I have been wondering, “researching” and experimenting how to make a good electrical distribution setup on my Onex. An electrical setup suitable for modern day delicate instruments and avionics. Very little information exist about this for the homebuilder. All the handbooks and info concerns exclusively about ancient old systems. The days of steam gauges, and where the most advanced electrical system is a solenoid operated master switch. The point is that such an ancient electrical power distribution just isn’t good enough for modern day instruments and avionics, not by a long shot.
More advanced and modern certified GA, such as Diamond aircraft with Austro/Tielert diesel probably have more modern systems, but these engines are unavailable for un-certified use. At the club we just got a Faeta with a Rotax 912 iS. In one of my first flights I had a snag with the EMS, which turned out to be nothing in the end. But, the previous owner came, and we dismantled the panel etc and at the same time he told about the 912 iS. That particular aircraft was one of the first to be equipped with the iS. Atec (the aircraft manufacturer) didn’t really understand how the iS worked, and the first iS’es had lots of snags, and all of them was upgraded to iS Sport (for free by Rotax). The previous owner had therefore been doing lots of wire work and upgrading of components to get everything up to the current standard.
Anyway, the Rotax 912 iS isn’t just a 912 with electronic injection and ignition. It has a fully redundant and modern electrical power distribution system tailor made for modern delicate instruments and avionics. It really is nothing like a 912 ULS at all in this aspect. It consists of:
This means the electric power coming out of the iS is always clean and at constant 12 (13.2) volt. To lose that power:
Even if that happens, there is still the possibility in the cockpit to use “uncleaned” electrical power (in case both power management systems should be dead)
The power management modules are cleverly interconnected with Lane A and Lane B in the ECU. Lane A and Lane B is somewhat similar to Mag 1 and 2 in an old aircraft, but that’s only on the surface. Lane A and B are the two ECU modules, and both of them “fires” all 8 spark plugs. They are tested individually during start up, and both are “ON” during normal operation, but only one of them actually do anything. The one not doing anything takes automatically over if the other stop working.
Installing a 912 iS, you don’t only get a state of the art electronically controlled engine, but also state of the art fully automatic and redundant electric power distribution system, with all harnesses and “black boxes” everything. They are intrinsic parts of the engine, and all data is fed through a double redundant CAN bus. Rotax has thought of everything in the iS. It really is a modern engine, tailor made for modern aircraft with glass cockpit regarding electrical power distribution. I’m impressed.
Looks good. And of course a serious saving of fuel, although that’s more an impressive % than an impressive LPH amount, because these engines sip fuel anyway 
The Thielert/Austro’s designs have a drawback in that you may get ‘ECU-A failure’ or ‘ECU-B failure’ annunciations, and there have been some reports of both ‘failing’. Obviously this creates some stress with the pilot, particularly if it’s a SEP. Nine out of ten these are not ECU failures as such but just a sensor spuriously exceeding a certain limit, after which that sensor signal reverts to a default value and the engine runs happily (although maybe not optimally) along. Once on the ground, a PC needs to be hooked-up, sensor identified, and re-set. Could create a logistics problem.. There have also been cases of the ECU itself failing although not often AFAIK, and there you have the advantage of having two and AFAIUI there is no single point failure that would cripple both ECU-A and ECU-B at the same time.
All I’m saying is that adding complexity also adds the risk of the above nuisance. But let’s hope that Rotax has succeeded to make a more robust electronics/sensor design than Thielert/Austro! Keep us posted about your experience on that front please!
aart wrote:
But let’s hope that Rotax has succeeded to make a more robust electronics/sensor design than Thielert/Austro!
Well, from what I gathered the 912 iS has had it’s share of snags from the very start. The latest (to my knowledge) being the alternator replacement. Being non-certified, it’s up to every owner to keep track on these things, and do the necessary fixes. Nevertheless, the package you get when buying a iS is way beyond anything I have seen. The redundant electric power supply/management (that actually works) is something you simply cannot purchase anywhere, and believe me, I have looked (and tried one of them). Installing a 912 iS, and you get it all. I guess that part is more or less the same BRP use for all their other stuff also ?
How do they produce a clean 13.2V with no battery connected?
How does their alternator and rectifier differ from the standard automotive 3 phase alternator and 3 phase full wave rectifier, as used in GA?
I don’t know the details here, but (as I understood it) there are two levels of how refined the electric supply is. There is the “standard” alternator/rectifier connected to the battery, but there is also voltage regulators from where the EFIS and EMS and other stuff are connected. In an emergency this can be bypassed, getting power directly from that “inner” circuit to EFIS and EMS. The core engine stuff (high pressure pumps, ECU, ignition) runs on the “inner” circuit (not sure about the ECU).
A full wave rectified 3 phase has a lot of ripple but if say you have a 12V alternator you could get slightly over 6V out of it with just a simple linear regulator:
If the ECU can run off 6V that would do. The car ones can because the battery drops to ~50% during starting and the ECU obviously needs to function all the way through.
Or you could get 12V (or anything else) out of it with a switch mode regulator. 12V 20A (240W) is a very small DC-DC converter these days.
Rotax use the Denso alternators which B&C use for the backup products. These have an internal 3 phase rectifier.
It would be curious if they fed that to the battery in the normal way and at the same time had a regulator which is hefty enough to power the whole aircraft but which normally powers just the ECU. That would only give you a smooth supply if the battery went open circuit; there would be no additional redundancy.
What would make a lot of sense, looking at the Diamond engine failures, would be an alternator dedicated to the ECU and not needing any battery at all. I think that would be a requirement for any certified “electronic ignition” anyway.
Peter wrote:
Rotax use the Denso alternators which B&C use for the backup products.
Not exactly. The main alternator is an integral part of the engine. It is mounted directly on the crank shaft on the aft. On the 912 UL it is a single output, and the rectifier is an external unit. On the 912 iS, the alternator is double output (black and grey as they call it), and the rectifier regulator is an integral piece. The “black and grey” outputs go to the “fuse box regulator” with a “black and grey” regulator corresponding to the outputs from the main alternator.
Both the UL and the iS can have an optional external generator (I have never seen one in the aircraft I have flown). It’s called “alternator F3A” in the illustrated parts catalog.
Peter wrote:
What would make a lot of sense, looking at the Diamond engine failures, would be an alternator dedicated to the ECU and not needing any battery at all. I think that would be a requirement for any certified “electronic ignition” anyway.
That is the way jet engine ECU’s are powered in Part-25 aircraft: the ECU has a dedicated generator driven from the engine gearbox as a backup to the aircraft bus feed, so the engine can run independent of all aircraft systems. Also the fly-by-wire engine controls are powered via the ECU and not the aircraft buses. So you can remove all electrical power from the aircraft systems and the engine will continue to run under limited control.
The principle is good until the software in the ECU’s have a certain bug which is replicated in all units on the aircraft and then the engine fail-safes to idle and you lose control of the engines a’la A400M and then…no matter how much redundancy you have you are going down quick…
Antonio wrote:
That is the way jet engine ECU’s are powered in Part-25 aircraft
Actually reading the Rotax manual clears up a bit, and confuses a bit as well. In normal engine operation, one alternator supplies the ECU while another supplies all the other stuff in the aircraft. This happens at idle speed, thus no battery is needed for anything when the engine is running. The start power switch (a rocker press and hold switch in the Atec) connects the EMS (Engine Management System, incl the “fuse box”) to the battery during start. The “backup battery switch” (a normal rocker switch in the Atec) does exactly the same, but stays in place. However, the manual sort of contradicts this at other places. It says if one of the alternators fail, the ECU automatically switches power to the functional one (Black or Gray circuit) in the EMS. If both fails, the engine stops, and the backup battery switch must be used. The manual also say that to “start” generator A, the RPM must be held above above 2500 RPM for at least 5 seconds. So, what these two alternators actually supply, and when, is a mystery.
Then there is the “Lane A” and “Lane B” in the ECU. Both are “ON” during normal operation. When both are “ON”, they are in auto-mode. Only one Lane does any work at any given time, and they fire all 8 plugs all the time. But (according to the manual), when manually switching on one or the other, only one ignition circuit fires.
LeSving wrote:
The manual also say that to “start” generator A, the RPM must be held above above 2500 RPM for at least 5 seconds
Ahh, think I got it. “Generator B” supplies the ECU by default. It “starts” immediately when the engine rotates, doesn’t mention any specific RPM. This is the main power to the ECU (unless the “start power switch” or the “backup battery switch” is pressed). “Generator A” is the one supplying the rest of the aircraft. It does not “start up” before the engine has run above 2500 rpm for at least 5 seconds. After you can run at lower RPM with generator A functioning. Then if Generator B should stop supplying, the ECU switches over to Generator A, and at the same time, according to the manual, the battery is not charged anymore. If Generator A also should stop, then the only option to get the engine running is to switch on the “battery backup switch”.
