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Integration of Rotax 912iS

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Eric Page

Joined: 15 Feb 2017
Posts: 249

Posted: Sat Oct 02, 2021 8:42 pm Post subject: Integration of Rotax 912iS

I'll be using a Rotax 912iS in my Kitfox build.  I'm considering how best to integrate it into a fault-tolerant electrical system architecture.  At first glance, it appears that Rotax has done much of the heavy lifting, but let me describe the engine's systems for anyone who is not familiar:



The engine is controlled by a dual channel Rockwell Collins ECU.  It has dual electronic ignition and redundant electronic fuel injection.  The fuel rail is fed by two electric fuel pumps.  There are two independent built-in alternators, "A" (16A) and "B" (30A), feeding two rectifier/regulators.  The engine's "Fuse Box" and ECU manage these components so that the "A" alternator is dedicated to engine loads and the "B" alternator to battery charging and airframe loads.



The 912iS Installation Manual contains the following relevant notes:

1. If generator A fails, generator B takes over its functions. The airframe electrical components and the instruments will be supplied by the battery. The battery will no longer be charged.

2. If generator B fails, the battery will no longer be charged. The engine still runs on generator A and the instruments will be supplied by the battery.



The operator's manual states: "Failure of both ... power supplies (alternator A/B) results in engine stoppage. Remedy: Switch “ON“ the Battery Backup switch (in this case the power supply is provided by the aircraft battery). Restart engine."



The Installation Manual lists engine electrical loads as follows:

- ECU and Fuse Box: ~1.6A

- Warning lamps (2): maximum 240mA [LED: 40mA]

- Fuel pumps (2): maximum 10A each



Some internet searching suggests that the actual combined running load for the fuel pumps is 7-9A, which seems reasonable, but I couldn't find a definitive answer.  In any case, the Operator's Manual only requires both pumps ON for takeoff and landing.  Let's assume the high end of the Rotax number and say the engine needs 10A in cruise with one fuel pump operating.



My endurance goal is to continue flight for 45 minutes after loss of both alternators (time equal to night VFR fuel reserve).  I plan to use an EarthX battery.  Their ETX900 discharge curve shows (with a bit of interpolation) that it can support a 15A load for ~60 min.  At 80% capacity (the figure that EarthX cites as end-of-life), it still meets my design goal, at ~48 min.



Assuming a maintained/tested ETX900 and an engine load of 10A, I would need to reduce continuous airframe loads to ≤5A if both alternators fail.  I can easily operate the battery contactor, comm radio, multi-function standby instrument and intermittent use of the trim motor and cabin lights within that budget, while saving the landing light for use on short final.



So, I propose the following: Let the Rotax hardware take care of the engine and alternators, and configure the airframe as shown in the attached simplified diagram.  In normal operation, with the Battery Master switch closed and the Battery Backup switch open, the Endurance Bus is powered directly from the Battery Contactor and the Main Bus is powered through a relay with redundant parallel contacts.  If both alternators fail and the Battery Backup switch is closed, the 40A relay is automatically opened, taking down the Main Bus and reducing the airframe to minimum endurance loads.



I don't want to carry the weight or maintenance burden of a dedicated EFIS backup battery just for this extremely unlikely scenario.  Engine start brownout protection will be provided by other means.



I'd love to hear what everyone thinks.  Thanks!



Eric



LINKS TO ROTAX & EARTHX MANUALS:

Rotax 912iS Installation Manual (7.24Mb): https://www.rotax-owner.com/pdf/IM_912_iSeries_ED2_R1.pdf

Rotax 912iS Operator's Manual (36.5Mb): https://www.rotax-owner.com/manuals/OM_912iSeries_ED2_R1.pdf

EarthX Battery User's Manual (1.16Mb): https://earthxbatteries.com/wp-content/uploads/2020/11/ETX_Manual_111017_Y-1.pdf

[ETX900 data on pages 12-13]

Simplified Electrical Architecture Rev B.pdf

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912iS Wiring Diagram.pdf

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Rotax 912iS Wiring Diagram

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user9253

Joined: 28 Mar 2008
Posts: 1931
Location: Riley TWP Michigan

Posted: Mon Oct 04, 2021 5:48 am Post subject: Re: Integration of Rotax 912iS

X3-3 is missing from your schematic.

I suggest that the relay be eliminated.  It is an unnecessary failure point.

Simpler is better.  If it is not installed, it can not fail.

Having 2 buses seems like a good idea, but complicates the electrical system.

If panel switches are arranged in order of importance from left to right, then in the event of alternator failure, the pilot can shut off switches on the right side to conserve battery energy.  There is no need to shut off a whole bus.  The pilot has the option of turning individual loads back on when needed.

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Joe Gores

Eric Page

Joined: 15 Feb 2017
Posts: 249

Posted: Mon Oct 04, 2021 6:45 pm Post subject: Re: Integration of Rotax 912iS

Thanks, Joe.  I appreciate you taking a look at this and posting your schematic.



 	  user9253 wrote: 	
		  X3-3 is missing from your schematic. 	


Yeah, I didn't bother with the Start Power switch since the Fuse Box wasn't shown.  I'll definitely include X3-2 and X3-3 when I flesh out the schematic.



 	  Quote: 	
		  I suggest that the relay be eliminated.  It is an unnecessary failure point.

Simpler is better.  If it is not installed, it can not fail.

Having 2 buses seems like a good idea, but complicates the electrical system.

If panel switches are arranged in order of importance from left to right, then in the event of alternator failure, the pilot can shut off switches on the right side to conserve battery energy.  There is no need to shut off a whole bus.  The pilot has the option of turning individual loads back on when needed. 	


That's certainly true, but the likelihood of relay failure is very remote and I much prefer a hidden relay to a row of additional switches on the panel.  I also like the simplicity of a 1-step response that properly configures the aircraft for maximum endurance.  When the engine quits my IQ will drop by at least half, and the Engine Failure Checklist will already be long enough:



- Best Glide Speed - ESTABLISH

- Landing Site - CONSIDER

- Fuel Quantity - CHECK

- Fuel Shutoff Valve - ON

- Fuel Pumps - BOTH ON

- Lane Switches - BOTH ON

- Backup Battery Switch - ON

- Engine Start Switch - PUSH



The relay module can easily be tested before each flight:



- Backup Battery Switch - ON

- Battery Master Switch - ON

- EFIS Display - CHECK OFF

- Backup Battery Switch - OFF

- EFIS Display - CHECK ON



It could fail two ways, open or closed.  If it fails open on the ground, it will be immediately apparent before flight.  If it fails open in flight, some avionics are lost but there is no jeopardy to the engine.  If it fails closed any time after the pre-flight test, it has no effect unless both alternators also fail (near-zero chance on the same flight), and the failure will be detected before the next flight.  The cost of components is ~$5 per module, so I would probably build two and keep a spare on the shelf.



I don't think shutting off a bus carrying only non-essential items is a major concern.  Losing the EFIS sounds ugly but I'm comfortable flying without engine instruments long enough to get on the ground, I'll still have a fully charged iPad to navigate with, and the comm radio will still be working, so I can get help if needed.



Eric

user9253

Joined: 28 Mar 2008
Posts: 1931
Location: Riley TWP Michigan

Posted: Tue Oct 05, 2021 4:19 am Post subject: Re: Integration of Rotax 912iS

It looks like the relay will be energized whenever the engine is running.  If the engine quits, you will turn on the backup switch.  So the relay will still be energized and depleting the battery that is needed to keep the engine running.  Or is my reasoning wrong?

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Joe Gores

yellowduckduo(at)gmail.co
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Posted: Tue Oct 05, 2021 5:14 am Post subject: Integration of Rotax 912iS

Figure 3.18: Wiring Powerside (optional)

in the install manual shows backup power bypassing the battery contactor 

which seems like the preferred architecture to me.



Interestingly I recently had a battery contactor go high resistance.  It 

would run the panel and lights but instantly went open circuit when the 

starter was engaged. Looked like new except the internal copper parts 

had a black coating.



Ken



On 05/10/2021 8:19 AM, user9253 wrote:

 	  Quote: 	
		   



 It looks like the relay will be energized whenever the engine is running.  If the engine quits, you will turn on the backup switch.  So the relay will still be energized and depleting the battery that is needed to keep the engine running.  Or is my reasoning wrong?



 --------

 Joe Gores





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=503416#503416

Eric Page

Joined: 15 Feb 2017
Posts: 249

Posted: Tue Oct 05, 2021 6:58 am Post subject: Re: Integration of Rotax 912iS

 	  user9253 wrote: 	
		  It looks like the relay will be energized whenever the engine is running. 	


Correct.  As long as the Backup Battery switch is open, the P-channel MOSFET is turned on by the pull-down resistor on its gate, allowing current to flow through the relay coil.



 	  Quote: 	
		  If the engine quits, you will turn on the backup switch.  So the relay will still be energized and depleting the battery that is needed to keep the engine running.  Or is my reasoning wrong? 	


No, turning on the Backup Battery switch applies ~12V to the MOSFET gate, turning it off, which stops current flow to the relay coil.  I've confirmed that there's no current path from pin X3-3 (normal power input) to pin X3-1, so there's no voltage on pin X3-1 in normal operation.



HOWEVER...  It turns out that pins X3-1 and X3-2 are shorted inside the Fuse Box, which makes sense given that the Backup Battery and Start Power switches do essentially the same thing.  This means that closing the Start Power switch will open my relay, which is definitely not the behavior I want.



Thanks for making me think this through, Joe.  VERY helpful!



Eric

user9253

Joined: 28 Mar 2008
Posts: 1931
Location: Riley TWP Michigan

Posted: Tue Oct 05, 2021 9:39 am Post subject: Re: Integration of Rotax 912iS

There might be voltage on X3-1 when the engine is running if using EMS Ground as the reference point.  That should not disable the transistor which uses airframe ground.  Isolated grounds make it confusing.

_________________
Joe Gores

Eric Page

Joined: 15 Feb 2017
Posts: 249

Posted: Wed Oct 06, 2021 4:21 am Post subject: Re: Integration of Rotax 912iS

 	  yellowduckduo(at)gmail.co wrote: 	
		  Figure 3.18: Wiring Powerside (optional)

in the install manual shows backup power bypassing the battery contactor which seems like the preferred architecture to me.



Interestingly I recently had a battery contactor go high resistance.  It would run the panel and lights but instantly went open circuit when the starter was engaged. Looked like new except the internal copper parts had a black coating. 	


That diagram shows several changes to the basic installation, including use of the RS Flight Systems EMU.  I presume you're meant to pick-and-choose the features you prefer, and modify the basic installation accordingly.



I'm not sure I understand the point of wiring the Backup Battery switch to the hot side of the Battery Contactor.  I presume it's intended to protect against a failed-open (or, as in your case, failed-high-resistance) contactor.  However, consider how and when the Backup Battery switch is used: both alternators have failed, which the Operator's Manual says causes engine stoppage.  The procedure says to close the Backup Battery switch, restoring power to the engine's electronics, then restart the engine.  If the Battery Contactor has failed, then the starter won't turn and you're no better off having the Backup Battery switch connected to the hot side of the Contactor than to the switched side.



I suppose if you have enough altitude you could dive the airplane and hope to restart the engine by windmilling, but you're sacrificing precious time/altitude/distance if it doesn't work.



Eric

Eric Page

Joined: 15 Feb 2017
Posts: 249

Posted: Mon Oct 10, 2022 11:16 am Post subject: Re: Integration of Rotax 912iS

Reviving this thread from a year ago: http://forum.matronics.com/viewtopic.php?t=16777418



I've fleshed out my electrical system architecture diagram and would appreciate another round of checks by the group.



Changes since my simplified diagram was posted last year:

- Renamed Main Bus to Airframe Bus.

- Eliminated the complicated module to switch power to the Airframe Bus, in favor of a simple relay-and-switch arrangement.

- Added an Endurance Bus with dual feed paths.

- Added an Overvoltage Protection Module in the path from the Rotax Fuse Box X3 Pin 3 to the Battery Bus.

- Developed wiring diagrams for the Airframe and Endurance Buses.



In addition to any problems you find, I have two specific questions:

1. The Rotax 912iS Installation Manual specifies 30A fuses at the Battery Bus, for the 12AWG wires to the Backup Battery Switch and the Fuse Box X3 Pin 3.  Can/should 16AWG fusible links be used instead?

2. Is my use of fusible links acceptable for the Endurance & Airframe Bus feeders at the Battery Bus and the hot side of the Battery Contactor?



Related links:

Previous simplified diagram: http://forum.matronics.com/download.php?id=48574

Rotax 912iS wiring diagram: http://forum.matronics.com/download.php?id=48575

LTC1696 data sheet: https://www.analog.com/media/en/technical-documentation/data-sheets/1696fb.pdf



Thanks!

Overvoltage Protection Relay Module (Rev B).pdf

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Electrical Architecture - Kitfox 5 - Rotax 912iS.pdf

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Electrical System Architecture Diagram

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Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Tue Oct 11, 2022 9:58 pm Post subject: Integration of Rotax 912iS

 	  Quote: 	
		   Hi Eric,

  

 I am not familiar with a Rotax installation, so some of my comments may not apply.

 Here are some things to review:

  

 1) There is a 22AWG powering the endurance bus feed relay coil, direct from the battery, but not marked *. Perhaps add a fuse?

 2) There does not appear to be a power feed to the relay coil of the airframe bus feed relay.

 3) Perhaps add an indication of the start relay engaged, in case it sticks?

 4) Perhaps add a 15A fuse at the battery bus to the endurance bus feed (14AWG not marked with *).

  5) Perhaps add a 15A fuse at the battery bus to the airframe bus feed (14AWG not marked with *).

 6) I didn't find the backup battery anywhere, but perhaps it is fed from the Rotax fuse box?

 7) I dislike inline fuses. There are two inline 1A fuses. Why not make just include them in the fuse box for the battery bus?

  

 Nicely done diagram. Easy to follow with lots of good notes.

  

 Jeff Page

 Dream Aircraft Tundra #10

 

  

 ________________________________  Message 3  _____________________________________Time: 12:17:02 PM PST USSubject: AeroElectric-List: Re: Integration of Rotax 912iSFrom: "Eric Page" <edpav8r(at)yahoo.com (edpav8r(at)yahoo.com)>Reviving this thread from a year ago: http://forum.matronics.com/viewtopic.php?t=16777418I've fleshed out my electrical system architecture diagram and would appreciateanother round of checks by the group.Changes since my simplified diagram was posted last year:- Renamed Main Bus to Airframe Bus.- Eliminated the complicated module to switch power to the Airframe Bus, in favorof a simple relay-and-switch arrangement.- Added an Endurance Bus with dual feed paths.- Added an Overvoltage Protection Module in the path from the Rotax Fuse Box X3Pin 3 to the Battery Bus.- Developed wiring diagrams for the Airframe and Endurance Buses.In addition to any problems you find, I have two specific questions:1. The Rotax 912iS Installation Manual specifies 30A fuses at the Battery Bus,for the 12AWG wires to the Backup Battery Switch and the Fuse Box X3 Pin 3.  Can/should16AWG fusible links be used instead?2. Is my use of fusible links acceptable for the Endurance & Airframe Bus feedersat the Battery Bus and the hot side of the Battery Contactor?Related links:Previous simplified diagram: http://forum.matronics.com/download.php?id=48574Rotax 912iS wiring diagram: http://forum.matronics.com/download.php?id=48575LTC1696 data sheet: https://www.analog.com/media/en/technical-documentation/data-sheets/1696fb.pdfThanks!Read this topic online here:http://forums.matronics.com/viewtopic.php?p=508051#508051Attachments: http://forums.matronics.com//files/overvoltage_protection_relay_module_rev_b_221.pdfhttp://forums.matronics.com//files/electrical_architecture__kitfox_5__rotax_912is_171.pdf

Eric Page

Joined: 15 Feb 2017
Posts: 249

Posted: Wed Oct 12, 2022 8:27 am Post subject: Re: Integration of Rotax 912iS

 	  Tundra10 wrote: 	
		  1) There is a 22AWG powering the endurance bus feed relay coil, direct from the battery, but not marked *. Perhaps add a fuse? 	


That wire (and the one on the Airframe Bud Feed Relay) will be about an inch long, just jumping between two pins of the relay.  I've marked them with a *.



 	  Quote: 	
		  2) There does not appear to be a power feed to the relay coil of the airframe bus feed relay. 	


Oops!  Good catch.  Fixed.



 	  Quote: 	
		  3) Perhaps add an indication of the start relay engaged, in case it sticks? 	


Good idea.  That signal can be fed to a general purpose input on the Dynon EMS-221 for display on the EFIS.  I've added that to the diagram.



 	  Quote: 	
		  4) Perhaps add a 15A fuse at the battery bus to the endurance bus feed (14AWG not marked with *).

5) Perhaps add a 15A fuse at the battery bus to the airframe bus feed (14AWG not marked with *). 	


Those 14AWG bus feeders are protected by 18AWG fusible links at the Battery Bus.  That's one of my questions: is this an appropriate use of fusible links?



 	  Quote: 	
		  6) I didn't find the backup battery anywhere, but perhaps it is fed from the Rotax fuse box? 	


"Backup Battery" is the inelegant name that Rotax uses for the switch that provides power from the Battery Bus to the ECU, via the Fuse Box, in the event that both internal alternator/regulator systems fail.  The electrical system has only one battery.



 	  Quote: 	
		  7) I dislike inline fuses. There are two inline 1A fuses. Why not make just include them in the fuse box for the battery bus? 	


Funny you should mention that, as I don't like them either.  They're very difficult to service, but I couldn't immediately think of a better solution.  Unless one of the experts here tells me that I'm abusing the purpose of fusible links, the Battery Bus will be a stud block like this...



https://www.ebay.com/itm/151830064157



...to connect multiple ring terminals.  Perhaps a small 4-position ATC fuse block like this...



https://www.ebay.com/itm/314111877309



...mounted next to the Battery Bus would be a better option for those fuses.  If I am abusing fusible links, then the whole Battery Bus can be a fuse block.



 	  Quote: 	
		  Nicely done diagram. Easy to follow with lots of good notes. 	


Thanks, Jeff!  I appreciate you taking a look.



An updated diagram is attached to this post.

Electrical Architecture - Kitfox 5 - Rotax 912iS - v2.pdf

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user9253

Joined: 28 Mar 2008
Posts: 1931
Location: Riley TWP Michigan

Posted: Wed Oct 12, 2022 9:25 am Post subject: Re: Integration of Rotax 912iS

Eric,

I admire your ability to design your own over-voltage protection circuit. Can it be designed to just open the relay without blowing a 1 amp fuse?



Your Skyview must not have a backup battery since there is a brownout booster.



Is the autopilot switch momentary in both directions, center off?



Airframe bus shed: 

I believe that one of the 3 relays inside of the Rotax fuse box is used for that function.  No external control is necessary.



 	  Quote: 	
		  Those 14AWG bus feeders are protected by 18AWG fusible links at the Battery Bus. That's one of my questions: is this an appropriate use of fusible links? 	
  Yes it is.  Having fuses in series is not a good idea, even if they are different sizes.



Big Mistake:

Regulator "B" ground lug should NOT be grounded to the airframe.

When running, the Rotax iS engine has an isolated ground.



Suggest that the battery bus and airframe bus be combined into one bus called the MAIN BUS.  Eliminate the relay.  Replace the 12 position fuse block with a 20 position.



Two switches can be reduced to one.  See attached picture.

Rotax 912iS SCHEMATIC.png

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Joe Gores

Eric Page

Joined: 15 Feb 2017
Posts: 249

Posted: Wed Oct 12, 2022 5:05 pm Post subject: Re: Integration of Rotax 912iS

 	  user9253 wrote: 	
		  I admire your ability to design your own over-voltage protection circuit. Can it be designed to just open the relay without blowing a 1 amp fuse? 	


Oops, that's another mistake. I meant to power that module through a resettable circuit breaker instead of an inaccessible fuse. I believe that's Bob's recommendation anyway.



 	  Quote: 	
		  Your Skyview must not have a backup battery since there is a brownout booster. 	


Correct.  I don't want the expense or maintenance burden of a backup battery.  All I care about is keeping the EFIS alive during engine starts.  It's a VFR-only airplane.



 	  Quote: 	
		  Is the autopilot switch momentary in both directions, center off? 	


Yes.  That little circuit is lifted directly from one of Bob's designs: http://www.aeroelectric.com/Reference_Docs/Misc_Pdf/AP_Disconnect_B.pdf



 	  Quote: 	
		  Airframe bus shed: 

I believe that one of the 3 relays inside of the Rotax fuse box is used for that function.  No external control is necessary. 	


Well, sort of.  If the B alternator fails or is switched to engine duty to replace a failed A alternator, the Fuse Box will cut power to the rest of the ship.  However, since the Airframe Bus can get electrons from the battery, the Fuse Box switching won't cut it off.



 	  Quote: 	
		   	  Quote: 	
		  That's one of my questions: is this an appropriate use of fusible links? 	
  Yes it is.  Having fuses in series is not a good idea, even if they are different sizes. 	


Excellent, thanks.



 	  Quote: 	
		  Big Mistake:

Regulator "B" ground lug should NOT be grounded to the airframe.

When running, the Rotax iS engine has an isolated ground. 	


I think you might have the A & B regulators mixed up.  The engine normally runs on the A regulator.  The Rotax diagram shows the "Regulator A Plate" tied to "EMS Ground" and the "Regulator B Plate" tied to "Airframe Ground."



 	  Quote: 	
		  Suggest that the battery bus and airframe bus be combined into one bus called the MAIN BUS.  Eliminate the relay.  Replace the 12 position fuse block with a 20 position. 	


I thought about that, but I designed the Airframe Bus to power everything that I don't need to continue flight to a safe landing.  By separating the buses, I can maintain the Backup Battery function and feed the Endurance Bus, but I'm able to shed all unnecessary items by throwing one switch.



 	  Quote: 	
		  Two switches can be reduced to one.  See attached picture. 	


Yeah, that would definitely work.  From a human factors perspective (simplicity and reduction of confusion), I'd prefer to keep the Backup Battery and Start Power functions separate.  Switches are cheap, and the odds of both failing on the same flight are about nil.



Thank you, Joe!

Eric Page

Joined: 15 Feb 2017
Posts: 249

Posted: Wed Oct 12, 2022 6:03 pm Post subject: Re: Integration of Rotax 912iS

Diagram updated as follows:



- Added annotation describing two 30A fuses at Battery Bus (Littelfuse MIDI 498 bolt-mount).

- Removed 1A inline fuse to EMS voltage sense pin and replaced with 24AWG fusible link and 20AWG wire.

- Removed 1A inline fuse to OVP Module and replaced with 24AWG fusible link and 20AWG wire to panel-mounted 1A circuit breaker, then 22AWG wire to OVP Module.

- Added OVER VOLT TRIP circuit breaker to switch panel depiction.



10/21 EDIT:

- Revised annotation describing ENDURANCE BUS ALTERNATE FEED switch.



Posting this time without 2nd and 3rd pages to reduce file size.

Electrical Architecture (Rev K-AEC).pdf

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Last edited by Eric Page on Fri Oct 21, 2022 10:39 pm; edited 1 time in total

user9253

Joined: 28 Mar 2008
Posts: 1931
Location: Riley TWP Michigan

Posted: Wed Oct 12, 2022 6:18 pm Post subject: Re: Integration of Rotax 912iS

 	  Quote: 	
		  I think you might have the A & B regulators mixed up. The engine normally runs on the A regulator. The Rotax diagram shows the "Regulator A Plate" tied to "EMS Ground" and the "Regulator B Plate" tied to "Airframe Ground." 	


OOPS, my Big Mistake.  You are right.

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Joe Gores

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