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Thread: 912is wiring discrepancies...has anyone noticed this?

  1. #1
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    Default 912is wiring discrepancies...has anyone noticed this?

    I've been scratching my head a bit this weekend trying to finish up my wiring. I've noticed that if I compare the wiring schematic shown in the rotax manual to the one kitfox provides there is a glaring discrepancy. I'll get into detail - but in short - kitfox has a single fuse on the 12v input to the switch protecting both start options (normal start power and backup start power). The Rotax manual has individual fuses to each on the 12v.

    Let me talk out loud here to provide more detail/context. But first, I'm no expert, so maybe it doesn't matter?

    Here's the Rotax manual bit (cut out the stuff that doesn't matter to this discussion). It's figure 3.14.

    RotaxManual.jpg

    Here, the first big difference is they are using two switches rather than one. Lets ignore that for now. If we break it down a bit, lets first look at the S2 circuit. Here we have F2 connected to 12v which then feeds into X3-3 and switch S2 when on (momentary) completes the connection of X3-2 to X3-3 (or X3-2 to 12v to be more exact). The way I see this is that when we are holding on switch S2 we are getting the 12v required on X3-2 for starting the engine, once we let go and the engine (eventually) switches alternators, X3-3 will be providing the charging and bus current back to the system, with the F2 breaker protecting it all. If F2 blows for any reason you would loose the alternator to your main bus and charge as well as the ability to start the engine using S2.

    So now lets assume F2 is blown. We could use S3 to pull 12v from the battery to start the engine using X3-1. I would assume here that the use of X3-1 also tells the ECU not to switch alternators and only to keep essentials running on the first alternator. (Even if that's not the case it doesn't matter here).

    Now we clearly see the opposite here too. If F1 was blown, you can still use S2 through F2 to start the engine.

    Lets now look at the Kitfox schematic. I hand drew it out to resemble more closely how Rotax drew it out (but should be the same - hopefully I didn't screw that part up).
    20191118_201343.jpg

    Lets look at this and try to compare it. First if we look at X3-3 output back to the main bus/12v it's protected with a 30a fuse F2, which is the same as Rotax. Next, lets compare the S2 Rotax circuit. The Kitfox schematic we see that the DPDT switch connects X3-2 to 12v through a second (F1) fuse, which then loops back to X3-3 through F2. I think electrically this bit works the same, you just now have two fuses.

    Now if we look at S3 from Rotax. The Kitfox diagram has X3-1 connect through the switch to 12v which is protected by F1. On it's own, this is the same as Rotax. (Side note, Rotax uses two 30A fuses, not a 30A and a 20A. For simplicity I show both as 30A here).
    So far so good. The problem that I now see is that if F1 in the kitfox diagram blows you have no way to use either start function and if F2 blows you can still use both.

    Thus, my concern is what issues or possible problems could arise with the Kitfox diagram as is? I suspect there are some very good reasons Rotax has things protected that way, but nothing I can find says why (surprise). I did talk to somewhere here in Canada who is forwarding this question to the higher ups.

    Finally, not being an expert, the only way I can see to use a DPDT switch and have the same protection and usage as Rotax is shown below. You need a third 30a fuse and need to move the existing F1 in the kitfox diagram. Basically you have fuses on both the X3-1 and X3-2 lines rather than just the power input to the switch.

    20191118_201358.jpg

    Now maybe this is all for not and there is no problem - but I don't like not knowing and just blindly following because it's been done before.

    I'd love to hear inputs and thoughts from those with 912is installs or those with perhaps electric engineering backgrounds.

    Thanks
    912 IS
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  2. #2

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    Default Re: 912is wiring discrepancies...has anyone noticed this?

    Well, here goes. Pin three of the relay box connector is the output of the alternator. F2 is there to protect the alternator form shorts on the 12v buss. The connection shown on the Rotax diagram at pin 3 was just a easy place on the diagram to show that the start power get voltage from the buss through the fuse F2. It is true that if F2 were blown there would be no start power, Hence one of the reasons for the backup battery switch, which gets power from the battery through F1 and also connects EMS ground to aircraft ground so that the computers can see the raw battery voltage. My SS7 is wired almost as shown the Kitfox firewall forward manual (document 64913.500. I built my kit in 2012/2013 and have 280 hr. on it now. I had available a extra battery so I modified the wiring slightly to include a "standby battery" with a guarded switch type circuit breaker . I am attaching a diagram of my system. I do not suggest that it should be used as it adds complexity to the system with more options in the event of an electrical emergency.
    Attached Files Attached Files

  3. #3
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    Default Re: 912is wiring discrepancies...has anyone noticed this?

    Thanks for the input. Looking at your diagram, from what I can see, you are also doing as I suggest as the "fix". You have your alternator on a breaker (the x3-3 connection at the fuse box), your start switch power has a breaker and then so does your backup power switch. Now it's also on it's own backup battery, but ignoring that it would be as i suggest - i think.
    912 IS
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  4. #4

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    Default Re: 912is wiring discrepancies...has anyone noticed this?

    To clarify; Normal operation , when starting the engine the standby battery ckt brkr is off. All others except the electronics are on. With start power switch held on, press starter button- engine springs to life. After airborne and have determined that aircraft is in fact flying then turn standby battery switch ON to top off standby battery, This can be observed by reference to the load in amps of the B alternator on the EFIS and the voltage level of that battery (volts 2 on the EFIS). when topped off, standby battery ckt brkr to off. Problem operation; a typical issue might be, in the event the main battery volts was decreasing to a unacceptable level and even might have popped the Alternator ckt breaker, the remedy would be to turn the standby battery on and isolate the main battery by turning the master switch off and then see if the B alternator is meeting the load after checking the alternator ckt brkr. There are other situations which would require a different sequence of switch actions. My check list includes several of them .As I said earlier , I am not advocating every one should configure their system as I have. I am comfortable with it after 20 years of avionic repair followed by 20 years of marine electronics repair and I understand the system. In the heat of an airborne calamity it could be very confusing to decide which switch to put where.
    ,

  5. #5
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    Default Re: 912is wiring discrepancies...has anyone noticed this?

    My diagram below is obviously not as in-depth as your proposal but may help you to see how the Eurofox guys do it in the UK for a 912iS. They've done many aircraft and know their stuff. Instead of using a Start Power switch, a one minute timer relay is used instead.

    The key switch is, effectively, the battery master switch. First position of the switch gives me the 'avionics bus' (the avionics CBs are powered). When ready to start the key switch is selected to the second position. This does two things: (a)it 'adds' the main bus to the battery (so now we have avionics bus and main bus powered and (b)it starts the one minute start timer. The start timer just does the job of the Start Power switch and then, after one minute, reverts the system to how it would look if you turned off the Start Power switch. The key switch is left in the second position for the rest of the flight.
    Attached Images Attached Images

  6. #6
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    Default Re: 912is wiring discrepancies...has anyone noticed this?

    Thanks. It's good to see things that are different from the "factory" defaults.

    I don't have any reply from Rotax yet and don't expect one anytime soon.

    That said, there's something very very simple that I just didn't clue into until about 5 minutes ago while driving the kids to school. Even though the Kitfox schematic only has a single breaker to the switch - that's fine. Here's the obvious (now - only after way too many hours thinking about this) reason. The fuse is only going to blow if the switch is on (either start or backup). If it does, you can simply place the switch in the off position and reset the breaker. Magic, now you can try the opposite start function with my "third" fuse. Sometimes overthinking things and not seeing the simple bits can be frustrating. This is one of those times.
    912 IS
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