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G3X Bootup if GSU is Offline

iamtheari

Well Known Member
I finally started wiring my panel in earnest. I've kept a spreadsheet for the past year or so, planning every wire. But I have a sanity check for my backup power options.

The system is a G3X Touch two-screen setup, two GSU 25's, one G5, one GEA 24, GMA 245R, GTR 20, GTX 45R, GAD 29, and GTN 650Xi. For power supplies, I have a VP-X Pro and a TCW iBBS. The G5 has its piggyback backup battery.

I had planned that the iBBS would power the PFD, GEA 24, primary GSU 25, and MFD. The MFD made the cut because it could be used to troubleshoot the VP-X if it is alive but not supplying power to the VP-X. My panel also has an alternate trim switch that uses iBBS power and a momentary up/down switch on the panel to operate the pitch trim servo in the event the primary trim circuit fails.

But now I am second-guessing the GSU part of the backup power plan. I have both GSU's on separate VP-X circuits, so I can shut one off if it provides inaccurate data (the failed GSU can be identified by cross-checking the G5, PFD, and MFD). Having backup power to the #1 GSU means that I would have to turn off the backup battery to fully power it down if it is the failed unit.

If both GSU's are offline, the PFD and MFD should get attitude and air data from the G5. So if I do not provide backup power to either GSU, I only have to hope that my main electrical system and G5 don't both die in IMC at the same time.

The question is this: If the G3X PFD and MFD are booted up (iBBS switch on) and the GSU's come online either during or after the bootup (main power turned on after iBBS), will they sync up okay or will the system complain and make me intervene? (Or worse, silently use the G5 instead of the GSU data.)

Other sanity check items for consideration:

1. Should I remove the MFD from the iBBS power? The iBBS would last maybe 75% longer with only one screen to power. I'm not 100% decided on the balance between the power usage vs. losing the MFD in a main power failure scenario and welcome any input.

2. If I provide backup power to the GTR 20 and wire it as comm #1 on the GMA 245R, I will have a failsafe comm radio when the main power goes offline. Will the PFD tuning control for the GTR 20 work okay when the GMA is offline, or are the on-screen controls for the remote radio and remote audio panel too integrated to tolerate that?
 
It sounds like you are planning to fly some hard IFR and want complete redundancy, which is fine. I'm not going to try to persuade you in any other direction but I will share what I decided to do. I looked at these questions as well when designing my panel. First off, the IBBS powers both my PFD & MFD with only one switch. The difference in mine is I'm not a VPX believe... I like the old school circuit breakers. But for you as you stated, you can simply turn off one of them manually via VPX. I decided to use the G5 backup battery so I don't have to worry about the G5 sucking juice out of the IBBS. Finally I decided in scenario where I lose my alternator, and my primary battery dies I will have IBBS and G5 internal battery for PFD/MFD but no radios. So I decided that I would just close my eyes and hope for the best at that point :D ... No I'm just kidding, I decided that having 2nd main aircraft batteries on separate switches would provide additional time to run all of the avionics to get back on the ground and serve as redundancy on the ground for cranking (both batteries shouldn't die at the same time unless you leave both On for a long period of time). Having 2 batteries does add additional weight but also gives me assurance that if things go bad and I lose my Alternator during IFR, I can immediately turn off one of the batteries to save power, shed things that aren't necessary for continued flight, and double my time running off of battery.
 
It sounds like you are planning to fly some hard IFR and want complete redundancy, which is fine. I'm not going to try to persuade you in any other direction but I will share what I decided to do. I looked at these questions as well when designing my panel. First off, the IBBS powers both my PFD & MFD with only one switch. The difference in mine is I'm not a VPX believe... I like the old school circuit breakers. But for you as you stated, you can simply turn off one of them manually via VPX. I decided to use the G5 backup battery so I don't have to worry about the G5 sucking juice out of the IBBS. Finally I decided in scenario where I lose my alternator, and my primary battery dies I will have IBBS and G5 internal battery for PFD/MFD but no radios. So I decided that I would just close my eyes and hope for the best at that point :D ... No I'm just kidding, I decided that having 2nd main aircraft batteries on separate switches would provide additional time to run all of the avionics to get back on the ground and serve as redundancy on the ground for cranking (both batteries shouldn't die at the same time unless you leave both On for a long period of time). Having 2 batteries does add additional weight but also gives me assurance that if things go bad and I lose my Alternator during IFR, I can immediately turn off one of the batteries to save power, shed things that aren't necessary for continued flight, and double my time running off of battery.
Statistically, I'm more likely to have a failure at night than in hard IFR, although in the desolate places I fly there isn't a practical difference between the two.

Have you tested what happens when you turn on the PFD and MFD with the IBBS switch but wait a little while before turning on the ADAHRS?
 
GSU 25 Power

I finally started wiring my panel in earnest. I've kept a spreadsheet for the past year or so, planning every wire. But I have a sanity check for my backup power options.

The system is a G3X Touch two-screen setup, two GSU 25's, one G5, one GEA 24, GMA 245R, GTR 20, GTX 45R, GAD 29, and GTN 650Xi. For power supplies, I have a VP-X Pro and a TCW iBBS. The G5 has its piggyback backup battery.

I had planned that the iBBS would power the PFD, GEA 24, primary GSU 25, and MFD. The MFD made the cut because it could be used to troubleshoot the VP-X if it is alive but not supplying power to the VP-X. My panel also has an alternate trim switch that uses iBBS power and a momentary up/down switch on the panel to operate the pitch trim servo in the event the primary trim circuit fails.

But now I am second-guessing the GSU part of the backup power plan. I have both GSU's on separate VP-X circuits, so I can shut one off if it provides inaccurate data (the failed GSU can be identified by cross-checking the G5, PFD, and MFD). Having backup power to the #1 GSU means that I would have to turn off the backup battery to fully power it down if it is the failed unit.

If both GSU's are offline, the PFD and MFD should get attitude and air data from the G5. So if I do not provide backup power to either GSU, I only have to hope that my main electrical system and G5 don't both die in IMC at the same time.

The question is this: If the G3X PFD and MFD are booted up (iBBS switch on) and the GSU's come online either during or after the bootup (main power turned on after iBBS), will they sync up okay or will the system complain and make me intervene? (Or worse, silently use the G5 instead of the GSU data.)

Other sanity check items for consideration:

1. Should I remove the MFD from the iBBS power? The iBBS would last maybe 75% longer with only one screen to power. I'm not 100% decided on the balance between the power usage vs. losing the MFD in a main power failure scenario and welcome any input.

2. If I provide backup power to the GTR 20 and wire it as comm #1 on the GMA 245R, I will have a failsafe comm radio when the main power goes offline. Will the PFD tuning control for the GTR 20 work okay when the GMA is offline, or are the on-screen controls for the remote radio and remote audio panel too integrated to tolerate that?

Booting up the GSU 25's after the GDU's will not be a problem. You have the ability to select which GSU 25 each GDU is using in flight, but you do not have the ability to tell the GDU to display G5 data. That only happens in the absence of good data from your GSU 25. When the GDU's are using G5 data, we paint an AHRS REVERT CAS message on the PFD so you will be alerted to the condition.

If the GTR 20 and GDU are online, and communicating via CAN bus, you will be able to tune frequencies on the GTR 20. The status of the GMA 245 would not affect this. If the GMA 245 is down, you would not have the ability to switch COM's you would be tied to COM 1.

Thanks,

Justin
 
1. Should I remove the MFD from the iBBS power? The iBBS would last maybe 75% longer with only one screen to power. I'm not 100% decided on the balance between the power usage vs. losing the MFD in a main power failure scenario and welcome any input.

2. If I provide backup power to the GTR 20 and wire it as comm #1 on the GMA 245R, I will have a failsafe comm radio when the main power goes offline. Will the PFD tuning control for the GTR 20 work okay when the GMA is offline, or are the on-screen controls for the remote radio and remote audio panel too integrated to tolerate that?

I have a very similar configuration, but with single battery and single alternator. I have the PFD on the main and emergency busses with the MFD on the avionics buss - avionics buss <off> is my primary load reduction (I can then pull breakers to select and reactivate the avionics buss). Lots of airplanes fly without an MFD - in fact I use my MFD very little and can easy fly IFR without it.

I powered my GTR 20 off my main (and E- busses) with a separate panel switch. This gives me power management (can use the radio, then turn it off in a power conservation scenario). However, the big benefit is I can power only my main bus and Com 1 (GTR 20) on the ground before engine start and with the failsafe mode on the GMA 245, Com 1 is active for the pilot to listen to ATIS/AWOS and contact Clearance delivery before engine start.

Final comment, when the AHRS reverts to the G5 (failed or power off), the PFD annunciates this as <AHRS Revert>, with a master caution alarm.
 
Booting up the GSU 25's after the GDU's will not be a problem. You have the ability to select which GSU 25 each GDU is using in flight, but you do not have the ability to tell the GDU to display G5 data. That only happens in the absence of good data from your GSU 25. When the GDU's are using G5 data, we paint an AHRS REVERT CAS message on the PFD so you will be alerted to the condition.

If the GTR 20 and GDU are online, and communicating via CAN bus, you will be able to tune frequencies on the GTR 20. The status of the GMA 245 would not affect this. If the GMA 245 is down, you would not have the ability to switch COM's you would be tied to COM 1.

Thanks,

Justin

Thank you, Justin. I figured that it would be designed that way but I can't test it until I wire everything. This information is very helpful.
 
Okay, now I'm having some new doubts about what's best. I can't find an answer in the manuals. The G3X Touch Pilot's Guide says:

If the ADAHRS source selection is set to Auto and an attitude miscompare condition is detected, the flight director cannot be engaged until the pilot determines which source of attitude data should be used. When the pilot selects a specific ADAHRS source other than Auto, the flight director and autopilot may be used (or re-engaged) even if the miscompare condition persists.

I checked both the installation manual and pilot's guide but I can't find other references to "source selection" or anything else about selecting which ADAHRS to follow in the event you want to ignore one, due to a miscompare or for any other reason.

I know from previous threads here that the default is for a system with two GDUs and two GSUs to display ADAHRS #1 on the pilot side and ADAHRS #2 on the copilot side, but you can change that and choose an ADAHRS to display on the screen. What is the procedure to do that? Is it two taps on the screen or an hour in the menus?
 
ADAHRS Source Selection

Okay, now I'm having some new doubts about what's best. I can't find an answer in the manuals. The G3X Touch Pilot's Guide says:



I checked both the installation manual and pilot's guide but I can't find other references to "source selection" or anything else about selecting which ADAHRS to follow in the event you want to ignore one, due to a miscompare or for any other reason.

I know from previous threads here that the default is for a system with two GDUs and two GSUs to display ADAHRS #1 on the pilot side and ADAHRS #2 on the copilot side, but you can change that and choose an ADAHRS to display on the screen. What is the procedure to do that? Is it two taps on the screen or an hour in the menus?

It requires one button push to bring up the AHRS Selection options on the PFD. Touch the HSI, which brings up the PFD Options page. The Source Selection settings can be seen in the top left hand corner of the right side of the screen.

Alternatively, Press the MENU key once, which will bring up the same PFD Options menu. You can then use the rotary knobs to change the ADAHRS source selection. Press the Back button to exit the menu. This is a good example of how you can use the knobs and buttons to accomplish the same task, in rough air.

i-jzZMTCp.jpg


Thanks,

Justin
 
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