RV-12iS EMS and generator operation

kshunz · #1 01-18-2023, 11:40 PM

We have two new RV-12iS aircraft coming soon to our flying club. I've never flown one, but as the Ops Officer I need to understand them.

I believe there's a switch on the upper console marked "EMS Backup Battery". As far as I can tell, there's no backup battery; this switch allows the main aircraft battery to power the ignition in case both generators fail. Do I have that right?

If Generator A fails, Generator B powers the EMS but the rest of the airplane is running off the main battery. If the battery voltage drops too low to run the fuel pumps, the engine stops. Correct?

The POH advises to turn off non-essential electrical equipment with a generator failure. I wonder if that ought to include one of the fuel pumps.

If both generators fail, I assume the engine stops until you flip on the EMS Backup Battery switch and then restart the engine.

These will each have a G3X display, and I understand there's an actual backup battery for that. But if aircraft power is lost, the fuel pumps stop and the engine stops. So this backup battery is just to keep the EFIS running during the final glide? This makes me wonder whether there's something I don't understand.

Can someone who knows this system confirm that I'm on the right track?

Mich48041 · #2 01-19-2023, 04:48 AM

Scott, you have a very good understanding of the operation of the Rotax 912iS operation. Everything that you wrote is correct. The chances of both alternators and the aircraft battery failing on a single flight are remote. Pilots should never takeoff with a weak battery. The battery should be load tested annually. http://forums.matronics.com/viewtopi...r=asc&start=20

thiggins · #3 01-19-2023, 06:06 AM

https://youtu.be/xo2JTX2Rwk8

Here’s a great explanation, with wonderful graphics.
Yes, a 915, but I believe this is identical to the 912.

Piper J3 · #4 01-19-2023, 08:55 AM

Quote:

Originally Posted by Mich48041

Pilots should never takeoff with a weak battery. The battery should be load tested annually.

I installed new PC680 last year (RV-12 w/ 912ULS) and it lasted about 8 months. On the day it failed, engine started as usual followed by 30-minute flight with shutdown. No cranking for restart. Odyssey replaced battery under warranty. Everything looks good....until it doesn't....

kshunz · #5 01-19-2023, 10:33 AM

Quote:

Originally Posted by thiggins

https://youtu.be/xo2JTX2Rwk8

Here’s a great explanation, with wonderful graphics.
Yes, a 915, but I believe this is identical to the 912.

Great video, and if it applies to the 912iS then I think it points out an error in my understanding. It shows the fuel pumps being supplied directly by the generators. With a failure of either generator, you might run the main battery dead but the pumps (and engine) will continue to run. The POH procedures and the EFIS backup battery provision make a lot more sense with this insight.

Thanks!
Scott

sparkcrafter · #6 01-19-2023, 11:47 AM

There is a backup battery. See the attached schematic.

-Jerald

rcarsey · #7 01-19-2023, 12:58 PM

You seem to have a good understanding of how the system works.

Quote:

Originally Posted by kshunz

If Generator A fails, Generator B powers the EMS but the rest of the airplane is running off the main battery. If the battery voltage drops too low to run the fuel pumps, the engine stops. Correct?

The pumps are not powered how you'd probably normally think. Think of them as part of the ECU/EMS. They would continue to be powered by Gen B.

Quote:

Originally Posted by kshunz

The POH advises to turn off non-essential electrical equipment with a generator failure. I wonder if that ought to include one of the fuel pumps.

Definitely leave both running. They're being powered by Gen B (which is now dedicated to running the engine only at this point, after Gen A died). Everything else in the aircraft is now running off battery -- so kill things like landing lights, nav lights, USB power, etc. as safety permits.

Quote:

Originally Posted by kshunz

If both generators fail, I assume the engine stops until you flip on the EMS Backup Battery switch and then restart the engine.

Exactly. Only at this point, would I even think about turning off one fuel pump -- and that would be in some ridiculous circumstance where the next suitable field is far away and I'm trying to conserve as much power as possible to make sure I can get there.

Quote:

Originally Posted by kshunz

But if aircraft power is lost, the fuel pumps stop and the engine stops. So this backup battery is just to keep the EFIS running during the final glide? This makes me wonder whether there's something I don't understand.

If Gen A, Gen B dies, and you activate the EMS Battery Backup switch and restart the engine... Eventually your main battery will die. You'll have the EFIS battery to rely on for the rest of the way down.

You should realize too, that there are other ways in which the EFIS battery may be useful. Suppose there is a short on the circuit which is powering "EFIS" (PFD, MFD, AHARS, Magetometer).. that would take out all the displays. In that case, the EFIS battery could be used to power the PFD/AHARS using a separate power line that wouldn't be shorted.

I also need to mention here that I brought a wiring bug up to the attention of Support that hopefully will be fixed through a SB soon (my proposed fix to them was a very simple 1 hour job, so nobody panic!). In this last case I mention, the EFIS battery actually fails to work at all.

BobbyLucas · #8 01-19-2023, 03:45 PM

Quote:

Originally Posted by sparkcrafter

There is a backup battery. See the attached schematic.

-Jerald

That is only for the EFIS, not the engine.

jrtens · #9 01-21-2023, 10:43 AM

Welcome to VAF Scott. I think you will find that the 12iS is a great little airplane. Where is your flying club?

subpar_bucker · #10 01-22-2023, 01:00 PM

I have a dumb question prompted by a question I was asked by a friend: the 912iS engine's electrical system is powered by 2 generators correct? It sounds like it is possible to install external alternators if desired based upon this video:

https://www.youtube.com/watch?v=vgtq289Wj_Q

I was also wondering if there's a diagram that shows where on the 912iS the generators are located. Google so far has failed me.

When I answered my friend's question they pushed back with, "no new engines are using generators any longer, yours has to have an alternator..."