I have questions regarding how the Rotax 912is/915is EMS power supply system works.

Could it please be explained the difference between start power(terminal2 of conx X3) and backup power(terminal1 of conx X3) in respect of 

how the EMS manages these 2 sources of power.

What would be the effect of selecting or having the  Backup Power selected ON with the engine operating with no faults  on LaneA/B?


Thanks for the assistance.


  • Re: 912is/915is EMS POWER

    by » 4 years ago

    This is going to be long!

    - - -

    For reference, The X3 connector on the Fuse Box has 3 terminals.

    Terminal #1 is the Backup Power connection

    Terminal #2 is the Start Power Connection

    Terminal #3 is the 12 Volt Power Bus Connection

    - - -

    First the easy one.

    BACKUP Power is connected directly to the EMS power bus without any intermediate components.

    If you connect the battery to this terminal, the EMS will be powered no matter what.

    This is what the Backup power switch is for.


    The start power terminal is a little more convoluted.

    Within the Fuse box are two heavy-duty (50Amp) Relays that I will label "A" and "B" for simplicity.

    The A Relay has the function of switching the EMS POWER BUS between the Start Power connection and the "A" Generator.

    The B Relay has the function of switching the "B" Generator between the EMS POWER BUS and the X3-3 12 volt Power Bus Connector.

    Before the engine starts the A relay defaults to connecting the Start Power to EMS BUS.  The EMS comes alive!

    And the B Relay defaults to connecting the B Generator to the EMS BUS.

    After engine start, The A Relay transfers the A generator to the EMS POWER BUS and then the B Relay transfers the B generator out to the  X3-3 12 volt Power Bus Connector to power the Airframe and charge the battery.

    The A gen powers the EMS

    The B gen powers the Airframe


    Now, you ask:

    What would be the effect of selecting or having the  Backup Power selected ON with the engine operating with no faults on LaneA/B?

    Nothing apparent would happen!  However, the A and B Generators, The EMS POWER BUS, and the Airframe and Battery would all be in parallel all happily cross powering each other.

    As long as nothing goes wrong, it will not be a problem.


    Now let's consider a problem, and let's make it really, really bad!  Worst case bad! Never gonna happen in a million years bad!

    BOTH Generators Fail simultaneously!!!

    With everything all connected together, nothing appears to be wrong initially because the battery is still supplying power to everything.

    The EMS BUS is still powered so the engine keeps running.

    The A and B Generator outputs are still showing the 12 volts being supplied by the battery so no Gen Fail warnings are generated to alert the pilot of the failures and the plane flies on until...

    The Battery LOW Voltage Alarm activates as the battery becomes depleted.

    Keep in mind that this engine does NOT have Magnetos and a reliable 12 volt supply is needed to power the EMS and the Fuel Pumps.

    By the time you realize the battery is about to die, you have already used up most of the battery reserve that was supposed to get you on the ground safely.  Your options are now very limited.

    With the START Power and the BACKUP Power switches OFF during normal flight, If either generator fails, you will get an immediate LANE Warning light and still have plenty of time to get on the ground safely.


    A poorly kept Secret Work-around.  . . . If either generator fails, the EMS will commandeer the working generator to keep the engine running leaving you with only the battery to power everything else.

    If only one generator failed and especially if the A Gen failed, the B Gen will be keeping the engine running, but it has twice the capacity of the A gen, so there is considerable unused gen capacity which you can steal some of if you do it early.

    By turning ON the Backup Power Switch you can tap into the EMS BUS to keep the battery charged and power a limited equipment set.

    Maybe the Radio and the Transponder but not the NAV Lights and other non-essential equipment.

    Keep in mind I said "KEEP the Battery Charged" and NOT "CHARGE the Battery".  You need to do this before the battery wants to draw any larger charging currents.

    Keep your eye on the 12v bus voltage.

    If it starts to fall below 12.6 volts, you are stealing too much current. Turn OFF more equipment!


    The question that wasn't asked was...

    If the A relay disconnects the START POWER from the EMS POWER BUS after the engine starts, why is it even necessary to turn OFF the START Power switch after starting the engine?

    Again nothing will be a problem until something goes wrong.  (Isn't it always that way?)

    Remember that once the engine is running, the EMS and the Fuel Pumps are power by the internal generators.  All is well.

    If you happen to be having another really really Bad Day and rollover the plane on landing; when the prop hits the ground and the engine stops, so do the fuel pumps.

    If the START POWER Switch was still ON when the Generators died, when the A Relay drops out, the EMS and the FUEL PUMPS will once again become powered by the battery via the START Power Switch.

    The last thing you want in a rollover is the Fuel Pumps dumping gas at a half-gallon per minute onto a hot engine.


    You can "PLAY" with the START Power and BACKUP power switches all you want and you won't damage anything.

    But for the Safety of Flight, keep BOTH Switches OFF unless there is a specific need to have them ON!

    - - -


    25477_2_RotaxFuseBox.PDF (You do not have access to download this file.)

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

  • Re: 912is/915is EMS POWER

    by » 4 years ago

    Thanks Bill,

    Very clear and informative description of the functionality of the system.


  • Re: 912is/915is EMS POWER

    by » 4 years ago

    Is it designed like this with electrical isolation between vital and non-vital systems because of the risks of power surges or some type of electrical fault killing the ECU or fuel injectors or something? Other engine manufacturers even ones with dual ECU don't do this even if they do have redundant generators. But Rotax seems to be super cautious about sharing electricity between vital and non-vital systems.

  • Re: 912is/915is EMS POWER

    by » 4 years ago

    Dear all

    caution and chicken broth do no harm to anyone.
    If there are switches and there is a clear operating procedure, follow them.
    The good news is that if any one of them is not positioned accordingly, nothing will happen except in cases of failure as explained brilliantly by Bill.

    Just make sure that everything is in accordance with the procedure and in the event of forgetfulness, have the habit of carrying out a new check, after takeoff in a cruise flight situation to be on the safety side.

    Thank you said by: RotaxOwner Admin, Jussi Pesonen

  • Re: 912is/915is EMS POWER

    by » 4 years ago

    Good explanation Bill. I would like to challenge you on one point. You say that if Backup Power is ON and both generators fail, "nothing apparent would happen." 


    In fact, wouldn't there be a drop in system voltage? Before the failure, both generators would be connected to the battery and the main bus, so it seems like the voltage would read whatever the output is of the highest voltage generator. When both generators fail, it seems to me that voltage would drop to that of battery level voltage (rather than generator output voltage).


    Does that sound correct, or am I missing something. If true, it seems to me that makes the voltage readout the best single indicator for monitoring the health of the electrical system.

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