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CTLSi Electrical System Questions


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As I believe everyone knows, the CTLSi has a different electrical system than the planes with carbureted engines.  One of the tradeoffs with the "i" version of the plane is that if there were a total electrical failure, the engine stops running.  For this reason, I want to be especially diligent in understanding what happens in various failure modes.  Therefore, a few questions:

  1. Alt A normally services the engine ignition, and Alt B runs everything else, including charging the battery.  When Alt A fails, Alt B automatically services the engine and it stops running the other things, such as avionics, lights, flaps, etc.  Under this scenario, those need to be run off the battery, if they're to be run.  The question I have is if Alt A fails and Alt B is servicing the ignition, is the battery still being charged by Alt B?  The reason I ask this is if there's a failure of both Alt A and Alt B, upon activating an emergency switch, the engine's ignition will run off the battery; if Alt A failed, I'd be concerned about maintaining sufficient battery charge to run the ignition should Alt B also fail.  If the battery weren't being charged in this circumstance, I'd be aggressively shedding load on the battery.
  2. How do you manually activate/deactivate Alt A and Alt B?  In the plane I previously flew, there were designated breakers for each alternator.  Based on the wiring diagram in the POH, and my experience, it appears that Alt A is turned on when the ignition switch is turned on, and Alt B is activated when the "Gen" breaker on the pedestal is pushed in and the engine is initially taken to a minimum RPM. I believe that the "Gen" breaker only impacts Alt B.  Is all this correct?
  3. What's the best emergency procedure for shedding battery load?  For sure if both alternators fail and the engine is running only on the battery, shedding load would be critical.  Further, depending on the answer to #1 above, it would also be important with an Alt A failure.  One way to do it would be to turn off the battery switch, in which case everything but the engine function would be shut down (other than devices' internal backup batteries), and I believe would have the maximum battery duration for the engine.  Alternatively, the battery switch could be left on, and the avionics master could be turned off, along with turning off unnecessary lights.  I would tend to favor the second scenario, as it would make flaps and lights readily available if needed, and if there were a fast need for avionics (i.e. getting landing clearance at a controlled airport) it could be done pretty quickly.  Thoughts?
  4. If Alt B fails, does Alt A take over charging the battery?  My belief, looking at the wiring diagram, is that the answer is no.  If this is the case, if there were an Alt B failure it would also be important to shed load.

Any input on all this would be very much appreciated!


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I like to think of Lane A and Lane B as Computer A and B.  With sufficient altitude you can select Lane A only (with the starter switch) and see if your getting a charge on the meter, then switch the starter switch to Lane B and see if the engine keeps running and check the meter again to see if you are charging.  That should answer your charging question.  Shedding battery load? Turn everything off and pull breakers as necessary after making your radio call.

Bottom line - land as soon as possible. 

Also we need to do lunch sometime. drjefts@juno.com

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Lane A&B are separate systems and if they fail then you have your emergency switch power. There are 4 trigger coils and then the battery for power. If all power and the plane is totally dead which as far as I know has never happened in a 912iS engine then the decision has been made for you. You glide to a landing.

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  • 3 weeks later...

Here are some answers:

Alt A fails and Alt B is servicing the ignition, is the battery still being charged by Alt B?

No - unless you engage the "backup battery" switch.  In this case "B" will charge the battery and run the engine.

How do you manually activate/deactivate Alt A and Alt B

For safety reasons, you can not manually activate or deactivate; however; you can tie each bus together with the "Start Power" which disengages when power comes from "B" (prior to 2500rpm on startup) or with the "Battery Backup" which continuously ties each bus together.

What's the best emergency procedure for shedding battery load?

Turn off all breakers except those critical to the safety of flight, and land ASAP.  For example you might engage the flap breaker for landing if  you need it.  You might power the radio if required to call the tower.  Something real bad must have happened to wipe our both your electrical systems, so land at the nearest suitable field, and be prepared for an engine failure at any time (keep your altitude and note emergency landing areas).

If Alt B fails, does Alt A take over charging the battery?

Again, only if you engage the "Battery Backup" which is a bus tie.  In this case it will - however you will only have 16A to charge the battery, and power the fuel pumps/ECU.  I would only do this is a dire situation, for example: where both your Dynon batteries have discharged,  you might engage the EFIS breaker to land.  In this case, be sure all breakers are off except for what you must have, and be prepared for engine failure when you engage the battery backup.  Perhaps when you are within glide distance of the runway.  Whatever took out B, might just take out A when you tie them together with the backup switch.

Hope that helps,

John Hurst



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