G5 as backup of g3x -> GFC500

[Ulysse]

In an aircraft with g3x + G5 as backup, how do you switch from g3x to G5 to drive the autopilot if you suspect the g3x AHRS is faulty?

[ArtVandelay]

Start pulling circuit breakers?

[Pinecone]

AFAIK, it switches when the G3X stops working, so CB pulling is probably the way to go.

I also understand, when it switches, the AP drop off, and you have to reengage it.

[PT20J]

The G3X includes a separately powered GSU 25 ADAHRS. The G5 has an internal ADAHRS. Both ADAHRS output to the CAN Bus. The GFC 500 software is contained in both the G3X and the G5. The G3X normally uses the GSU 25 and the G3X software to control the FD/AP.

In the event of a hard failure of the GSU 25, the G3X will switch to using the G5 ADAHRS. I believe that the autopilot disconnects during the switchover and will have to reengaged.

If the G3X GDU itself fails, the autopilot will disengage and can be reengaged using the G5.

In the event of a soft failure of the GSU 25 ADAHRS (e.g., a miscompare error annunciated), it's a bit more complicated because you will have to decide whether the G3X or G5 is the aberrant unit. I installed a AV-20S as a tie breaker for this purpose. Then, assuming that the GSU 25 is the bad unit, you could pull the GDU circuit breaker disabling the G3X, or you could pull the GSU circuit breaker disabling only the ADAHRS. In the first case, you would revert to using the G5 including the autopilot. In the latter case, you would still have the G3X and autopilot available with the G3X reverting to using the G5 ADAHRS as described above. The rub here is that the G3X STC calls for installing the GSU 25 and the GMU 11 magnetometer on the same circuit breaker, so pulling that breaker also disables the magnetometer. I asked Garmin support why they did this and did not get a reasonable answer.

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[Ulysse]

Thank you very much for the detailed explanation of the g3x/g5 /gfc500 interaction.
I will try to disconnect the GSU and then the GDU next time I fly to check if I can run the AP from the g5. 
One big issue in disconnecting the GDU is that I would lose my engine parameters since I have EIS. I think garmin pilot can display engine data but I don't have Pilot. Is there a standalone software that could do that ? I suppose switching the GDU off would not affect the EIS except that it would not have a display.

Although the error message I was getting conistently was "AHRS magnetic anomaly" which would suggest something wrong with the magnometer, why would this fault affect the attitude indicator which displayed absurd situation like high angle descent and turn to the left etc ?

On 7/31/2023 at 6:48 PM, PT20J said:

I installed a AV-20S as a tie breaker for this purpose

Could the AHRS in the GTX 345 displayed on an ipad be used for this purpose?

 

Edited August 3, 2023 by Ulysse
Typo

[PT20J]

1 hour ago, Ulysse said:

Thank you very much for the detailed explanation of the g3x/g5 /gfc500 interaction.
I will try to disconnect the GSU and then the GDU next time I fly to check if I can run the AP from the g5. 
One big issue in disconnecting the GDU is that I would lose my engine parameters since I have EIS. I think garmin pilot can display engine data but I don't have Pilot. Is there a standalone software that could do that ? I suppose switching the GDU off would not affect the EIS except that it would not have a display.

Although the error message I was getting conistently was "AHRS magnetic anomaly" which would suggest something wrong with the magnometer, why would this fault affect the attitude indicator which displayed absurd situation like high angle descent and turn to the left etc ?

Could the AHRS in the GTX 345 displayed on an ipad be used for this purpose?

 

I believe that the Garmin Pilot gets EIS data from the G3X Bluetooth. Losing engine data is not a big issue for me in my J, but with a turbo it is more of a concern.

The GTX 345 AHRS is disabled in a G3X installation. While the GTX outputs AHRS data on Bluetooth, the source is the G3X or G5 AHRS.

The GSU AHRS, as all MEMS AHRS, uses various external inputs for aiding including the magnetometer data, pitot-static data, and the GPS navigator data. Garmin doesn't explain well what happens when one of these sources is degraded which is what you are apparently experiencing. 

I would discuss this with Garmin. I don't see an error message "AHRS magnetic anomaly" described in the Pilot's Guide, the Maintenance Manual or the Installation Manual, so perhaps it is pretty rare.

[Ulysse]

54 minutes ago, PT20J said:

The GSU AHRS, as all MEMS AHRS, uses various external inputs for aiding including the magnetometer data, pitot-static data, and the GPS navigator data

This means that if the magnetometer fails then both the g3x and the G5 are affected and become potentially unusable? This is scary.

 

57 minutes ago, PT20J said:

I believe that the Garmin Pilot gets EIS data from the G3X Bluetooth

So if the GDU is switched off, there will be no Bluetooth, thus no way of getting the engine parameters? 

 

[PT20J]

9 hours ago, Ulysse said:

This means that if the magnetometer fails then both the g3x and the G5 are affected and become potentially unusable? This is scary.

The G5 works differently. The GMU 11 is optional for a stand alone G5, so the internal AHRS does not require and uses GPS and pitot-static only.

Garmin does’t explain all the possible soft failure modes. That’s why I installed an AV-20S. All the jets have three attitude indicators and I wanted a tie breaker made by a different manufacturer that’s completely independent of any other systems. Probably overkill, but it was cheap and I sleep better knowing it’s there.

9 hours ago, Ulysse said:

So if the GDU is switched off, there will be no Bluetooth, thus no way of getting the engine parameters? 

I believe that’s true. If the engine instrumentation us critical, I believe the Garmin solution is a second GDU. 

[ArtVandelay]

The AV20S is not approved under a STC, but under NORSEE standards and is only legal as a secondary instrument. I too use it as a 3rd option, just in case the Garmin network of instruments went kaput. It also has AOA functionality if you’re interested in that too.

[201Mooniac]

39 minutes ago, ArtVandelay said:

The AV20S is not approved under a STC, but under NORSEE standards and is only legal as a secondary instrument. I too use it as a 3rd option, just in case the Garmin network of instruments went kaput. It also has AOA functionality if you’re interested in that too.

Just wondering if you have gotten the AOA to work reliably.  I also added one as a tie-breaker but haven't bothered to try to calibrate the AOA yet as I've never heard of one working properly in a Mooney.

[Ulysse]

4 hours ago, PT20J said:

The G5 works differently. The GMU 11 is optional for a stand alone G5, so the internal AHRS does not require and uses GPS and pitot-static only.

this is good to know. thank you

4 hours ago, PT20J said:

That’s why I installed an AV-20S.

Great idea. I am going to order one too

[ArtVandelay]

Just wondering if you have gotten the AOA to work reliably.  I also added one as a tie-breaker but haven't bothered to try to calibrate the AOA yet as I've never heard of one working properly in a Mooney.

Good question, I had problems in the beginning, it kept giving me warnings when it shouldn’t. I adjusted the settings but haven’t actually tried to test it by doing slow flight.

[PT20J]

4 hours ago, 201Mooniac said:

Just wondering if you have gotten the AOA to work reliably.  I also added one as a tie-breaker but haven't bothered to try to calibrate the AOA yet as I've never heard of one working properly in a Mooney.

Mine works, sort of. Since it doesn’t have a probe, it takes a bit time to come up with a calculated solution. So, if you fly a very stable approach in calm air, it seems reasonably accurate. But if your actual AOA is varying due to changing airspeed or turbulence, the lag makes it not very useful.

I never could get the Aspen AOA calibrated correctly even after several tries and verifying the accuracy of the pitot system, but it was much more stable than the AV-20S.

Someday I might install an Alpha Merlin on the glare shield.

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[gmonnig]

9 hours ago, 201Mooniac said:

Just wondering if you have gotten the AOA to work reliably.  I also added one as a tie-breaker but haven't bothered to try to calibrate the AOA yet as I've never heard of one working properly in a Mooney.

I haven’t got my AoA to work reliably either. I chalked it up as user error. There isn’t a good description or video on how to actually set it up. Super frustrating but I do like the display!  

[Jsavage3]

I’m having the G3X installed now and was planning on the G5 backup however several talks with Garmin at Oshkosh has caused me to lean heavily towards the 275 as a backup instead.  Has anyone else heard of this idea or followed thru with it?

[PT20J]

3 hours ago, Jsavage3 said:

I’m having the G3X installed now and was planning on the G5 backup however several talks with Garmin at Oshkosh has caused me to lean heavily towards the 275 as a backup instead.  Has anyone else heard of this idea or followed thru with it?

Until recently, GI 275 had severe limitations when used as standby for the G3X, but I believe that Garmin has finally fixed that. Still, I would carefully review the current G3X installation manual with your dealer to compare the G5 and GI 275 capabilities as a G3X standby (the manual is dealer only, but they can go over it with you, they just can’t give you a copy). 

That said, I would still chose the G5. While the GI 275 has a better display, it has shorter backup battery run time. The G5 display is entirely adequate and I never look at it when the G3X is operational. Also, the G5 costs less. 

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[Jsavage3]

The Garmin guys at Oshkosh were very deliberate in saying the new 275 has fixed all the G3X backup issues.  

Personally, I just like the look of the 275 better.

 The useful battery time is an interesting topic.  I’ll be looking forward to finding out more about that comparison.

[201Mooniac]

While the battery life for the GI275 is less then the G5, I tested mine and it lasted 88 minutes which I expect is easily enough to get back on the ground if needed.

[ArtVandelay]

The G5 can drive the GFC 500, can the 275?

[PT20J]

9 hours ago, ArtVandelay said:

The G5 can drive the GFC 500, can the 275?

Yes.