GI-275 and magnetometer for revisionary mode

[shawnd]

I am getting two GI275s installed in my Mooney to replace the stock attitude & HSI and found out that there's only one GMU-11 magnetometer  being  installed that will feed  data  to  both 275s. Given the recent case in Canada, and the unit wanting  to re-align mid-flight, I thought a dual GI-275 installation should include dual  GMU-11s to truly support revisionary mode. Am I wrong in assuming this?

[EricJ]

47 minutes ago, shawnd said:

I am getting two GI275s installed in my Mooney to replace the stock attitude & HSI and found out that there's only one GMU-11 magnetometer  being  installed that will feed  data  to  both 275s. Given the recent case in Canada, and the unit wanting  to re-align mid-flight, I thought a dual GI-275 installation should include dual  GMU-11s to truly support revisionary mode. Am I wrong in assuming this?

The GMU-11 just provides heading alignment.    I don't think it's needed for attitude indication, only heading.    You have a compass to back that up and those are pretty reliable. 

 

[shawnd]

Thanks @EricJ, the two units are ADAHRS and ADAHRS+AP.  I may have been looking for redundancy everywhere!

[PT20J]

You have to check what the STC allows. It will be very specific about approved configurations. I went through the same exercise when planning my G3X/G5 installation. The problem when you have two of any sensor is determining which one is bad in the case of a subtle failure. I ended up replacing the clock with an AV-20-S to give me a third AHRS for a majority vote.

In the G3X (assuming the GI 275 ADAHRS uses a similar scheme), the most important thing is a GPS input. I would look to see what options allow you the most redundant GPS combinations using the GI 275 GPS  and panel mount GPS.

Skip

[David Lloyd]

13 hours ago, shawnd said:

I am getting two GI275s installed in my Mooney to replace the stock attitude & HSI and found out that there's only one GMU-11 magnetometer  being  installed that will feed  data  to  both 275s. Given the recent case in Canada, and the unit wanting  to re-align mid-flight, I thought a dual GI-275 installation should include dual  GMU-11s to truly support revisionary mode. Am I wrong in assuming this?

The recent case in Canada, the HSI even with an ADAHRS and battery was configured as an HSI only and the AHRS source was selected in the ADI.  Any other reasonable installation would have reconfigured the second unit as a Standby ADI/HSI and been required to have a reversionary switch (in addition to being an automatic function).  And the AHRS source set to internal unless there was an operational problem.

[EricJ]

55 minutes ago, PT20J said:

You have to check what the STC allows. It will be very specific about approved configurations. I went through the same exercise when planning my G3X/G5 installation. The problem when you have two of any sensor is determining which one is bad in the case of a subtle failure. I ended up replacing the clock with an AV-20-S to give me a third AHRS for a majority vote.

In the G3X (assuming the GI 275 ADAHRS uses a similar scheme), the most important thing is a GPS input. I would look to see what options allow you the most redundant GPS combinations using the GI 275 GPS  and panel mount GPS.

Skip

I wonder what it does if the GPS and magnetometer are bad but Air Data is good?

 

[flyboy0681]

23 minutes ago, EricJ said:

I wonder what it does if the GPS and magnetometer are bad but Air Data is good?

 

The air data computer is fed by the magnetometer only.

[PT20J]

18 minutes ago, EricJ said:

I wonder what it does if the GPS and magnetometer are bad but Air Data is good?

 

It’s supposed to maintain valid attitude. But, as you know “bad” can range from “doesn’t quite meet specified error tolerance” to “no output.” I’m pretty sure that the AHRS failure detection circuitry can detect catastrophic failures. But with only two AHRS it cannot detect more subtle ones. The G3X Pilot’s Guide alludes to this by describing  various “miscompare” annunciations triggered when the G3X and G5 ADAHRS do not agree but the logic cannot tell which is bad. The manual advises the pilot to manually select to “good” ADAHRS. After imagining myself trying to navigate G3X menus while recovering from an unusual attitude, I decided on the AV-20-S. It’s probably overkill, but it’s one less thing to worry about.

Skip

[EricJ]

1 hour ago, PT20J said:

It’s supposed to maintain valid attitude. But, as you know “bad” can range from “doesn’t quite meet specified error tolerance” to “no output.” I’m pretty sure that the AHRS failure detection circuitry can detect catastrophic failures. But with only two AHRS it cannot detect more subtle ones. The G3X Pilot’s Guide alludes to this by describing  various “miscompare” annunciations triggered when the G3X and G5 ADAHRS do not agree but the logic cannot tell which is bad. The manual advises the pilot to manually select to “good” ADAHRS. After imagining myself trying to navigate G3X menus while recovering from an unusual attitude, I decided on the AV-20-S. It’s probably overkill, but it’s one less thing to worry about.

Skip

Ah, that last page says, "Likewise, loss of magnetomer (heading) data, in combination with a loss of GPS data, results in loss of all attitude and heading information."

So even if the air inputs are good, it fails, according to that.   Although uAvionix and a few others provide attitude indication with only AHRS/IMU and air data, it appears that Garmin does not.   Maybe the difference is standby vs primary use?

Sounds like keeping an AV-20-S there is not a bad idea.

[PT20J]

5 minutes ago, EricJ said:

Ah, that last page says, "Likewise, loss of magnetomer (heading) data, in combination with a loss of GPS data, results in loss of all attitude and heading information."

So even if the air inputs are good, it fails, according to that.   Although uAvionix and a few others provide attitude indication with only AHRS/IMU and air data, it appears that Garmin does not.   Maybe the difference is standby vs primary use?

Sounds like keeping an AV-20-S there is not a bad idea.

You are correct -- I misread the flow chart. I talked to an engineer at a company that builds standalone AHRS for various applications a while back. As I understand it, the best aiding comes from velocity data from GPS. Some AHRS units have built-in 3-axis magnetometers and can meet drift specs without GPS. I believe that's how the RCA 2610 works. It's better if you also have pneumatic airspeed. I think that's how the original Aspen and the AS-20-S work. It's hard to find anyone at these companies that will talk to me and that really understands the details. 

Skip

[RobertGary1]

2 hours ago, David Lloyd said:

The recent case in Canada, the HSI even with an ADAHRS and battery was configured as an HSI only and the AHRS source was selected in the ADI.  Any other reasonable installation would have reconfigured the second unit as a Standby ADI/HSI and been required to have a reversionary switch (in addition to being an automatic function).  And the AHRS source set to internal unless there was an operational problem.

But that is exactly what I had and I still had a dual AHRS failure. Garmin engineers took my plane for a week debugging it and never came back with anything. Director of engineering called me and only said my installation was not at fault.

-Robert

[EricJ]

9 minutes ago, PT20J said:

You are correct -- I misread the flow chart.

The flow chart is a bit ambiguous at first glance about that case, which is why I was wondering.   The text clears it up, assuming it is correct. 
 

[David Lloyd]

29 minutes ago, RobertGary1 said:

But that is exactly what I had and I still had a dual AHRS failure. Garmin engineers took my plane for a week debugging it and never came back with anything. Director of engineering called me and only said my installation was not at fault.

-Robert

Shawnd asked about the Canadian failure, that is what I answered.  There is still no reason given for the initial ADI realignment.

Your failure does have me personally concerned and I addressed that concern to Garmin directly.  That the Director of Engineering phoned and said your installation was not at fault only deepens the mystery.  I can only imagine "installation" referred to the physical installation, the configuration (shop access) and setup (pilot access).  Garmin needs to step up and admit there has been a problem of this nature with a small number of installations, what has been found, what is being done and possibly a warning to current users of what conditions to avoid.

Several years ago, I had a Dynon Skyview shut down in flight and would not restart until all power was removed from that unit.  It was the first flight after a software update, months later it was tracked to problem with that software.  Even though it only involved one of the two displays, it was a year before they had my trust.

[shawnd]

Thank you all for the feedback, and sorry for the delayed response. Plans have changed a bit due to circumstances and this has been put on hold for a bit.

[Bigdaddie]

What did you find out about the magnatrometer installation?  I just got my airplane back from the avionics shop last month. I have 2x G500txi + a GI-275 for backup and one magnatrometer. What concerns me is that the G500 powers the magnatrometer, so when in reversionary or standby mode, the backup GI-275 has no heading information. It seems to me that the GI-275 should be primary driver for heading and transmit that info to the G500s.  But, I guess the GI-275 could fail just as much as the G500. To me, heading information should be always available from the standby instrument, even on battery power.

[PeteMc]

54 minutes ago, Bigdaddie said:

so when in reversionary or standby mode, the backup GI-275 has no heading information.

I'm no avionics expert, but it sure sounds like something is configured or wired wrong.  Or maybe there's a mode you need to switch on the GI275.  If it is a true backup, I totally agree that if you loose the G500 or for whatever reason the GI275 goes into its stand-alone mode, it should be running off its own magnetometers. 

Except for GPS data, mine is a separate instrument, not connected to anything else.  So it's interesting that if it meets the requirements of a "Backup" it was wired where another device can be interfere with its functionality/integrity. 

[Bigdaddie]

27 minutes ago, PeteMc said:

I'm no avionics expert, but it sure sounds like something is configured or wired wrong.  Or maybe there's a mode you need to switch on the GI275.  If it is a true backup, I totally agree that if you loose the G500 or for whatever reason the GI275 goes into its stand-alone mode, it should be running off its own magnetometers. 

Except for GPS data, mine is a separate instrument, not connected to anything else.  So it's interesting that if it meets the requirements of a "Backup" it was wired where another device can be interfere with its functionality/integrity. 

That's my thought also.  Shop said they talked to Garmin and the setup was correct and my other option was to install a second magnetometer.  I understand that there's always the compass, but who knows how to fly off those things these days?  Especially in an no s**t emergency and you're down to that one 275 on battery power, I don't need to be trying to scan up to the compass to fly simple headings.

[PT20J]

24 minutes ago, Bigdaddie said:

That's my thought also.  Shop said they talked to Garmin and the setup was correct and my other option was to install a second magnetometer.  I understand that there's always the compass, but who knows how to fly off those things these days?  Especially in an no s**t emergency and you're down to that one 275 on battery power, I don't need to be trying to scan up to the compass to fly simple headings.

You don't really need heading as long as the GPS is working. You just fly GPS track. Check the AFMS, but the G3X/G5 combo reverts to displaying TRK when the GMU 11 fails. TRK is better than HDG because you don't have to compensate for wind.

[PeteMc]

1 hour ago, PT20J said:

You don't really need heading as long as the GPS is working.

But if it's an electrical issue, then no GPS.  I guess if the G500Txi has the optional Battery Backup it is not as much of an issue.  But if it doesn't, I wonder if the configuration is the same (per Garmin). 

I'd interested to hear Garmin's logic on making the GI275 dependent on the PFD.  I always believed the point of a Backup is to have a separate independent source in case things look off and you want a second source.  I'd want that to also be true if you're in a full power loss situation and on battery. 

 

[PT20J]

Seems to me that if I had an electrical failure that took out both the GMU and the GPS that I’d be worried about more than heading because I probably would not have nav’s or coms or anything else but a single G5. But, in my case, I have a vertical card compass that’s well damped and easy to read. 

[201Mooniac]

While the GI 275 can use the heading information from the GMU 44 with the G500 TXi, you can also install a GMU 11 with the GI 275 that will act as a backup (still use the GMU 44 as primary so no heading discrepancy between G500 TXi and GI 275) and it is powered from the GI 275 so it will still function if power is lost and your GI 275 has a backup battery.

[Ragsf15e]

On 8/20/2024 at 3:23 PM, PT20J said:

Seems to me that if I had an electrical failure that took out both the GMU and the GPS that I’d be worried about more than heading because I probably would not have nav’s or coms or anything else but a single G5. But, in my case, I have a vertical card compass that’s well damped and easy to read. 

Additionally, a G5 has an internal gps as well.  It can be set up iaw the STC with a small antenna under the glareshield (it also has an internal antenna that my hangar elves found works great).  If it’s connected to the panel gps, that’s primary but it can use internal gps in a failure, so it doesn’t lose gps.  Does the -275 also have internal gps? 

[201Mooniac]

2 hours ago, Ragsf15e said:

Does the -275 also have internal gps? 

Yes, it does.  Uses the same antenna option as the G5.  Works quite well.

[shawnd]

On 8/20/2024 at 3:09 PM, PeteMc said:

I guess if the G500Txi has the optional Battery Backup it is not as much of an issue. 

Just a note - the backup battery GBB-54 on the G500TXi is only allowed if you have a dedicated MFD display. If you have PFD and EIS - the GBB is not allowed to be backup. Why? Joys of Garmin....

[shawnd]

On 8/20/2024 at 4:35 PM, 201Mooniac said:

While the GI 275 can use the heading information from the GMU 44 with the G500 TXi, you can also install a GMU 11 with the GI 275 that will act as a backup (still use the GMU 44 as primary so no heading discrepancy between G500 TXi and GI 275) and it is powered from the GI 275 so it will still function if power is lost and your GI 275 has a backup battery.

Turns out I have a spare GMU-11 from the anticipated older GI-275 setup. Will ask the shop to see if I can have this be installed in the right wing (left one has the GMU-44). And if not, then help me find a better backup solution. Though I agree with @PT20J, the vertical card compass is the last resort in case of electrical failure that would affect the GTNs as well as rest of the avionics once the battery dies.