Dual G3X: why G5 backup?

[hais]

Hi, question for IFR setup.

I understand that G3X dual screen setup works in reversionary mode, and that G3X can drive the GFC500, and that a ship like M20R with dual batteries can drive either display to a landing should the alternator fail.

So why is G5 installed as ADI backup? 

[Mooney Dog]

If i had to guess it would be for a second AHRS system. Thinking to some of the G1000 installs where there's only one AHRS for the screens and they too have a secondary AI/mag display. 

[hais]

Thanks, makes sense. Hopefully Garmin will have a dual AHRS setup in the future because that will clean up panels further.

1 hour ago, Mooney Dog said:

If i had to guess it would be for a second AHRS system. Thinking to some of the G1000 installs where there's only one AHRS for the screens and they too have a secondary AI/mag display. 

 

[Ragsf15e]

10 hours ago, hais said:

Thanks, makes sense. Hopefully Garmin will have a dual AHRS setup in the future because that will clean up panels further.

 

Real answer is total electrical failure.  Yes you’ve got a robust solution with dual batteries and the alternator, but you need a backup ADI that can function without any power.  G5 or GI275 both have battery backup.  G3x doesn’t.

Even the 3 screen G1000 with gen, alternator, and battery I fly for work has an Aspen as the backup because the Aspen is backed up by it’s own internal battery.

Edited November 8, 2021 by Ragsf15e

[Wayne Cease]

Part of it is different software. That way if there is a bug and the G3X has an issue both units could have the same issue. A different model will have different software and extremely unlikely to suffer the same problem in the same situation.



Wayne

[Mooney Dog]

9 minutes ago, Ragsf15e said:

Even the 3 screen G1000 with gen, alternator, and battery I fly for work has an Aspen as the backup because the Aspen is backed up by it’s own internal battery.

Every G1000 plane ive flown has it on its own backup battery bus. Does yours not? 

[Ragsf15e]

2 minutes ago, Mooney Dog said:

Every G1000 plane ive flown has it on its own backup battery bus. Does yours not? 

It doesn’t have a backup battery for the g1000.  Only the ship’s battery.  There is an emer bus, but if the poop hits the fan and your electrical system fails, no pieces of the g1000 have their own internal battery.  Only the Aspen has that.  While that’s a remote possibility, I bet it’s a certification requirement.  
 

All the G1000 airplanes I’ve seen have an ADI backup with their own power or vacuum system.  

[Mooney Dog]

2 minutes ago, Ragsf15e said:

It doesn’t have a backup battery for the g1000.  Only the ship’s battery.  There is an emer bus, but if the poop hits the fan and your electrical system fails, no pieces of the g1000 have their own internal battery.  Only the Aspen has that.  While that’s a remote possibility, I bet it’s a certification requirement.  
 

All the G1000 airplanes I’ve seen have an ADI backup with their own power or vacuum system.  

Ah i think i misinterpreted what you originally posted. Its not backup power, its the backup battery for the module itself. My mistake. 

[Ragsf15e]

44 minutes ago, Mooney Dog said:

Ah i think i misinterpreted what you originally posted. Its not backup power, its the backup battery for the module itself. My mistake. 

Yeah that’s what I meant.  Sometimes I write stuff and think it’s perfectly clear to everyone exactly what I meant, but it’s only perfectly clear inside my own head, and even then…

[hais]

1 hour ago, Wayne Cease said:

Part of it is different software. That way if there is a bug and the G3X has an issue both units could have the same issue. A different model will have different software and extremely unlikely to suffer the same problem in the same situation.



Wayne

 

The dual GI275 failure comes to mind ... Electrical and AHRS issue aside, why is that configuration legal, but G3X isn't?

[RobertGary1]

2 hours ago, Ragsf15e said:

It doesn’t have a backup battery for the g1000.  Only the ship’s battery.  There is an emer bus, but if the poop hits the fan and your electrical system fails, no pieces of the g1000 have their own internal battery.  Only the Aspen has that.  While that’s a remote possibility, I bet it’s a certification requirement.  
 

All the G1000 airplanes I’ve seen have an ADI backup with their own power or vacuum system.  

In the g1000 the essential buss will power the pfd and essential items including attitude etc. The big limitation with the g1000 is the single AHRS. We have a vacuum gyro as the backup. Neither the 275 nor the g5 is approved by the faa to replace the vacuum gyro in that install. There is no stc. 

[Mooney Dog]

2 hours ago, Ragsf15e said:

Yeah that’s what I meant.  Sometimes I write stuff and think it’s perfectly clear to everyone exactly what I meant, but it’s only perfectly clear inside my own head, and even then…

If i dont know what im thinking, clearly no one else will so no one can stop me! 

[Ragsf15e]

8 minutes ago, RobertGary1 said:

In the g1000 the essential buss will power the pfd and essential items including attitude etc. The big limitation with the g1000 is the single AHRS. We have a vacuum gyro as the backup. Neither the 275 nor the g5 is approved by the faa to replace the vacuum gyro in that install. There is no stc. 

What powers the essential bus during a total electrical failure?

I agree with your single ahrs issue, but even systems with multiple ahrs (mine has 2), still need a backup adi with it’s own internal power.

[RobertGary1]

31 minutes ago, Ragsf15e said:

What powers the essential bus during a total electrical failure?

I agree with your single ahrs issue, but even systems with multiple ahrs (mine has 2), still need a backup adi with it’s own internal power.

If the main buss (ship battery and alternator) are dead there is a second large standby battery behind the panel. So other than fire you should have a total of well over an hour of flying. Preflight test includes a load test on this standby battery . 

[Ragsf15e]

42 minutes ago, RobertGary1 said:

If the main buss (ship battery and alternator) are dead there is a second large standby battery behind the panel. So other than fire you should have a total of well over an hour of flying. Preflight test includes a load test on this standby battery . 

I guess I’d like to see the certification guidelines because the battery you’re talking about powers an essential bus with several items on it, not specifically the PFD.  From the airplanes Ive seen with g1000, there’s always a backup ADI with its own integral battery or a vacuum driven backup.

The g1000 in the Meridian I fly has multiple sources of power as well, from different buses and yet there’s still that battery powered Aspen sitting next to it.

[PT20J]

So, in summary: Two things

1. Unlike the Experimental G3X, the certified G3X is limited to a single GSU 25D ADAHRS, so it needs some sort of backup.

2. The G3X does not have a backup battery.

Skip

[Ragsf15e]

1 minute ago, PT20J said:

So, in summary: Two things

1. Unlike the Experimental G3X, the certified G3X is limited to a single GSU 25D ADAHRS, so it needs some sort of backup.

2. The G3X does not have a backup battery.

Skip

Skip, if someone wanted too, could they use 2xG3x, both having an ahrs?  Say like 2 7” ones but both adhrs?

[RobertGary1]

14 minutes ago, Ragsf15e said:

I guess I’d like to see the certification guidelines because the battery you’re talking about powers an essential bus with several items on it, not specifically the PFD.  From the airplanes Ive seen with g1000, there’s always a backup ADI with its own integral battery or a vacuum driven backup.

The g1000 in the Meridian I fly has multiple sources of power as well, from different buses and yet there’s still that battery powered Aspen sitting next to it.

Are we saying the same thing?

[RobertGary1]

12 minutes ago, Ragsf15e said:

Skip, if someone wanted too, could they use 2xG3x, both having an ahrs?  Say like 2 7” ones but both adhrs?

Not sure but Garmin oddly lacks certification as backup for instruments it’s certified as primary. 

[Ragsf15e]

1 minute ago, RobertGary1 said:

Are we saying the same thing?

Probably.  I was just starting to think the same thing…

[Ragsf15e]

12 minutes ago, RobertGary1 said:

Not sure but Garmin oddly lacks certification as backup for instruments it’s certified as primary. 

Yeah it’s strange how a G5 can be primary by itself and backup for a G3x, but it can’t be a required backup for other systems (single screen Aspen).

Edited November 8, 2021 by Ragsf15e

[ArtVandelay]

Yeah it’s strange how a G5 can be primary by itself and backup for a G3x, but it can’t be a required backup for other systems (single screen Aspen).

That’s up to Aspen, but they rather you buy a 2nd unit so don’t hold your breath.

[Ragsf15e]

25 minutes ago, ArtVandelay said:

That’s up to Aspen, but they rather you buy a 2nd unit so don’t hold your breath.

I don’t think so.  Maybe it’s changed in Rev 69.xx, but the G5 installation manual says it can’t be used as a required backup attitude indicator.  I just used Aspen as an example.

[Ragsf15e]

34 minutes ago, ArtVandelay said:

That’s up to Aspen, but they rather you buy a 2nd unit so don’t hold your breath.

Maybe you’re right and they got rid of that restriction because there’s plenty of threads on it on the internet, but I don’t see the restrictions in the “limitations “ Section of the install manual anymore.

[PT20J]

5 hours ago, Ragsf15e said:

Skip, if someone wanted too, could they use 2xG3x, both having an ahrs?  Say like 2 7” ones but both adhrs?

I don't believe the STC allows a certified G3X system to have more than one ADAHRS no matter how many GDUs (displays) it has.

The G5 and GI 275 can serve as backups for the G3X because of the system design. The G3X, G5 and GI 275 each have ADAHRS, GPS and GFC 500 autopilot software and the G5 and GI 275 have backup batteries.

It doesn't appear that Garmin was particularly interested in the G5 or GI 275 as standby instruments for non-Garmin primary instruments. Since these instruments are not TSO'd, they are installed under STC and Garmin controls what the STC allows. Garmin doesn't say you can't use the G5 as a standby for an Aspen -- ,they say that you cannot use their STC as a basis for approval. Someone could always go for a field approval. But the sticking point is likely the software. Since Garmin updates the software frequently, it might be hard to get a field approval for a moving target. 

Skip