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Dual G3x with dual AHRS vs adding separate G5

F

fwlarsen

I'm New Here
Thoughts?

We have a couple options:
- two G3x screens with dual AHRS and backup battery

- two G3x screens with single or dual AHRS with backup battery and G5 with backup battery

Have the autopilot and GTN 750.

As an aside, reading 91.205(d).3 it says something about not needing a "gyroscopic rate of turn indicator, except on the following aircraft:
(i) Airplanes with a THIRD ATTITUDE INSTRUMENT SYSTEM"

If we want to fly IFR, do we by reg NEED a backup G5 to the redundant G3x's??

Thanks for the help!
 
Should add mission

Mostly VFR flying (live in AZ) but instrument rated and definitely want IFR capability.
 
Multiple screens with dual GSU 25s was my choice.
Here’s a video from Midwest Avionics on back up instruments.
 
The only major avionics gizmo that failed during my -7A's recent first flight was the GSU 25C ADAHRS, so I was very grateful that I had the G5 as a backup. Those are the only two sources of aircraft attitude I have. I have steam gauges that serve as third-backups for airspeed and altitude.

I got to observe what Garmin's software does when it detects and corrects a conflict between two sources of aircraft attitude: The erroneous artificial horizon (in my case, from the single GSU) gets displayed, and then at some point you get an alert message "ATT MISCOMP". Finally, when Garmin's software magic decides to stop trusting the GSU, you get an alert message "AHRS REVERT", and the display begins using the attitude from the G5 instead. At some point later, the display may start trusting the GSU and switch back to using it. I'm sure this is all documented in the manual somewhere, but it was interesting to observe it first hand.

Now I'm wondering whether having a third AHRS source would serve as some kind of automatic tie-breaker in the Garmin system.
 
fwlarsen said:
Thoughts?

We have a couple options:
- two G3x screens with dual AHRS and backup battery

- two G3x screens with single or dual AHRS with backup battery and G5 with backup battery

Have the autopilot and GTN 750.

As an aside, reading 91.205(d).3 it says something about not needing a "gyroscopic rate of turn indicator, except on the following aircraft:
(i) Airplanes with a THIRD ATTITUDE INSTRUMENT SYSTEM"

If we want to fly IFR, do we by reg NEED a backup G5 to the redundant G3x's??

Thanks for the help!
Click to expand...

G5 with its own, four hour battery is the way to go. It hooks up to the CANbus and keeps you going if the ADHRS fails. (The ADHRS provides mag heading, AOA, OAT, TAS, and wind. In case of failure, you don't *need* any of those).

If you have a G5, you don't *need* the complexity of backup batteries for the G3X.

Dual G3X screens will help if a screen fails, but of course, are no help if the ADHRS fails, so they do not meet IFR redundancy requirements.

I have dual G3X Touch, G5, GTN650 and autopilot and it meets my needs. Then again, one element of my risk profile is that I don't fly IFR above an overcast lower than 1,000 feet, so if the engine goes poop, I have at least a few moments when I pop out of the clouds to look for something soft.
 
I did dual screen, dual GSU, and G5. If I had to give up some of that, I would keep the G5 rather than the second GSU and screen. The G5 is self-contained. If my amateurish wiring all goes bad and the screens can’t find the GSUs on the CAN bus or backup channel, the G5 will still show me which way is up.
 
Ed_Wischmeyer said:
Dual G3X screens will help if a screen fails, but of course, are no help if the ADHRS fails, so they do not meet IFR redundancy requirements.
Click to expand...

What IFR redundancy? Not trying to argue personal safety choices, but by the regs, we aren't required to have redundant ifr systems.
 
They are all good choices. Wallets and panel space seem to rule a lot of these choices.

Just remember if all the bells and whistles **** out, a half full water bottle on top of the glare shield can be used for attitude.
 
fwlarsen said:
As an aside, reading 91.205(d).3 it says something about not needing a "gyroscopic rate of turn indicator, except on the following aircraft:
(i) Airplanes with a THIRD ATTITUDE INSTRUMENT SYSTEM"
Click to expand...

This is a mis-quote. 91.205 says you DO need a rate of turn indicator, unless there are three attitude systems….
Since the faa has ruled that an electronic instrument may replace a ‘gyroscopic’ one, don’t you have a rate of turn displayed? So, none of this (d.3) applies.
 
Webb said:
Just remember if all the bells and whistles **** out, a half full water bottle on top of the glare shield can be used for attitude.
Click to expand...

Since I don’t see a smiley face, I wonder if you’re serious. At the very least this post should be labeled a bad joke, so it doesn’t confuse a neophyte.
 
Using water bottles for attitude reference does not work in IMC and could kill you.

Folks, the advice to use a water bottle as an attitude reference in IMC will not work and could get you killed if you follow this advice. I hate to be so blunt but that is the truth and air safety trumps diplomacy in this case. The forces in flight will cause the water to orient itself to flight g forces and not the gravitational pull of Earth. Fill a bucket with water. Tie a rope around the handle and start swinging the bucket in a circular motion. You will find the forces exerted on the bucket overcome gravitational forces and the water will not fall out of the bucket even when inverted. In an aircraft, the forces of flight will do the same thing as g loads accumulate. The half filled water bottle will orient to whatever flight forces act upon that bottle. Hence, it is a useless attitude reference.
 
I will save all the ugly comments, but to the posters that suggested the water bottle, please observe your bubble level in the T&B during a turn. Given that you referenced that specific part of the instrument to make your point, I will help by pointing out that in a coordinated turn, that bubble level will show as level, even in a 60* bank. That is working on the same principal as the water bottle. An attitude indicator uses a gyroscopic principal, which is NOTHING like a water or bubble level. It is using a very different principal and NOT GRAVITY or G forces, like a level does. I suggest reviewing your private pilot or instrument pilot study materials as it is covered there and every airman should understand these principals, ESPECIALLY if one is relying upon them to potentially save their lives in IMC. If things turn ugly with your instruments in the soup, there is no one to call for help. YOU MUST understand how this stuff works to sort out problems and to know what to trust and how to deal with it and that also includes understanding how the pitot and static systems work as well. This is why partial panel work is such a big part of IFR training.

Just one more example why trust but verify is so critical when using the internet as your knowledge source, especially with safety critical issues such as the one discussed here.

Larry
 
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Webb said:
Posts removed. The water bottle can only show a constant climb or descent.
Click to expand...

No, it can't even do that. In a completely unaccelerated airplane, it can show the attitude. If the water is at the aft end, you have nose high. But you might be in a constant climb, level flight (slow flight), or a constant descent (again, nose high). You can't tell by the water bottle. And even in perfectly level flight, the water bottle will show nose high if you add power and accelerate forward.
 
I feel like an idiot and I know better.

In my temporary moment of insanity, I remember an old instructor 20 years ago saying you might make it down with a bottle and a compass. It just goes to show we sometimes don’t think. I didn’t.

I owe everyone an apology. Thank you for not flaming me too badly. Please let this die. I’ve beaten my self up today more than any of you could.
 
I asked a Garmin engineer that question at Oshkosh. He recommended the G5 as a backup because the programming was completely different than in the G3X. Bugs that may affect one unit would likely not affect the other because of the dissimilar programming. He indicated that was intentionally made that way.
 
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