For you folks with all glass cockpits out there, I'm guessing you're powering your EFIS from the essential (backup battery) bus only for startup? This would be to prevent having the EFIS on the same battery as the starter and any voltage fluctuations that could damage it. Seems like you really need a backup battery to power the EFIS in order to monitor oil pressure after startup.

Maybe EFIS on the radio bus (switched off during startup) and EIS Engine monitor on essential bus.


thanks in advance.

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Back when the Wright brothers were experimenting and I was learning to fly, most if not all of the planes I flew had mechanical oil pressure and was taught to only turn on the avionics master after engine start and turn it off prior to shutting down "to avoid damaging the electronics."  However, I recall reading "somewhere on the internet" (how's that for an authoritative source? Ha!) that modern avionics are pretty much immune to the voltage spikes and fluctuations experienced during start and shut-down.  Maybe someone with electronics expertise can chime-in and verify this?

FWIW, I routed my MGL XTreme EFIS and EMS through a Composite Design power panel with its electronic filters and they are powered when my master is on for engine start.  They seem totally unaffected by engine start and I can therefore monitor the rpm and oil pressure (all electronic on the EMS) during and immediately after start.  I put the remainder of the electronics (radio, GPS, transponder, AHRS, magnetometer, ADSB, etc.) on the avionics master on the power panel.  Works for me!



Hi John,

Yeah that was my plan too until I found this in the GRT Sport 10.1 manual:

"If all the buses that power the display unit(s) and AHRS are also used to supply power to
the engine starter, then the display unit(s) and AHRS should be turned off during engine
start-up. This maximizes the current available for the starter and prevents undesirable
voltage fluctuations being applied to the display unit when it is booting up."

I know it says "when booting up" so it maybe a mute point once it's running. I'd rather not damage my very expensive new EFIS though.

I'm also installing the Composite Design Radio Stack Power Panel also. They way they have the breakers arranged (EFIS and ENG data) on the essential bus really drives the need for a backup battery otherwise these things wouldn't be on the Radio master switch like the radio, gps and xponder).  Maybe the best thing to do is confirm it with GRT that once it's booted it's okay to start the engine on the same battery as the EFIS.

curious what others have done too.


Hi Rob, 

In my Corvair powered 750 I have the GRT Sport EFIS fed from the avionics master and the GRT EIS on the master so I have RPM and Oil Pressure displayed on the EIS for startup and then they are also displayed on the EFIS after the avionics master is on and the EFIS boots up. 



Hi Vince,

thanks! I like idea.

If others have something different still please let me know.


Now that I think about it, I usually turn on the master first thing so the EFIS and EMS have already booted before engine start since I usually pause and verify fuel quantity on the EFIS and run through my checklist, so as you say, perhaps it's a moot point.


With Dynon use the internal battery during start.

Internal battery eh, so Dynon prefers to not have the engine started on the same battery either I guess.

Hey Rob, as much as I am aware, Dynon doesn't take a position on the issue.  Practically, I'm not sure it matters to most modern electronics anymore operating in a well designed electrical distribution system.   I have a dual screen D700 setup, each on a separate bus  which are both powered up with a keyed master ( no avionics master).  Like John A, I power up the screens and everything else during my interior preflight and by the time I'm done my requisite checks and ready for engine start, everything has booted, acquired GPS data, etc. and I can monitor start parameters.  Engine start is from a dual EarthX battery setup running in parallel for the start so there is plenty of energy for everything even during the start.  So far so good...

The Dynon Skyview installation schematic recommends at least one display be connected to the master bus so that you can monitor engine parameters during start with the Avionics bus turned off.

Dynon Wiring Schematic

Hi Orlando,

Yeah that makes good sense to monitor the engine during startup. Which is what I want to do but with an all glass cockpit (and according to the GRT folks) you really shouldn't be powering the EFIS solely on the same battery as the starter. There is potential that the EFIS not have enough power which could cause it to reboot. Not a horrible thing if it happens a few times but repeated occurrences could damage it.

Now the EIS Eng Monitor from what I've read is OK to have on the same battery. So perhaps just using it until the engine is started like what Vance has done is a good idea. Or install a second battery  to provide as a second feed to the EFIS. The EFIS will use most of the current from whatever source has the greater voltage (I'm guessing Dynon does this too) but I was trying to keep the weight off and an extra 4 Ah battery maybe a pound or two.

My Dynon D-180 has been on for every engine start over the past 12 years without missing a beat.  

This is from their Users Guide:  "The FlightDEK-D180 requires between 10 and 30 volts DC for operation and has inputs for an external backup power supply and a keep-alive voltage. It is acceptable to have the FlightDEK-D180 turned on during engine start."  I imagine their other products are the same.

Dave G


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