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Rotax 912iS fuseboxes fuses

rcarsey

Well Known Member
I was recently troubleshooting an alternator-out/intermittent issue. In the process of tracing things, I examined the Rotax fusebox mounted on the firewall. During building, I never bothered to open it or even look inside.

Below is a photo of what I found. The engine has 3 hours on it. The two fuses pictured, (I believe, though haven't consulted Rotax manuals yet) protect Regulator A and Regulator B output. It's not clear to me what would have happened if both fuses fell out simultaneously.

I have pushed them back in (with minimal force I may add), and added generous beads of hot glue to make sure they don't come out. I would advise all owners to take a look through the protective frosted plastic next time they have the top cowl off.
 

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If the grip on fuse terminals is not strong enough to prevent a fuse from falling out,
then the grip is not strong enough to provide a good electrical connection either.
 
Don't think I'd use hot melt glue for anything on the firewall.

Or for anything at all important elsewhere on the airplane.

Dave
 
You might try putting a slight twist in the fuse blades. When I was a young Navy nuke reading the the core tech manual they discussed the tendency of control rods to vibrate in the coolant flow. It was solved by “mechanical bias”, a fancy term for bending to get a good interference fit. Hey, if it was good enough for Rickover you could at least try. Fuses are cheap.
 
I was recently troubleshooting an alternator-out/intermittent issue. In the process of tracing things, I examined the Rotax fusebox mounted on the firewall. During building, I never bothered to open it or even look inside.

Below is a photo of what I found. The engine has 3 hours on it. The two fuses pictured, (I believe, though haven't consulted Rotax manuals yet) protect Regulator A and Regulator B output. It's not clear to me what would have happened if both fuses fell out simultaneously.

I have pushed them back in (with minimal force I may add), and added generous beads of hot glue to make sure they don't come out. I would advise all owners to take a look through the protective frosted plastic next time they have the top cowl off.
Thanks for the heads up Rob. I’ve never thought of opening that up.
 
I had the same problem after my first flight on Feb 6th 2020. It took me quite a while and a lot of sweat to find the (frustratingly simple) cause of the problem. See https://vansairforce.net/community/showthread.php?t=180536

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Thanks. I'm not sure how I had missed your prior post. I reported my (and your) issue, along with our photos to Van's Support so that they are aware of it being more than a one-off builder issue.

I'd think they would want to consider taking action if more than 1 builder is having fuses fall out (in a fusebox that doesn't get opened/inspected routinely).
 
It does not help that the 'window' is opaque, and that you cannot inspect the innards of the fuse box without removing the lid.
 
Perhaps you could put small piece (strip) of foam on inside of lid. When lid is in place the foam will press against fuses to hold them in place. Removal of lid allows complete unobstructed access.
 
Perhaps you could put small piece (strip) of foam on inside of lid. When lid is in place the foam will press against fuses to hold them in place. Removal of lid allows complete unobstructed access.

This could be done, but if the mechanical (friction) connection of the fuses is not enough to keep them from vibrating out, then I would not trust it to be a good electrical connection either.

If anyone else with a RV-12iS experiences this problem please use the notification procedure outlined in the Maintenance manual to let us know the details.

One thing I would suggest to anyone that experiences this is to immediately recheck the pitch of the prop blades to confirm the pitch angle is within .1 degree of each other, and then get a dynamic prop balance ASAP. It is possible that the grip friction of all of the fuse sockets is acceptable but the one moved was a bit lower than the others which allowed excessive vibration to displace the fuse.
 
It does not help that the 'window' is opaque, and that you cannot inspect the innards of the fuse box without removing the lid.

Actually the cover has a clear portion aligned over the row of fuses that allows for pretty easy inspection of whether a blown fuse indicator LED is lit or one of the fuses is displaced.
 
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I think the fuse issue was from before installing the engine. I pushed it back in place and had no problem with it since (but do check it once in a while!).
 
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One thing I would suggest to anyone that experiences this is to immediately recheck the pitch of the prop blades to confirm the pitch angle is within .1 degree of each other, and then get a dynamic prop balance ASAP.

This is a good point. Immediately after I solved my electrical problems, I started to do the dynamic prop balancing. The initial measurement was 0.26 ips @ 337. I think this is in the "not horrible, but needs improvement" range.

Now that the engine has a few hours on it, I will re-check the pitch before commencing the balancing process (and potentially start chasing my tail with blades that are pitched all wonky).
 
So my intermittent alternator problem has returned. I hesitate to even say it was ever fixed. I did take everyone's advice and twist the fuse blades slightly.. and replaced the hot glue with a bead of high-temp red RTV.

I've e-mailed Support on this one, as I'm out of things to try. I made a short video that shows you what I'm seeing.. I still have a feeling that the AN525 that was rolling around under the HIC module has something to do with this.. However, I inspected it and the traces on the module and they look great.. and the couple resistors and MOSFETs on the board seemed to test out OK too.. though I lacked the equipment at the hangar to do a proper test of the chips.

https://www.youtube.com/watch?v=ihHIc_CMAFc
 
I was not aware of the operating behavior of the regulators -- dissipating unused power. I'm glad that you brought that up.. its counterintuitive if you ever start experiencing regulator problems in flight.. turn more equipment on!

I may be on the wrong train of thought here.. but I think both regulators are operating OK. When first starting the engine, the battery is used, of course. Main Bus Power is supplied to the engine over Pin 2 of the EMS Fusebox X3 connector (the connector with the 3 thick white wires).

But once started, the fusebox switches the ECU over to Generator B/Regulator B.... and during that time, B is unavailable to supply aircraft loads. I'd also imagine that this Pin 2 wire should then get de-energized, as its not needed anymore.

Once you hit 2500RPM for a few seconds, then the ECU gets switched over to the smaller Generator A/Regulator A... and B becomes available for aircraft loads.

If Regulator A were having problems, the ECU can move back over to Generator B (and deprive the aircraft its power), but that is only allowed to happen ONCE during the flight. engine is allowed to switch the ECU back over to Gen B, but ONLY one time.. and its stuck there permanently until shutdown. If regulator B were having problems, I'd expect the engine would die quickly after a start attempt (or immediately switch it over to Gen A) ... which doesn't seem to be the case.
 
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