertical mode (as necessary)

OK, I'll pick a new lateral mode by selecting a ground based nav source, but can't as only heading and pitch modes are available. They should be very clear by stating ground based navigation is not an option.

Not correct. Heading mode and vertical modes (IAS, VS) work without GPS.

 

[NordicDave]

@Hang 4 Of course Heading mode and vertical modes work without GPS and I said so in the post. What does not work is ground based nav sources which was the point of the post.

Re-quoting the post:

..OK, I'll pick a new lateral mode by selecting a ground based nav source, but can't as only heading and pitch modes are available. They should be very clear by stating ground based navigation is not an option.

 

[Hang 4]

@NordicDave - You're right. I misread your post as many have said it reverts to pitch and roll mode only, I read too quickly.
Having said that, you can still fly your green needles using those modes. The G5 or G3X will still show the indication. Not that big of a deal to turn the knob to align with the course and use VS to go down a glideslope. That's what you give up for the price. The GFC600 has the capability to "smooth out" some of the noise associated with following a localizer or VOR. The 500 uses the GPS to accomplish the same thing.

 

[morleyz]

Example: Loss of GPS Information; SELECT different lateral or and/or vertical mode (as necessary)

OK, I'll pick a new lateral mode by selecting a ground based nav source, but can't as only heading and pitch modes are available. They should be very clear by stating ground based navigation is not an option.

But...you can't actually do that with the GFC-500. Garmin is pretty clear that with the loss of GPS information, the NAV button goes away as an option. Maybe there's a different version yours has, but mine doesn't not have a VOR lateral mode button. If I want to use ground based navigation, I have to change the output from the GPS unit. My assumption (which could be wrong) is that the GPS unit actually stops outputting NAV information upon the loss of GPS, so the GFC isn't getting any NAV data.

This was also explained to me by the avionics shop that did the install and the reason I kept my analog CDI hooked up to my 2nd NAV/COM, because the GFC-500 can't even use a standalone ground based signal. Yes, we could have fed the 2nd NAV/COM into the GPS unit, but if GPS signal was lost, it wouldn't run the GFC-500 anyway.

I never felt like this was obfuscated by Garmin (or the installer for that matter).

 

[mandm]

I think you’re more likely to have an issue with electrical than GPS going out. At least you can still maintain a heading and altitude which is key. But if you have a 650 or 750, I imagine you can still see the lateral guidance and vertical guidance on a LOC and ILS?

 

[midlifeflyer]

I think you’re more likely to have an issue with electrical than GPS going out. At least you can still maintain a heading and altitude which is key. But if you have a 650 or 750, I imagine you can still see the lateral guidance and vertical guidance on a LOC and ILS?

Yes, you will see both lateral and vertical guidance on your CDI/HSI. It's an autopilot automatic tracking issue, not a GPS navigator issue. You can even use the autopilot's HDG/VS/IAS modes. The limitation is that NAV/APR won't be available to follow either automatically.

 

[Groundpounder]

Or you could hand fly the approach......

 

[midlifeflyer]

Or you could hand fly the approach......

No. That's not allowed.

 

[Martin Pauly]

This was also explained to me by the avionics shop that did the install and the reason I kept my analog CDI hooked up to my 2nd NAV/COM, because the GFC-500 can't even use a standalone ground based signal. Yes, we could have fed the 2nd NAV/COM into the GPS unit, but if GPS signal was lost, it wouldn't run the GFC-500 anyway.

How does the analog CDI help? The display of your #1 VOR or ILS on the primary HSI (e.g. G5r or G3X) is not the issue; just the autopilot's ability to steer from those signals.
Unless of course you want a backup for your primary HSI, but that's a different story...

- Martin

 

[morleyz]

How does the analog CDI help? The display of your #1 VOR or ILS on the primary HSI (e.g. G5r or G3X) is not the issue; just the autopilot's ability to steer from those signals.
Unless of course you want a backup for your primary HSI, but that's a different story...

- Martin

My point was just that without the GPS, the GFC-500 does not have the capability to navigate from ground based radio navigation of any source type.

In the conversation with the avionics shop, we decided to just leave the analog CDI hooked to the NAV/COM2 vs. changing to send it into the GPS and removing the CDI, displaying both sources on the G5 HSI. The primary reasoning was that I only have 1 GPS unit, so if it had a complete failure, my understanding is that the G5 HSI wouldn't get either of my two radio NAV signals, taking out the ability to navigate by GPS and VOR with a single point of failure. So, yes it's a backup for a total GPS unit failure vs. the HSI. If I had a 2nd GPS, I probably would have made a different decision.

 

[Hang 4]

The primary reasoning was that I only have 1 GPS unit, so if it had a complete failure, my understanding is that the G5 HSI wouldn't get either of my two radio NAV signals, taking out the ability to navigate by GPS and VOR with a single point of failure.

I don't believe that's correct. The G5 will display green needles with or without GPS from a NAV source. The GFC500 uses GPS to augment ground based sources to smooth out the inputs of VOR/ILS, not for primary navigation. As long as there is a good nav input to the G5, it will give you valid course indications, it just won't drive the AP using those inputs.

 

[morleyz]

I don't believe that's correct. The G5 will display green needles with or without GPS from a NAV source. The GFC500 uses GPS to augment ground based sources to smooth out the inputs of VOR/ILS, not for primary navigation. As long as there is a good nav input to the G5, it will give you valid course indications, it just won't drive the AP using those inputs.

I'm not the expert here. What I understood was that I could drive the GFC-500 from the NAV/COM2 unit (a standalone radio/nav unit whose model escapes me) by routing the signal to the GPS unit and tossing the CDI. In that case, if the GPS were dead (not just GPS signal loss), it would not display the green needle on the G5. The whole conversation was in the context of driving the GFC-500 from a second analog NAV source, so you may be correct that the needle would still be displayed, but I didn't understand it that way.

I understand I can send the NAV/COM2 directly to the G5, but it didn't seem worth the expense to move the needle 4 inches to the left.

I don't have any way to confirm or deny as we went ahead and only setup the GFC-500 to run off the IFD, which is an amazing upgrade over my dual auto-pilot configuration that the plane came with.

 

[Martin Pauly]

... that I could drive the GFC-500 from the NAV/COM2 unit (a standalone radio/nav unit whose model escapes me) by routing the signal to the GPS unit and tossing the CDI.

That's not how these signals are routed. The second NAV/COM radio's output is not routed through a primary GPS/NAV/COM; otherwise you'd lose everything if that primary GPS/NAV/COM goes belly up. Instead, those two radios send their data to the PFD (and the backup PFD if available) where NAV source selection happens. The pilot then selects from the first or the second radio, and (if applicable) GPS or VLOC, and that is then the navigation signal which is used to drive the HSI as well as the deviation bars often superimposed on the electronic attitude indicator, and also to drive the autopilot (and the flight director, if applicable).

Keeping an old mechanical CDI around when upgrading to a glass cockpit can make sense if a single point of failure can bring down the only indication of lateral and vertical deviations. But there is so much redundancy in most glass cockpits that there is usually a backup available, including lateral deviations displayed on the GPS unit itself for RNAV approaches.

NAV source selection through the PDF is really a beautiful thing. It ensures that the pilot and the autopilot/flight director are always on the same page as to what guidance is being followed. In the old days, there were all sorts of creative solutions to make an autopilot follow this or that radio for guidance, and could be a source of confusion. Much simpler these days.

- Martin

 

[morleyz]

That's not how these signals are routed. The second NAV/COM radio's output is not routed through a primary GPS/NAV/COM; otherwise you'd lose everything if that primary GPS/NAV/COM goes belly up. Instead, those two radios send their data to the PFD (and the backup PFD if available) where NAV source selection happens. The pilot then selects from the first or the second radio, and (if applicable) GPS or VLOC, and that is then the navigation signal which is used to drive the HSI as well as the deviation bars often superimposed on the electronic attitude indicator, and also to drive the autopilot (and the flight director, if applicable).

Keeping an old mechanical CDI around when upgrading to a glass cockpit can make sense if a single point of failure can bring down the only indication of lateral and vertical deviations. But there is so much redundancy in most glass cockpits that there is usually a backup available, including lateral deviations displayed on the GPS unit itself for RNAV approaches.

NAV source selection through the PDF is really a beautiful thing. It ensures that the pilot and the autopilot/flight director are always on the same page as to what guidance is being followed. In the old days, there were all sorts of creative solutions to make an autopilot follow this or that radio for guidance, and could be a source of confusion. Much simpler these days.

- Martin

I understand that the actual signal routing may not work the way I stated/understood, but I don't think that changes my understanding of how the functionality affects the GFC-500. In the scenario where my single GPS unit fails, the APR, NAV, TRK, and VNAV functions on the GFC-500 all become unavailable and if the AP was active on a NAV signal, it would revert to HDG and ALT, even if I was right in the middle of the AP flying a RNAV.

If my IFD were to totally fail, I would not be able to use the radio either, because it's my NAV/COM1 so that wouldn't show up on the HSI anyway (nor could I display it anywhere else). I would also not be able to have the GFC-500 fly NAV2 in any scenario where the single GPS is out.

So basically this comes down to the decision of whether or not I display the green arrow/vertical guidance on the G5 HSI or keep the existing CDI which is literally one spot to the right in my panel. The only misunderstanding I may have had is that the green arrow/vertical guidance would still be displayed on the G5 if the IFD were totally dead.

Am I really misunderstanding this? Did I miss out on some great functionality by keeping the CDI connected to NAV/COM2?

 

[Hang 4]

I understand that the actual signal routing may not work the way I stated/understood, but I don't think that changes my understanding of how the functionality affects the GFC-500. In the scenario where my single GPS unit fails, the APR, NAV, TRK, and VNAV functions on the GFC-500 all become unavailable and if the AP was active on a NAV signal, it would revert to HDG and ALT, even if I was right in the middle of the AP flying a RNAV.

If my IFD were to totally fail, I would not be able to use the radio either, because it's my NAV/COM1 so that wouldn't show up on the HSI anyway (nor could I display it anywhere else). I would also not be able to have the GFC-500 fly NAV2 in any scenario where the single GPS is out.

So basically this comes down to the decision of whether or not I display the green arrow/vertical guidance on the G5 HSI or keep the existing CDI which is literally one spot to the right in my panel. The only misunderstanding I may have had is that the green arrow/vertical guidance would still be displayed on the G5 if the IFD were totally dead.

Am I really misunderstanding this? Did I miss out on some great functionality by keeping the CDI connected to NAV/COM2?

As long as you have two nav sources, then the 2nd CDI is fine. In your failure scenario on an RNAV approach, I'd have the G5 heading bug on the inbound course (which is a good practice regardless) and if the GPS failed, I'd hit HDG on the GFC 500 and VS to line up with the glide slope, or just hand fly the vertical part. Otherwise hand fly it. The only downside to your 2 sources I can see is you would need to tune NAV2 to the ground based frequency ahead of time. I'd hate to be fiddling with that on an approach. Having said that, not a bad practice I guess as a back up to a total NAV1/GPS failure.

 

[Martin Pauly]

In the scenario where my single GPS unit fails, the APR, NAV, TRK, and VNAV functions on the GFC-500 all become unavailable and if the AP was active on a NAV signal, it would revert to HDG and ALT, even if I was right in the middle of the AP flying a RNAV.

That is correct if you indeed only have one GPS receiver on board. Many installations have multiple GPS receivers; but even then the concern discussed in this thread (with regard to the autopilot) isn't the failure of one of those GPS receivers, but a failure of the overall GPS system (i.e. a problem outside of your aircraft like jamming or spoofing) in the area you are flying in. Meaning you'd want to revert to VOR navigation and maybe an ILS approach at your destination, but your autopilot won't track a VLOC signal without GPS working, so you'll have to revert to heading mode. And having a mechanical CDI for the #2 NAV/COMM radio does not change that.

- Martin

 

[morleyz]

That is correct if you indeed only have one GPS receiver on board. Many installations have multiple GPS receivers; but even then the concern discussed in this thread (with regard to the autopilot) isn't the failure of one of those GPS receivers, but a failure of the overall GPS system (i.e. a problem outside of your aircraft like jamming or spoofing) in the area you are flying in. Meaning you'd want to revert to VOR navigation and maybe an ILS approach at your destination, but your autopilot won't track a VLOC signal without GPS working, so you'll have to revert to heading mode. And having a mechanical CDI for the #2 NAV/COMM radio does not change that.

- Martin

I feel like we're talking the exact same thing, using different words. My whole rationale for sharing my setup was to respond to the OPs presumption that Garmin was doing something sneaky by requiring GPS for the GFC-500. My GFC installer was quite clear about that requirement for radio navigation.

Since I only have 1 GPS and the CDI was connected to COM/NAV2 already prior to the GFC installation, I didn't see any value in eliminating the CDI (at some actual AMUs to me) when it couldn't be used by the GFC without a properly working GPS anyway. The only change would be moving the display for COM/NAV2 from the CDI to the G5 in my actual panel.

 

[Pinecone]

The difference would be not having the lateral and vertical deviation displayed on the heading indicator versus have to use the needles on the CDI and the heading from the G-5. Basically losing the integration of all the info into the single display.