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  #1  
Old 11-14-2022, 02:56 PM
snoop9erdog snoop9erdog is offline
 
Join Date: Oct 2005
Posts: 289
Default Avionics master needed?

Hi All. I'm finalizing and wiring up all my Garmin boxes, plus I have IBBS with PFD,GTN625, ARINC, ADAHRS, EMS, and MAGnetometer on the IBBS, most which take minimal amps. Items in an emerg to get down. MFD is on the Garmin Gad 27 Keep Alive. G5 (another contingency) is on separate backup battery.

Startup procedure is to switch on the IBBS backup battery first to check that those items come up and check charge, then Master/Batt/Alt on, so with at least 14.4 V or more the systems runs off main buss. IBBS switch stays on and only kicks in backup once the main batt is rundown/depleted to a min voltage.

So the question is with that startup process is an avionics master even needed? My avionics master is essentially my backup battery switch that powers up most everything, minus my Comm1 because it has no secondary power feed.

Convince me that in 2023 an avionics master is needed. Id appreciate your thoughts
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Last edited by snoop9erdog : 11-14-2022 at 03:17 PM.
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  #2  
Old 11-14-2022, 03:19 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
Posts: 3,363
Default

I think the main purpose of an avionics master switch is to protect avionics from big spikes in voltage that can occur as the engine is starting or stopping. I don't claim to understand what in the system can cause those, but....

An older airplane I had for awhile had a Terra transponder with an internal fuse that would routinely blow if you shut the engine down with the avionics master on. I'm sure it's the case that the Terra transponder was an unusually poor device (a POS really) that was overly sensitive, and very likely that most modern avionics boxes have much better power supplies that are less likely to be damaged, but I still think it is good practice to be able to isolate those systems.

No doubt some EE guys will enlighten us as to why, or why not.
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  #3  
Old 11-14-2022, 03:26 PM
sailvi767 sailvi767 is offline
 
Join Date: Sep 2010
Location: Charlotte NC
Posts: 1,527
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I like to have a avionics master to allow quickly shutting down a large segment of powered items and wiring that could be causing a smoke or fire event inflight.
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  #4  
Old 11-14-2022, 04:01 PM
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MacCool MacCool is offline
 
Join Date: Aug 2020
Location: central Minnesota
Posts: 1,476
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If I turn the avionics master on after I start the engine it will be up to 30 seconds during EFIS boot-up before I can see the oil pressure after the engine has started, so I turn the avionics master on after the Master Switch before startup. I turn it off before shutting the Master Switch off after shut down. I don't know if that's necessary anymore these days in a diode-protected circuit with modern avionics, but I have been told that the big voltage spikes that might harm avionics are more likely to occur with the collapsing circuit as the Master Switch is turned off. Not that they will, only that they could. Used to be mantra, now I'm certainly prepared to believe that it's just legend.

I'm old school, but I guess I'd prefer a separate avionics master switch. It is on my list of things to ask Stein at the next Fairbault fly-in, however.
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Last edited by MacCool : 11-14-2022 at 04:12 PM.
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  #5  
Old 11-14-2022, 04:01 PM
Desert Rat Desert Rat is offline
 
Join Date: Aug 2018
Location: Wichita KS
Posts: 1,103
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My avionics shop is owned by a former Garmin engineer. I had this discussion with him when I was in the design phase. His opinion was that we install avionics masters just because we've always done it that way, not because they are necessary with todays avionics. He was adamant that there is 0% chance of modern Garmin avionics being damaged by being on during startup/shutdown.

He did recommend the ability to reboot anything with a processor in case it locks up, and with Garmin that's pretty much everything.

Based on those criteria, I elected to not have an avionics master. Just an IBBS for AHRS and engine sensors, then breakers for stuff I want to be able to reboot or load shed individually.

I will admit that I was initially resistant to the idea, but after thinking about it for a while I don't really see a downside and it simplifies the system design quite a bit.
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  #6  
Old 11-14-2022, 04:11 PM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Southwest, USA
Posts: 2,602
Default No avionics master

I dont think a avionics master is needed for modern Glass. I have a Garmin panel with the GAD27 box. The box has a power circuit that prevents the critical boxes from browning out during engine start. But I have circuit breakers for the major boxes; they have been useful for hangar power ups when I dont need everything on.
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  #7  
Old 11-14-2022, 04:24 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
Posts: 3,363
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Yes, it seems that the EMS (or EFIS with engine info displayed) is the one box that you want to power up with the main master, not on an avionics master. That way it is up and running for engine start.

Quote:
Originally Posted by MacCool View Post
If I turn the avionics master on after I start the engine it will be up to 30 seconds during EFIS boot-up before I can see the oil pressure after the engine has started, so I turn the avionics master on after the Master Switch before startup. I turn it off before shutting the Master Switch off after shut down. I don't know if that's necessary anymore these days in a diode-protected circuit with modern avionics, but I have been told that the big voltage spikes that might harm avionics are more likely to occur with the collapsing circuit as the Master Switch is turned off. Not that they will, only that they could. Used to be mantra, now I'm certainly prepared to believe that it's just legend.

I'm old school, but I guess I'd prefer a separate avionics master switch. It is on my list of things to ask Stein at the next Fairbault fly-in, however.
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 725
also
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2022
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  #8  
Old 11-14-2022, 04:40 PM
BobTurner BobTurner is online now
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 8,694
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I agree with John. Modern avionics should be well protected against over-voltage, mostly because itís so easy and cheap these days. Iíd be more worried about low voltages - switching power supplies that stop switching, or fluorescent backlighter tubes that keep trying to strike back on - which a GAD 27 type device hopefully prevents.
I have a GRT EIS (engine instruments) box on the main buss, separate from the avionics buss. It does re-boot on engine starts but is so simple electronically that it re-boots in 2 seconds - and shows a big red light if thereís no oil pressure.
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  #9  
Old 11-14-2022, 04:45 PM
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Walt Walt is offline
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,997
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Here we go again, a quick search will give you some different opinions, personally I put avionics masters in all panels I build.
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  #10  
Old 11-14-2022, 04:58 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 4,024
Default

Why would anyone be one broken switch away from a dark panel in IFR?

What I do:
- Half the panel (one EFIS, one Comm, etc.) on one battery. The other half on the other.
- Either side can be powered from either battery - multiple backup modes.
- Engine startup is the left EFIS on (to get EMS data), the right EFIS is off, as well as the touchy GTN-650.
- Engine start is via a master solenoid on each battery feeding the starter solenoid. Easy.

Carl
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