What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

G5 power source question

fixnflyguy

Well Known Member
I have an RV-4 I built with all steam gauges, and recently, I took the leap to a G5 for my primary centered instrument but retained all my other steam stuff for now. I never had a AF Horizon, and now the G5 has me all set. Since the G5 is often used as a back-up, power source seems to be best coming from the main bus the way I see it, and in the event my AVX (com,xpndr,ect) becomes a smoke generator, I want them off-line(AVX bus). Being all that said, I have the G5 wired to the aircraft master side of power, which means the G5 comes alive with the master prior to start of the engine, and I am curious if anyone has had issues that would damage the G5 due to voltage drop and drain during engine crank? I really don't want to physically turn on/off the G5 with every flight cycle. The install manual isn't very clear on preferred/recommended or precautionary choices. How are others wiring theirs?
 
I have dual G5's in my Cessna 170. The attitude indicator G5 is on the master bus and it comes on before engine start and the HSI G5 is on the avionics bus and is not on during engine start. No issues so far (about 2 years).
Both have a backup battery.

Doug
 
Last edited:
possible option if needed?

Wouldn't a TCW back-up battery or a dedicated G5 back-up battery be an appropriate consideration if brown out is a concern?
 
Does the G5 have a battery? If not you may find one useful. It will keep the G5 alive during cranking and if all the magic electric smoke ever escapes. If no battery then the G5 will probably reboot during engine start, it is unlikely to be damaged. I wired mine to come on with the master but I have a battery.
 
I have the back-up battery

Mine has the back-up battery..my only concern is spiking or voltage fluctuation during start-up, and it seems others have wired like mine without any issues. I don't want to fry my new piece of equipment and hear the "I told you so"..Thanks for all the replies from real life experience.
 
I wired my G5 as follows:
Main aircraft power to G5 pin 7 - Aircraft Power 1
TCW backup battery to G5 pin 8 - Aircraft Power 2
I already had the TCW battery installed as part of my G3X system so I did not use the dedicated G5 backup battery.
 
Mine has the back-up battery..my only concern is spiking or voltage fluctuation during start-up, and it seems others have wired like mine without any issues. I don't want to fry my new piece of equipment and hear the "I told you so"..Thanks for all the replies from real life experience.

I don't think that modern avionics are at risk for those kind of voltage drops on startup anymore. I have an avionics master switch, but it's generally on during engine start...and in two years, no issue. My other avionics, including my Advanced Flight EFIS's and my IFD440 likewise are on during engine start, as recommended in the manual.
 
Last edited:
I just went through all this with the design of my all Garmin all glass panel.

Levi Self who owns Midwest Avionics in Lees Summit Missouri designed everything and fabricated the avionics wire bundles. Levi teaches the AEA course that G3xpert is promoting in another thread right now. He's also a former Garmin engineer.

Based on all that I trust his opinion, and when it came to the discussion of needing an avionics master, his opinion was that I didn't need one. I have a backup avionics battery for the PFD and engine stuff, which will prevent a potential brownnout condition during start, but the G5 just comes on with the master and isn't hooked up to any sort of redundant power supply other than it's own backup battery. FYI- if you don't have a backup battery, the garmin boxes have a pinout that also allows for brownout protection during start via a different wiring scheme.

I questioned this in detail due to 30 years of flying airplanes with an avionics master, but he was confident that with modern Garmin avionics, there is no potential of harming them during startup.

I'm a few months from putting this to the test, so time will tell.
 
There are no voltage spikes during engine start. However, there are voltage sags. Transistors made in the 1960s were vulnerable to voltage sags. Thus the warning to shut off avionics during engine start. That warning has been passed along from generation to generation and persists even today. Transistors were improved in the 1970s. They are no longer vulnerable to voltage sags. However, a voltage sag will cause avionics to reboot. I do not have an avionics switch in my RV-12. The radio and transponder and GPS and D-180 have always been on during engine start for the past 12 years and hundreds of hours. Bottom line: Modern avionics will not be damaged if left on during engine start.
-
P.S. I have flown over 300 Young Eagles, so there are an extra 300 engine starts with the avionics turned on.
 
Last edited:
Another good reason for an avionics master is to reduce battery load during start. Why have an additional 10 amp load sucking valuable energy from the battery when you crank the starter. Do you start with lights, strobes, pitot heat on?

How about smoke emanating from your 430 [this just happened the other day to a friend]. If you turn off the master things just got exciting.

Milspec switches/relays are extremely reliable.
Everything is a trade off, pick your poison.

Personally I’m a control freak, I like the ability to turn things on/off.
 
An example From the Cirrus SR22 flight manual, certainly one of the most modern GA aircraft around.

Avionics Power Switch
A rocker switch, labeled AVIONICS, controls electrical power from the circuit breaker panel bus to the Avionics Bus. The switch is located next to the ALT and BAT Master switches. Typically, the switch is used to energize or de-energize all avionics on the Avionics Non-Essential and Essential Buses simultaneously. With the switch in the OFF position, no electrical power will be applied to the avionics equipment, regardless of the position of the master switch or the individual equipment switches. For normal operations, the AVIONICS switch should be placed in the OFF position prior to activating the MASTER switches, starting the engine, or applying an external power source.
 
Cirrus makes a nice aircraft but I wouldn’t copy everything they do. Their electrical systems utilize master control units, field control units, etc. (hopefully I got the names right). Per the COPAs I know, these make for nice operation when they behave; however, they tend to be problematic and expensive. Maintenance shops like to LRU them versus troubleshoot them to add to the pain.

Guessing the OP will keep any related actions simple.
 
Why have an additional 10 amp load sucking valuable energy from the battery when you crank the starter. Do you start with lights, strobes, pitot heat on?
Every plane is different. Yes, I start with everything on. But my Rotax 912ULS starts so quickly that it doesn't matter. As soon as I turn the start key, the engine is running. An observer might notice a quarter turn of the propeller before it is turning too fast to see.
 
Avionics master switches are single point failures that take down all of services powered from the switch if (when) it fails. If nothing else install an alternate supply path.
 
Back
Top