Finalise electrics - whether to have a separate "avionics" bus

The avionics switch sits on the main power bus. The question is what items should reside on the avionics bus. Should I even have an "avionics" bus?

Traditionally, the avionics bus powered the radio and transponder, as these were the only electronics in the aircraft. More detailed and recent Sling schematics (oriented around Garmin) move much more of the electronics/avionics onto the avionic bus.

In the new style the EFIS (GDU) is located on the avionics bus, making it necessary to select avionics on prior to engine start, so that the oil pressure etc can be checked immediately after switch on, and the engine revs set.

In the old style the EFIS is on the main bus, allowing the avionic turn-on to be left until after the engine was started, the traditional usage. The KAI reflects the traditional arrangement, with only the radio and the transponder on the avionics bus.

How should things be arranged? One guide for my Dynon based installation is that a substantial amount of avionics is powered by the Dynon network bus:

• AHAHRS
• EMS220
• Autopilot control panel
• Magnetometer

So they all are powered along with the EFIS (Dynon HDX1100)

The following "avionics" items are powered separately:

• Autopilot servos
• Com Radio control panel
• Com Radio
• IFC-10 Flap controller
• Transponder
• ADS-B receiver
• Flarm Fusion
• Flarm trafficView80
• Backup EFIS (Dynon D30)

The following are also powered separately, but not directly connected with "avionics"

• USB outlets
• Landing and Taxi lights
• Interior lights

On my preflight walk around, traditionally, I need to check fuel, state of battery, so I need the EFIS for that. I also check lights.

After shutdown, I often need to set the flaps.

I also want to be able to operate these with only the master on, without also having to turn avionics on. This leans towards the traditional pattern.

On arrival at the field, I might also wish to keep phones and iPads charged, so this would also suggest having the USB circuits on the main bus.

I might also want to have access to the interior lights

The transponder, com radio, and ADS-B receiver are a natural combination to be on the avionics bus, and they will not have their own switches. The Autopilot servos is the odd-one out, and should have its own switch

The backup EFIS is really only useful in the air, but given the specs call for its own switch an breaker, it might as well be on the main bus

The flarm+trafficview is placed on the avioncs bus, as it mirrors the ADS-B functionality

So the following are on the main circuit (and will each have separate switches and breakers):

• EFIS
• Backup EFIS
• IFC flap controller
• USB outlets
• Interior lights
• Landing lights (incl wig-wag)
• Taxi lights
• IFC-10 Flap controller
• Autopilot servos

This leaves the following as possible candidates for the avionics bus (no separate switches, a single breaker)

• Com Radio control panel
• Com Radio
• Transponder
• ADS-B receiver
• Flarm fusion
• Flarm trafficView80

One issue with the avionics bus as defined here is that it runs over the panel as well as right to the rear of the aircraft for the ADSB-In. This complexifies the possible failure modes. There is a case for having the ADSB-In on a separate breaker. However the same objection also applies to the magnetometer, which is powered from a common Dynon network bus.

So this is what I will do: I will dump the avionics bus, and move everything onto the main bus, but with additional breakers and grouping

"Radio" switch with its own breaker

• Com Radio control panel
• Com Radio

"Transponder" switch with its own breaker

• Transponder
• ADS-B receiver

"Flarm" switch with its own breaker

• Flarm fusion
• Flarm trafficView80

SUMMARY

References Circuit breaker planning - We Build Planes

No avionics bus!

UPDATE 250703:

Further thoughts on the avionics bus. I am going to introduce a backup battery to keep the following alive in the event of a main bus failure

• Com Radio control panel
• Com Radio
• Transponder
• ADS-B receiver
• Flarm fusion
• Flarm trafficView80

• IFC-10 Flap controller
• Autopilot servos
• Dynon autopilot panel (for the trim motors)

and also

• EFIS
• Backup EFIS

I plan to use a TCW IBBS 6AH LiFePO4 backup battery. The EFIS and backup EFIS will be on this backup so in the event of a main bus failure there will initially run normally off the IBBS. Only when the IBBS is exhausted and the above items shut down will the EFIS (and backup EFIS) fall back onto their own backups leaving a more limited panel functionality - a kind of progressive degradition. At this point, trim, radios, transponder, flaps, etc will be lost.