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Glass panel with no mechanical backup

dbaflyer

Well Known Member
I think this should have been done as part of flight testing, but now that I've got a new G5 and thinking about getting an IFR rating, I thought it necessary to understand what happens in the cockpit when various switches are turned off. Yes there is the theory of your electrical diagram, but then there is the reality of what happens.
I have a dual display G3X touch system with GMC305, GTN650, G5, GTR200 and audio panel. Dual alternators feed the main bus. The G5 has a backup battery and an IBBS keeps the displays, GEA24 and GSU25 powered up.
The avionics buss is feed from the main buss with with an avionics switch and relay. A second essential buss switch feeds the same avionics buss from an always hot battery buss.

Testing out scenarios like:
1. Turn off main alternator
2. Turn off main battery switch
3. Turn off backup battery (IBBS) switch

At this point everything is running very good and powered up. So I work up my nerves to actually turn off the essential buss switch which is going to make my entire panel go dark except the G5 which has its own battery. The single electronic ignition is running off the always powered battery buss so the engine is very happy.

I was doing this over the top of an airport with plenty of altitude so I went ahead and hit the switch. Like planned the panel went dark, but something happened that wasn't expected. Any guesses as to what it was?

My bird immediately went into a right handed turn and did a slight pitch down. Keep in mind before the panel went dark the auto-pilot was flying in both pitch and roll.

At the time I had know idea what jut happened, but now later can speculate that my airplane was not completely in trim and the AP servos were doing some work to keep the plane straight and level. When the servos powered down those forces went away.

If anyone has any other ideas on what could cause this let me know, but it was fairly scary at the time.
 
You need to hook up auto pitch trim, and keep the fuel “balanced”.
(When you disconnect the AP the airplane should stay pretty close to straight/level)
The AP servos are not supposed to be working to maintain trim.
 
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Testing

I would do the test again to confirm what happened, but first manually trim the plane before engaging the autopilot. And perhaps also do the test without the autopilot engaged at all, to prove that you can fly the plane by referencing the G5 and looking out the window.
 
I’ve found that the G3X isn’t quite as resilient under partial panel conditions as I had hoped.

When I first got the panel, we had a bad G5 component, and that caused the G3X attitude to show some really strange stuff. That was fixed easily, but more recently, I had a pitot line connector break, and the G3X attitude was inaccurate without airspeed input as well. This is obviously less optimal than a steam gauge panel where a bad pitot input wouldn’t effect the ADI or HSI.

Bottom line, these panels are great for VFR and light IFR, but know the failure modes before you decide to use them under more serious conditions. Bottom line, I’m not incredibly thrilled with the partial panel failure modes of the Garmin stuff that I’ve experienced so far.
 
Walt, that was my expectation. The G3X auto trim would trim to keep minimum forces on the servos so that when you do loose power to the AP the airplane should fly straight and level or nearly so I'm not sure that's what happened. If you just disconnect the AP with power still to the servos it probably does just fine. Will probably do some more testing at some point but nothing planned in the short term.
 
, I had a pitot line connector break, and the G3X attitude was inaccurate without airspeed input as well. This is obviously less optimal than a steam gauge panel where a bad pitot input wouldn’t effect the ADI or HSI.
.

Assuming you have a data card in the PFD and can remember when that occurred I would suggest sending that data file to the G3X folks so they can look at what happened, it may help figure out why it happened and possible fix.
 
Assuming you have a data card in the PFD and can remember when that occurred I would suggest sending that data file to the G3X folks so they can look at what happened, it may help figure out why it happened and possible fix.

I did have the data card installed. My understanding talking to a few different people at Sun N Fun was that the majority of glass panels (Dynon & Garmin alike) require airspeed for their attitude information.

If that’s not the case, I’d be interested in hearing G3Xperts opinion. I’ll give a shout and report back!
 
Both Garmin and Dynon require a speed - either airspeed or gps ground speed - to reach a stable long term attitude solution. Did you have gps data going into the units? They should have auto-switched to gps data if the pitot data failed.
Or, go with a grt EFIS, which does not require either gps or pitot data to reach a stable AI solution.
 
Walt, that was my expectation. The G3X auto trim would trim to keep minimum forces on the servos so that when you do loose power to the AP the airplane should fly straight and level or nearly so I'm not sure that's what happened. If you just disconnect the AP with power still to the servos it probably does just fine. Will probably do some more testing at some point but nothing planned in the short term.

If the autotrim is working, there is really no reason for the experience you had (assuming rigging is good and you can manually trim to S&L), so I would test it. Set the plane up in an out of trim condition, turn on the AP. After a minute of two turn it off. If the plane is now trimmed, it is set up and working. If not, you need to troubleshoot.

Larry
 
Did some testing this morning starting with letting the AP take over after take off and IAS to altitude and nav mode. Once leveled off and power was set, let the plane fly for a bit. Then disconnect the AP. The plane had a slight climb and right turn but not as dramatic as what I experienced as described in the start of this thread. Even though not as dramatic it wouldn't have take very long to be in a climbing right turn with air speed decreasing.

Then I manually trimmed the plane for straight and level for hands off flying. Activated the AP for awhile. After disconnecting the plane flew straight and level.

I'm concluding that the G3X auto-trim could be relying too much on the AP servos, but there could be other explanations I'm not aware of.
 
Did some testing this morning starting with letting the AP take over after take off and IAS to altitude and nav mode. Once leveled off and power was set, let the plane fly for a bit. Then disconnect the AP. The plane had a slight climb and right turn but not as dramatic as what I experienced as described in the start of this thread. Even though not as dramatic it wouldn't have take very long to be in a climbing right turn with air speed decreasing.

Then I manually trimmed the plane for straight and level for hands off flying. Activated the AP for awhile. After disconnecting the plane flew straight and level.

I'm concluding that the G3X auto-trim could be relying too much on the AP servos, but there could be other explanations I'm not aware of.

I suppose there could be differences across installations. However, in my 10, the G3X does a perfect job of trimming. When I disconnect the AP, it is in perfect trim, regardless of where it started. Sometimes I even turn the AP on for a minute to let it do the trimming, as it does so better and faster than me. The system reads the torque necessary for the servos to hold a given flight attitude and keeps trimming untill the minimum torque is achieved.

If you can manually trim to S&L, the G3x should be able to as well. Suggest that something is a bit off in your installation/configuration.
Larry
 
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