Load analysis and Garmin dual feeds

[ZAM]

Much to my surprise, load analysis isn't as boring a feared. Plugging load data into various schematics and playing with various scenarios is good for the mind. There's a couple of things I'm working through that I'd appreciate thoughts on. (Chatgpt was no help)

1. In a split bus scenario, if the Garmin dual feed LRU's are split for redundancy purposes, how would you do a load analysis? example: If you feed the audio panel POWER 1 pin from a LEFT bus and POWER 2 pin from a RIGHT bus, what bus would get the "normal load"? Whatever side has the higher alternator voltage? Is this a Schrodinger's cat situation?

2. Is there any good articles, threads or resources about best practices for deciding what is "continuous"? Example: Should the flaps and the comm transmitter be counted as "continuous". I assume not, but hey, I've assumed wrong wrong before.

 

[DerekS]

Higher voltage wins - for a diode-or circuit all the current will be from the side with the highest voltage at any given time.

From a load analysis perspective, though you have to assume all the load on both sides, since by design that what will happen if the higher voltage side drops off.

The ACs for certification tend to be really dry reads, but some good stuff in there.. e.g. AC 21-38 from new zealand - lots of stuff in there for big airplanes but you can read through that for smaller ones.

https://www.aviation.govt.nz/rules/advisory-circulars/show/AC21-11-AC91-23

Derek

 

[Walt]

Items like the audio panel and GTN, are not Power input 1 and 2, those should be fed from the same supply bus (same voltage), usually accomplished by splicing a larger gauge wire into the smaller 22ga wires. The purpose is because the smaller high density pins cannot handle the full load with just a single pin.

 

[ZAM]

Items like the audio panel and GTN, are not Power input 1 and 2, those should be fed from the same supply bus (same voltage), usually accomplished by splicing a larger gauge wire into the smaller 22ga wires. The purpose is because the smaller high density pins cannot handle the full load with just a single pin.

Thanks Walt. This is an important point. Once you see it, it seems pretty clear, but getting there took me longer than I wished. Although I never found an official "rule" for labeling and identifying the differences in LRU inputs, Garmin seems to very specifically and consistently identify when inputs are "diode/OR".

To expand on Walt's point for anyone new to this idea:

from the G3X install manual:

"The GDU 37X can operate using power from one or both inputs (AIRCRAFT POWER 1 AND AIRCRAFT POWER 2). The pins are internally connected using diodes to prevent current from flowing between the two power inputs. AIRCRAFT POWER 2 is for connecting to an alternate power source, such as on aircraft with two electrical buses."

and from the GMA245 install manual:

"The GMA 245 has four pins for aircraft power bus inputs. Use one wire for each of the pins connecting to the aircraft power and ground. Do not splice the power and ground pins at the unit and use only one wire to aircraft power and ground." The table underneath labels the inputs as pin 8 "AIRCRAFT POWER" and pin 9 "AIRCRAFT POWER". I understand this to mean that pins 8 and 9 are simply joined together for the purposes of higher power input.

 

[ZAM]

and further down the rabbit hole...

Higher voltage wins - for a diode-or circuit all the current will be from the side with the highest voltage at any given time.

Which leads me to consider, for the dual bus scenario, if it really matters which source is the "normal" provider. IF and only if the answer is "yes", are there sensible ways to encourage the use of one source over the other? Off the top of my head, setting one alternator slightly less than the other might do it...but I see other issues with that.
(edit: I guess on further thought, the total load of all the dual feed/Diode -OR LRU's is only about 5ish amps, moving this discussion into more of an academic discussion)

From a load analysis perspective, though you have to assume all the load on both sides, since by design that what will happen if the higher voltage side drops off.

Probably the easiest and simplest way to go, and makes my question above irrelevant.

Also, thanks for the link to the load analysis document. Very helpful. This lead me to a google hit on a similar doc from Australia.

 

[Avanza]

Depending on your mission VFR/IFR and the level of safety, the electric system may be built differently.
Failure of one instrument or radio is, under most circumstances, not a big deal. If you lose the one alternator or battery, this raise it to the level of an emergency. Flying at night/ IFR or having electronic ignition, fuel injection require more of the electrical system.
In this case one can build it as two parallell systems with two buses.
Each bus having one alternator and a battery. If you choose to install a power switch between the buses you can have both system fed from one alternator and the other from a standby alternator. If the normal alternator fails no action is required. The standby alternator will pick up the load and signal low voltage. If you open the power switch between the buses you
have two separate electrical systems.
This is what I have. I also have an emergency battery to the G5 and one for
the electronic ignition.

Good luck

 

[DerekS]

Which leads me to consider, for the dual bus scenario, if it really matters which source is the "normal" provider. IF and only if the answer is "yes", are there sensible ways to encourage the use of one source over the other? Off the top of my head, setting one alternator slightly less than the other might do it...but I see other issues with that.

If you can connect the two buses together (without or-diodes) in your architecture, setting the primary alternator a few tenths higher makes the system more defined and is a very common practice. If you are running on battery, the battery with the higher voltage will win, but no promises which one that would be.

One other consideration, if all of your equipment is diode-or'ed together a sufficiently high voltage on either bus could take out all of your equipment. Its a less common failure than no voltage but still happens. By separating devices and not using the diodes you increase the odds you will lose one of them, but decrease the odds you'll lose both. Not really an option when the busses are tied together all the time, but if you are building a dual bus, its one more thing to think about.

Derek