What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

GMU-22 + GMU-11 Magnetometer?

Dad's RV-10

Well Known Member
In the parts pile, I have a GMU-22 magnetometer, three GSU-25 ADAHRS units and two GDU-460 displays. I will likely only install two of the ADAHRS units.

I plan to buy a G5 and a GTN-750Xi (to replace a GTN-650 I already have). Am I correct in my understanding that a single GMU-22 can drive both GSU-25's, but not via the CAN bus? If so, will the G5 be able to source magnetic heading information from the GMU-22 via the two GSU-25 ADAHRS units?

If all of the above is accurate, it seems the weak link is the single GMU-22. How likely is a GMU-22 failure?

Is it possible to have a GMU-11 on the CAN bus in the above configuration? If so, what is to be gained or lost in choosing to install the additional magnetometer vs. staying with only the GMU-22?

I'm not trying to design Boeing level redundancy. I'm just trying to get an better understanding of how all of these devices interact and share information.

Thanks.
 
GMU22

I have one GDU460, one GDU450, one G5, one GSU25 and one GMU22.
The GMU22 is connected to the GSU25.
With this configuration I have ADAHRS redundancy but no redundancy
for magnetic heading in the G3X-system. I have a wiskey compass though.

With two GSU25 and one G5 you will have ADAHRS triple redundancy and
no magnetic heading redundancy in the G3X-system.
Depending on the failure mode of the GSU25 connected to the GMU22 you may or may not loose the GMU22.
The GMU11 communicate direct on the CAN-bus so this will provide better redundancy.
Yes it is possible to use both GMU22 and GMU11 in the same G3X-system.

Good luck
 
Last edited:
Thanks for the feedback.

I guess the remaining question is, what is the likelihood of a GMU-22 failure and how does a loss of magnetic heading data affect the remainder of the flight (if IMC)?
 
The G5 acts as a backup ADHRS for the G3X and will automatically revert if your GMU 22 or GMU11 fails.

Revert to what? The G5 is getting its magnetic heading information from (in my case) the GMU-22, by way of the GSU-25 ADAHRS units. If the GMU-22 fails, I think the G5 would only have track information and not have magnetic heading information. Unless I'm mistaken.

I understand that the G5 has its own internal ADAHRS, but it relies on external sources for magnetic heading.
 
Revert to what? The G5 is getting its magnetic heading information from (in my case) the GMU-22, by way of the GSU-25 ADAHRS units. If the GMU-22 fails, I think the G5 would only have track information and not have magnetic heading information. Unless I'm mistaken.

I understand that the G5 has its own internal ADAHRS, but it relies on external sources for magnetic heading.

Is it correct to use the term ADAHRS for an instrument that doesn't have an internal heading reference? Seems to me it would revert to an ADARS if it lost external heading data without one.

Or am I being too pedantic?
 
Is it correct to use the term ADAHRS for an instrument that doesn't have an internal heading reference? Seems to me it would revert to an ADARS if it lost external heading data without one.

Or am I being too pedantic?

I guess the engineers would have to comment on that. Both the GSU-25 and the unit internal to G5 are listed as ADAHRS on Garmin's site, yet both rely on external magnetometers for magnetic heading information.
 
Losing the GMU in IMC

Well, the flight planned route and the aircraft will still be displayed on the EFIS.
The HSI (heading) will be U/S but course deviation indicators will not be affected. I would not be too worried.

Good luck
 
GMU 11/22 Information

Revert to what? The G5 is getting its magnetic heading information from (in my case) the GMU-22, by way of the GSU-25 ADAHRS units. If the GMU-22 fails, I think the G5 would only have track information and not have magnetic heading information. Unless I'm mistaken.

I understand that the G5 has its own internal ADAHRS, but it relies on external sources for magnetic heading.

The GMU 11 (or 22) reads the magnetic field in the immediate area of the aircraft, and provides that information to both your G5 and GSU 25, which independently calculate magnetic heading, using the data from the GMU 11 as an input. So the GMU 11/22 is not generating magnetic heading in any case, and is simply providing an input each ADAHRS uses to determine magnetic heading.

If you have a GMU 11 on your CAN bus, and your GSU 25 were to fail, the G5 would still have access to the information from the GMU11 and continue to provide magnetic heading information. Since the GMU 22 is connected directly to a GSU 25, you do rely on that GSU 25 being in good working order for the rest of the system to receive the information from the GMU. That is one of the advantages the GMU 11 holds over the GMU 22.

Thanks,

Justin
 
... Since the GMU 22 is connected directly to a GSU 25, you do rely on that GSU 25 being in good working order for the rest of the system to receive the information from the GMU. That is one of the advantages the GMU 11 holds over the GMU 22.

Thanks,

Justin

Thanks Justin. So in my case, where I'll have two GSU-25's, the GMU-22 can provide information to both, correct?

What is the likelihood of the GMU-22 failing? And if it were to fail, with two GSU-25's and and a G5, what are the real world implications (if the GMU-22 were the only magnetometer installed)?
 
Last edited:
Is there any reason in an aircraft that isn't "high performance" (which I think Garmin defines as capable of a TAS over 300 kt) to choose a GMU22 over a GMU11?
 
I generally agree, but one reason is CAN bus length. With a GMU 11 out on the wing tip you have to be careful with your CAN bus routing to stay within the length limit on a RV-10. As the GMU 22 is not on the CAN bus, it's less of an issue.
 
Back
Top