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For the electronic junkies-there is a limit


Roger Lee

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Just installed a Garmin GDL39 (ADSB) and an ACK 406 ELT (uses a GPS signal) to slave them to a Garmin 696, AP in a CT.

 

Here is the warning:

You can add too many devices to the GPS and it will start dropping signals. Three most likely okay, four may be too many.Talked to Dynon and they said the same thing. You can ask too much from a single GPS output source and it weakens the signal to them.  I ask about a remedy and they said add another GPS and divide the devices or buy an RS32 signal booster.

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  • 4 weeks later...

This brings up another issue I have thought about...what is the limit of electro-wizardry you can add to a CT and not overload the rather puny 20A alternator?

 

Clearly the short answer is "20A worth!"...but what does that mean in practical terms, once you account for all the necessary and standard electrical gear? Surely a dual Skyview system, 796, autopilot, iPad, ADS-B box, etc, have to start taxing things...

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The CTLSi has dual alternators.  And a Hi Density Li Fe Po Battery.  Alternator A (220 Watt) powers the ignition.  Alternator B (420 Watt) powers the avionics and recharges the battery.

Yes, all of that is required because if the injected motor loses electrical power, it stops.

 

I'm talking about the single 18-20A unit in the majority of CTs. There must be a practical limit on load, just wondering what it is.

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It's not 20A.  I know what it says, but it is 18A peak and 16A continuous. Many of the CT's with all the electronics are close or at the max continuous now.

 

To help everyone understand: the ALTERNATOR (magneto-generator :P) is rated for 20 amps, but the regulator-rectifier we use is inefficient, and wastes about 15% of the power.

 

The defining difference between "continuous" and "intermittent", is how long the circuits are going to be used. The FAA (may not be the same as an ASTM built aircraft) defines that length of time as 2 minutes in standard aircraft.

 

When we consider aircraft power, we have to plan the circuits carefully. A lot of the dynon panels are rated for ~1.5 amps peak each for the D series, but use on average ~1 amp. Skyview uses almost 5 amps average (including charging battery) PER DISPLAY for the 10 inch series while charging the backup batteries! 3.5 amps when not charging.

 

If you have the old style landing light, that uses a few amps! The LED style, however, use maybe half an amp. Flaps will use a couple amps too.

 

This doesn't include the other little tidbits in our ULS aircraft. So as roger says, we're already pulling an awful lot of power if you have all the bells and whistles, which burdens the poor little ULS alternator decently already. Most aircraft coming off the assembly lines from big name manufacturers incorporate 40-60amp + alternators by default. Even in the 70's and 80's, they had 40 amps from the factory.

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Not true.  The dual electronic ignition system runs off the battery for several hours in case of alternator failure.  In the case of the CTLSi, there are two alternators, each is a backup of the other.  The ECU automatically redistributes alternator output of the remaining alternator to critical functions like the ignition system.

 

No backup battery is needed due to the low level of power needed and the capacity of the battery to keep the engine running in case of alternator failure.  Less than 5v in the case of the 912iS.

It is true, if the injected motor loses electrical power, it does stop.

It is not true that "each is the backup of the other".  There is no backup of B by A, only battery power to instruments in the event of B failure.

Yes there is battery backup if A and B fail: however, the engine does stop, and needs to be restarted after activating the "battery backup switch".

About the running for hours on the battery - procedures say land at next available opportunity. 

For those of us with years of experience, we know enough not to depend on any battery for no more than 30 minutes.

There are inexperienced readers on this forum Cecil.  Perhaps you should put a little more research in before  presenting information as fact.

 

3.2.9. Failure of the ECU power supply.

! If the standard ECU power supply from alternator A fails then the ECU automatically switches over to

alternator B for one time.

Caution: No charging of battery if ECU runs on alternator B! Failure of both ECU power sources (alternator

A and B) results in an engine stoppage!

! If loss of ECU power supply from Generator A occurs switch off unnecessary electric devices. Land at

next available opportunity.

! If both ECU power supplies fail switch on backup battery switch. In this case the engine electronic runs on the aircraft’s battery. Land at next

available opportunity.

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If you have the old style landing light, that uses a few amps! The LED style, however, use maybe half an amp. Flaps will use a couple amps too.

 

 

 

I got an eye opener when I had a battery failure.  We jumped the airplane off at a remote field, and I came home at dusk with the original halogen landing light on.  The flight went normally, but after landing, the flaps would not retract until the landing light was turned off.  Without the battery to provide  some extra power, there was insufficient electrical power to operate both the light and the flap motor simultaneously.

 

I have since gone to a Soraa LED landing light, but I always still wonder about the weak electrical output in our airplanes, especially on long cross country flights where I'm running/charging my iPad and a phone in addition to all the standard gear in the airplane.

 

BTW, by power calculations, the factory 80W, 12V DC light uses 6.66A, while the 11.5W, 12V DC uses 0.958A.  Making that one change saves 5.7A, or 35.6% of the entire 16A power budget!  If you have not made the change to an LED bulb, do it!!!  I now run the landing light all the time, which is at least one small thing to make my airplane more visible to others.

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My old halogen was 100W bulb. I would get low voltage warnings at idle with the alternator operating fine. It is worth noting that the ~16Amps available output from the RR is proportional to RPM and spec'd at max so 5500. at 2400 idle, you'll just get 7Amps or so

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My old halogen was 100W bulb. I would get low voltage warnings at idle with the alternator operating fine. It is worth noting that the ~16Amps available output from the RR is proportional to RPM and spec'd at max so 5500. at 2400 idle, you'll just get 7Amps or so

 

Wow, so a very electronics-heavy airplane will probably be draining the battery idling on the ground, even with the landing light off?  

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Wow, so a very electronics-heavy airplane will probably be draining the battery idling on the ground, even with the landing light off?

Very much so. Just Dynons and radio and transponder, I'm at about 6Amps myself. Running 1800RPM on the ground, I am slowly draining my battery
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I use a silent hektik RR which you can only order from Germany. About half as lossy. http://www.silent-hektik.com/Duc_R_Duc.htm

I also adjust up to charging voltage to 14.2V with the diode so that my battery is always topped off. I use a Shorai LFX18 LiFePO4 battery to save some weight and run the starter better. That would still only carry me for 1hr though if the alternator or RR failed.

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it is NOT true the engine will quit if you lose an alternator.  In fact on initial startup of the engine, only one alternator is operating.  The second alternator does not engage until the engine has been warmed up and the rpm increased to 2500 for a few seconds.

 

The engine will run with both alternators down.  The battery will supply enough current to make that happen.  And it is true that Lane A and Lane B are redundant and the software in the ECU will rebalance if one of the Lanes goes down, or one of the alternators goes down.  The system is setup to eliminate non critical components in the case of a Lane fail. 

 

A CTLSi equipped with a Garmin 796, there is a virtual six pack of steam gauges in the device.  The Garmin 796 has it's own internal battery which will keep the unit running in case of complete Avionics outage.

Who said the engine would quit if you lose an alternator?  I did not.  I just quoted the book that says if the injected engine loses electrical power it stops and that is TRUE. If that happens then you should be aware of how to restart you engine using the backup battery switch.

Rebalance what?  There is no rebalance - If either lane fails, the remaining one powers the engine and the battery powers the instruments. 

Redundant? Nope!

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Not addressing anyone in particular, but...

 

...it's imperative that all pilots fully understand their electrical systems.

 

When I was a Cirrus instructor I was involved with transition training into a Cirrus, or recurrent training of Cirrus pilots.

 

Many were fuzzy on how the electrical system worked - which in an all-electric plane is vital to survival if something goes wrong.

 

I would print out a handful of these (click on image to enlarge):

 

15364442495_12049d7a54_o.jpg

 

With a green and yellow highlighter I would have them trace the flow of electrons in various failure modes until I saw they understood the actions to take in each mode.

 

And, in spite of the redundancy, some Cirrus's did manage to have one ALT fail, followed a few minutes later by the second. Should not happen, but it did. It was then critical to know what was on the Essential Bus and how to minimize battery draw to get on the ground before everything went blank.

 

And then there was the famous, landmark TwinStar accident:

 

http://www.flightglobal.com/news/articles/accident-ignites-da42-engine-row-213371/

 

Worth a read to show how vital it is to understand why POH recommendations can be very important.

 

My point is, spend time with your POH and be sure you understand how your system works. VFR is not as critical as IFR, to be sure, but it still is good to know under what conditions, if any, your engine will stop.

 

And no system is foolproof.

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I use a silent hektik RR which you can only order from Germany. About half as lossy. http://www.silent-hektik.com/Duc_R_Duc.htm

I also adjust up to charging voltage to 14.2V with the diode so that my battery is always topped off. I use a Shorai LFX18 LiFePO4 battery to save some weight and run the starter better. That would still only carry me for 1hr though if the alternator or RR failed.

Specifically, which models did you use?
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