Attitude indicator/ADAHRS, how many do you need for IFR..

Maybe this write up will help someone else...

About a year ago during VFR flight, one of the dual AHRS rebooted resulting in EFISes displaying divergent flight information. I can totally see event could really become a fatal distraction if it ever happened at just the wrong time.

My panel includes 3 GRT HX EFISes driven by Dual AHRS and dual magnetometers. The electrical system is Z14 architecture (dual alternators and dual batteries). EFIS displays can be swapped but are normally set so that EFIS1 shows Primary Flight information, EFIS2 Map, and EFIS3 Engine. AHRS1 goes to the Primary flight EFIS1 display and AHRS2 goes to the EFIS2 display.

. . .

The first symptom that something was not right was "Dual AHRS" indication replaced by "AHRS x" which means something took a powder and only one AHRS was on line.

The Primary flight EFIS FROZE showing pitch up and roll left (takes several minutes to come back on line). But would you really trust it afterwards?

Ok, no problem. Just switch the backup EFIS2 (with AHRS2) to display Primary flight information. It showed good (?) data but it still disagreed with EFIS1.

It was SO TEMPTING to troubleshoot. The glaringly obvious problem was that neither EFIS could be trusted, so you REALLY need a tie breaker. Thankfully my panel includes backup AS, Alt and Attitude steam gauges. It wasn't difficult to switch over to steam gauges, but the EFISes were a huge distraction.

I'm very thankful this happened in VFR conditions and I learned a few valuable lessons.

1) Assume something will break and you will never be disappointed

Although my panel is very capable, I had assumed that my fancy multi-display, dual everything gizmos would always be there. Not so much!

2) Practice

Backups are worthless if you don't practice using them. We have SO much packed into EFIS displays. It's really distracting if one fails and there's little time to troubleshoot if in the soup. Even problems in VFR conditions can be equally fatal if they happen at the wrong time.

3) Remove the distraction

Do EFISes include an OFF switch? Mine don't and I didn't add any. Note to self, if an EFIS appears to fail, switch it to display that shows something else, such as engine parameters, or simply cover it up so there's no temptation to fixate on it.

Conclusion

After a good deal of troubleshooting on the ground, the reboot issue was traced to an AHRS power crimp. I fixed the crimp and then added a SECOND power wire input to each AHRS.

I now regularly practice switching to backups in flight... and have adjusted expectations that something will break regardless of every effort to prevent problems. Oh, and a totally low tech wet compass is still your best friend in case everything if all else fails.

Regards,
Jay
RV10 N433RV

Why don't these systems incorporate the ability to compare 3 AHRSs? So that if 1 shows different, but 2 agrees, it will still give you an AHRS error message but will continue to follow the 2 that are matching.

In agreement with Vic!

IMHO.......

Independent redundancy in technology via different manufacturers along with a minimum of a VSI, airspeed indicator and an altimeter has always been my preference. Whether GRT/Trutrak/Garmin or Dynon/Trio/Garmin, as I've used in the last two RVs I've built, these 2.25" instruments were MEL items for me personally and always will be.

With a myriad of technology possibilities, training in the use of all features and, more importantly, understanding the partial failure modes and the partial failure operation options, are as important, if not more important than any hardware redundancy.

While information presentation is important, information overload seems to be an issue, as well. Decades before EFIS/MFD/PFD, my instructor would routinely cover portions of the panel, reminding me how pioneers flew the world. It was training and practice that made the difference, no matter the circumstance.

Prayers for the lost souls in the Lancair. I hope other lives are saved by the lessons of this tragedy.

BHunt said:

Why don't these systems incorporate the ability to compare 3 AHRSs? So that if 1 shows different, but 2 agrees, it will still give you an AHRS error message but will continue to follow the 2 that are matching.

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Because, in the case of the OP, there were only two AHRS systems, not three.
As the OP pointed out, he had very good round dial backups; you just have to be willing to use them, and include them in your scan.

BobTurner said:

Because, in the case of the OP, there were only two AHRS systems, not three.
As the OP pointed out, he had very good round dial backups; you just have to be willing to use them, and include them in your scan.

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Yup, I'm aware that it only had two. I was asking why there isn't a 3 vote logic to use 3. It seems (from my MINIMAL research) that EFISs will compare two, but not three

BHunt said:

Yup, I'm aware that it only had two. I was asking why there isn't a 3 vote logic to use 3. It seems (from my MINIMAL research) that EFISs will compare two, but not three

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If I had to guess, they probably figure most builders are going to install an independent attitude unit vs. a third AHRS.

BHunt said:

Yup, I'm aware that it only had two. I was asking why there isn't a 3 vote logic to use 3. It seems (from my MINIMAL research) that EFISs will compare two, but not three

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Bottom line is I don't think they have the technology to do it yet, at least in GA world, most likely due to cost.

I've asked the same question as many of todays EFIS system will support 3 ADAHRS.

3 sources

I work on the big jets, and there are cases where we have 3 sources of the same data (think air data, inertial, and others). Sometimes even 4. To handle that correctly takes a lot of system engineering expertise. It seems like a simple task, but the devil is definitely in the details here.

To give you an idea, think about the different ways it can fail. If 1 signal disagrees with the other two, who agree with each other, thats pretty easy, just kick that one out. But what if they all disagree but by a small amount? Sometimes you get a very brief but high magnitude spike in 1 signal, those should be ignored. But what about a failure that looks like a slow ramp, a fast ramp, or oscillating at any number of frequencies/magnitudes? How do you determine what the acceptable level of disagreement is? A lot of things play into that like sensor accuracy, data transmission rates, comparison monitor execution rates, etc.

This is just a taste of signal management. People build careers on this topic alone.

unitink72 said:

I work on the big jets, and there are cases where we have 3 sources of the same data (think air data, inertial, and others). Sometimes even 4. To handle that correctly takes a lot of system engineering expertise. It seems like a simple task, but the devil is definitely in the details here.

To give you an idea, think about the different ways it can fail. If 1 signal disagrees with the other two, who agree with each other, thats pretty easy, just kick that one out. But what if they all disagree but by a small amount? Sometimes you get a very brief but high magnitude spike in 1 signal, those should be ignored. But what about a failure that looks like a slow ramp, a fast ramp, or oscillating at any number of frequencies/magnitudes? How do you determine what the acceptable level of disagreement is? A lot of things play into that like sensor accuracy, data transmission rates, comparison monitor execution rates, etc.

This is just a taste of signal management. People build careers on this topic alone.

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And I fly them (777) and you are exactly correct.

The system monitors so many different failure points, our manuals don't even cover/address most of them. When something happens, it's recognized and you get an alert and (usually) the system has already configured a workaround for you.

Those are the easy ones...

The hard ones are when the system can't figure out the source of the bad data and dumps it back into your lap for the decision...which then leaves it up to the pilot to look at the 3 different attitude/air data sources and determine which one is bad.

Hopefully, you aren't already inverted/stalled when that happens.

The books say while sorting these kinds of problems out, to try to remain in visual conditions while doing this if an attitude problem...or set a known pitch attitude/power setting if an airspeed one...

As we all realize, you don't get to pick when these failures occur...

Additionally, you're trying to sort all this out while usually getting inundated with EICAS messages/stall warnings/overspeed warnings/autopilot disconnects/etc.

It's a very challenging situation, even in air transport with a professional crew at the controls...as several very high profile accidents have shown.

We are getting MUCH more emphasis (and training) flying and troubleshooting these types of problems, but I am under no illusion that it would be anything other than one of the most difficult inflight situations I could ever be presented with...

As a single pilot operating IFR, that challenge is even greater...and we need to consider what level of risk we are comfortable with, potential failure modes, and train accordingly...

Rob S.

jaybob said:

2) Practice
Backups are worthless if you don't practice using them. We have SO much packed into EFIS displays. It's really distracting if one fails and there's little time to troubleshoot if in the soup. Even problems in VFR conditions can be equally fatal if they happen at the wrong time.
RV10 N433RV

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Could not agree more with this recommendation and my main reason against a different brand EIFS as secondary. One will not know it nearly as well in the time that really needs it. Steam gauges are my best option for redundancy, no matter how good one's electrical system is.

Bavafa said:

Could not agree more with this recommendation and my main reason against a different brand EIFS as secondary. One will not know it nearly as well in the time that really needs it. Steam gauges are my best option for redundancy, no matter how good one's electrical system is.

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Well that creates a problem. Most electronic backup indicators now look pretty much like the primaries - airspeed and altitude tapes on left and right (respectively), and an attitude indicator in the middle, probably with a heading indication on the top. That is pretty much standard EFIS these days. To keep the airplane upright,the backup looks just like the primary, no matter what brand it is.

Using round dials as a backup means that you have a totally different visual representation of your data, and you have to keep very current on that alternate method of display. So it can be argued that it is easier to maintain currency on your alternates if they are digital/electronic.

I am not talking about all the button pushing, approach setup, and moving stuff - but that's not really what the third source is for.

Paul

g3xpert said:

Walt,

Thanks for sharing your thinking on this. There is plenty of good backup going on in your RV.

We wanted to point out some advantages of your dual ADAHRS G3X system which others may not be aware.

1. The G3X system performs attitude miscompare monitoring when two or more ADAHRS units are installed, so a MISCOMP alert on your PFD is a warning that the system has automatically detected that there is significant disagreement between the ADAHRS units.
   
   Increased fault tolerance and miscompare monitoring are the primary advantages of having dual ADAHRS. Even if you are not monitoring the output of the second ADAHRS, the system is and will let you know if you should perform a manual comparison against the backup instrument to determine which ADAHRS to use on the primary PFD (and by the autopilot).
   
   
2. For a dual display, dual ADAHRS system such as yours, a reversion switch can come in handy on the MFD. With a simple flip of a switch you can display both ADAHRS solutions side-by-side making it easy to compare the G5 attitude with ADAHRS1 and ADAHRS2. The default ADAHRS source selection for PFD1 and MFD1 is ADAHRS1 using the AUTO selection, but you can flip the reversion switch and manually select ADAHRS2 to the MFD before you launch into IFR so that it is immediately displayed when the reversion switch is flipped, or just leave the ADAHRS source selection to AUTO and manually select either ADAHRS1 or ADAHRS2 to either display if you so choose. After selecting the backup ADAHRS for the second display, you can take it out of reversion and use it as a MFD knowing that it is ready to be used as an additional attitude display if you need it.

Thanks,
Steve

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Realize this is an old thread, but was curious if the compare feature applies if I have one ADAHRS and a G5. Also curious if the manual reversion function will work to bring up the G5 AHRS on the MFD screen.

Larry

lr172 said:

Realize this is an old thread, but was curious if the compare feature applies if I have one ADAHRS and a G5. Also curious if the manual reversion function will work to bring up the G5 AHRS on the MFD screen.

Larry

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Larry,
The GSU 25 and G5 do not at this time cross compare the attitude solutions.

The G5 attitude will display in the G3X Touch if there is no valid GSU. It will display in any PFD window in the G3X Touch display regardless of whether the display is a PFD or an MFD with a PFD window configured on.

You could force the G5 attitude to the G3X by pulling the circuit breaker on your GSU(s).

Thanks,
Levi Self

Graham D said:

Walt,
are you forgetting your partial panel training? Just refer to your DG or compass. If youre also turning, trust the AI that agrees with the turn.

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Correct, the manly way. Most panel GPS units also provide sufficient indication of a turn to see a turn occurring. Where it gets tricky is that many EFIS units depend on the primary GPS, and if the "second nav" is an SL30 you may have no such indication (unless like so many of us you use an iPad).

If you ARE an iPad user, for $250 the Merlin ADS-B/ADHARS leaves almost no excuse not to have that as a backup EFIS with iFly.

Walt said:

So how many folks think they could recognize then analyze this situation with their current glass panel and not end up in the same situation?

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GPS doesn't usually lie at the same time as your magnetometer. The problem sounds like complacent cross-check.