Question Avionics Circuit Breaker Switch Fail

[lrobichaux]

@Cactus Charlie , again really appreciate the discussion.

1) Understood, so far so good with my GDU.

2) Very good to know.

3) Very good comments. I think I'll upsize to a 15A CB switch and monitor.

4/5) OK I'll take the bait. In your opinion what are the critical "single points of failure" in the CC IFR panel? I definitely intend to fly the plane IFR and am interested in what modifications can be done to add redundancy and address failure modes? Adding a second Avionics CB switch seems like a good start, but if I'm gonna go down this path working with an avionics shop to make changes, I'd like to address everything that I can in one trip to the shop.

6) Excellent observation, I'm researching now. If it turns out that the GMA 245R has "pass through" capability and Com1 can be wired to that pathway, then I'm definitely going to look into redundant power for the GNC 355.

All this discussion of failure scenarios has also convinced me that I need to come up with a solution to be able to keep a hand-held radio within reach at all times. Currently, by hand-held is in my flight bag in the aft cargo area ... pretty much useless in an actual situation when it's needed.

 

[Cactus Charlie]

I had not intended to limit single point failures to the avionics. No single point failure of the avionics is an immediate threat to my safety even in IMC.

If I had to pick an avionics concern it would be Garmin's current design for reversion to alternate attitude data. It takes well over 10 seconds for a reversion to G5 data if the GSU 25C malfunctions and provides misleading attitude data. Keep that G5 in your scan.

Yes, a handheld radio is a useful backup but only if the headset can be connected to it.

 

[lrobichaux]

Agree, my hand-held has a direct LEMO plug receiver.

 

[lrobichaux]

@Cactus Charlie I've confirmed that the GMA 245R has a "fail safe" path that provides for a pass through of COM1 when the unit is deenergized. Thus, your idea of providing backup power to the GNC 355 is a great one. What's the process for doing that given that it's not designed to accept a second power source?

 

[Cactus Charlie]

Backup power for GNC 355:

First decide on your backup power source.

If you decide on a second IBBS the implementation is quite easy. The IBBS can be configured to pass power through when it is available and to switch to backup power when external power is not available. Only a single power feed from the new IBBS to GNC 355 would be required. (Note that the existing IBBS installation does not work that way as pass through power is not implemented).

If I added a second IBBS it would be used for emergency ignition. I consider the existing AGM emergency ignition battery to be inadequate and plan to use main battery for ignition as long as possible keeping the AGM battery as a desperate lost resort. Perhaps you need 3 IBBS to reach your desired comfort level.

Solutions that do not use a dedicated Avionics IBBS may be harder to implement. You don't want to use MAIN bus or battery direct to power the GNC 355 unless it is a manually switched reversion. GNC 355 should not be powered during engine start.

The existing IBBS will not enable backup output unless IBBS switch is on and the charging input is lost (MAIN bus). It doesn't know or care if Avionics bus is live. However, it is possible to force the enabling of IBBS backup power simply by pulling the IBBS circuit breaker.

 

[lrobichaux]

Not likely enough room for a new IBBS behind the instrument panel. Will it work to install a new (third) IBBS in the aft avionics bay, run wires up to the panel, charge the IBBS from the avionics bus, and connect the back-up power to the CB driving the GNC 355? Would the new IBBS require its own switch? Related question, if I'm going to this trouble to dig into the panel, does it make sense to upgrade the existing IBBS batteries from 3AH to 6AH? I realize there's a cost to this, but it may be worth it to double the time available to work through an electrical failure.

Pic of panel layout below. Really appreciate this discussion.

 

[Cactus Charlie]

I took another look at the TCW IBBS manual. It requires pass through power input pins to use the same source as the charging input. Since you want GNC 355 to be on Avionics bus for normal operation it means that the new Avionics IBBS must be charged from Avionics bus. This puts up to 2.5 A additional load on the Avionics bus switch/breaker.

If also means that GNC 355 would receive power from the Avionics IBBS whenever it was enabled and Avionics bus was not live. You would have to, by procedure, only enable Avionics IBBS after you would turn on Avionics bus.

Its easy to go down a rabbit hole.

 

[Cactus Charlie]

We doubled.

Do you already have 2 IBBS?

 

[lrobichaux]

Already have two IBBS batteries. One for the (R) IGN electronic ignition (see switch / CB on the left) and one IBBS that presently drives only the GDU 465 (best I can tell) activated by the IBBS switch top row and having a 5A circuit breaker below the G5. These are identical 12V 3AH batteries.

 

[Cactus Charlie]

Is the ignition IBBS now a CubCrafters option or it is something you specified yourself? Either way, much better than the AGM battery.

My IBBS discharge test showed over 2 hours operation of G3X touch system. I see no need for longer but a periodic discharge test is required to confirm capacity.

edit to add - Standard IBBS does not just drive the GDU 465. I gave the list of LRU earlier in the thread.

 

[Neal]

I think IBBS is only installed on Experimental. I don't think xcubs get it.

 

[lrobichaux]

I think IBBS is only installed on Experimental. I don't think xcubs get it.

It's my understanding that the IO-390-D3B6 (marketed by CC as the IO-393) is used in both the experimental and Part 23 certified XCubs and NXCubs. This installation has dual electronic ignition, with the dual IBBS set-up described above.

 

[lrobichaux]

Is the ignition IBBS now a CubCrafters option or it is something you specified yourself? Either way, much better than the AGM battery.

My IBBS discharge test showed over 2 hours operation of G3X touch system. I see no need for longer but a periodic discharge test is required to confirm capacity.

edit to add - Standard IBBS does not just drive the GDU 465. I gave the list of LRU earlier in the thread.

Ahhh gotcha. Forgot to answer your first question. The dual IBBS / electronic ignition / panel layout is all standard set-up from CC.

 

[Cactus Charlie]

The Avionics switch breaker protects the link from MAIN Bus to the Avionics switch. That link is wire MB11A12R which should be 12 AWG. If the Avionics switch /breaker was replaced by a plain switch then a short at the Avionics bus bar would likely blow the Main bus 40 A fuse if the link didn't burn out first.

This link is shown as AB42A18 (18 AWG) in the G3X schematic.

Before changing any breaker ratings check the actual installed wire size. The available schematics may not reflect the actual build configuration.

Pete at CC told me the actual Avionics breaker rating is 10A and it is not increased for the IFR option.