Second Alternator question

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gdball

Second Alternator question

Post by gdball »

If a DA40NG has factory (or other permanent install I guess) electric air conditioning with a second alternator, does that alternator potentially provide a second source of non-battery power if the primary alternator fails?

Its not a deal breaker, but don't totally like the "get down in 30 minutes or else it gets really quiet" that the AFM mentions. Better than other types of failure I know are still possible. I suppose I'm just getting used to the power and ECU dependence. I can almost hear my Dad's reaction were he still around. :)

I've never had an alternator itself fail that I remember, but I have had a belt fail and hope that if the answer to the previous question is yes, that it drives from a different belt. As best I can tell it does off the front, with the original gear driven on the back?

Greg
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Re: Second Alternator question

Post by Rich »

A rational question. While my DA40 alternator continues to hum right along, in the past I've had 3 alternator failures on 2 different airplanes and 1 generator failure at night over NE Arizona. I've never had a belt fail, though one of the alternator failures was the pulley coming off it :scratch:
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Re: Second Alternator question

Post by chili4way »

The alternator that powers the RACC (factory air conditioner) is dedicated to that purpose and does not back up the primary alternator. Each alternator has its own mechanical connection to the engine.

In the DA40NG, the alternator is the primary source of electric power and charges the main battery and the EECU backup batteries. Both the main battery (first) and the EECU backup battery (second) need to be exhausted before the engine "gets really quiet". 30 minutes is what is left when only the EECU backup battery is left and is all that is keeping the EECU and engine operating. (The "Emergency Battery" that is turned on via the guarded switch and is common in all DA40 G1000 systems is a separate system that only powers the emergency portions of the panel.)

The total time you can keep flying in a DA40NG after an alternator failure depends on how quickly you detect the alternator failure (and reduce load) and the condition of your main battery. It's very likely more than only 30 minutes (not that you would want to delay getting on the ground).
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Re: Second Alternator question

Post by Charles K »

Hmmmm - my old lawnmower technology magnetos will keep flying till I run out of gas :)
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Re: Second Alternator question

Post by chili4way »

Charles K wrote: Tue Nov 24, 2020 1:03 am Hmmmm - my old lawnmower technology magnetos will keep flying till I run out of gas :)
True, and we are much more likely to run out of gas than have an electric system failure (Nall report).
gdball

Re: Second Alternator question

Post by gdball »

chili4way wrote: Tue Nov 24, 2020 12:54 pm
Charles K wrote: Tue Nov 24, 2020 1:03 am Hmmmm - my old lawnmower technology magnetos will keep flying till I run out of gas :)
True, and we are much more likely to run out of gas than have an electric system failure (Nall report).
My Dad and Grandfather spent a fair bit of time cleaning points and plugs too - on a number of engine types. :) I know the old tech has its issues and failure points. I should never read Mike Bush's engine books before bedtime. Never live in fear, but if there is something you can reasonably do to minimize either the mechanical OR human mistake...

Thanks guys for answering, even if the engineer in me thinks another buss tie would be logical. Oh, wait, certification issues, sigh. I think the NG is overall a safer engine and system design. Though should I ever have the funds/opportunity to purchase a 42 or 62 I'm not passing it up.

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Re: Second Alternator question

Post by Rich »

Does that A/C alternator run at the same voltage as the main one?
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Re: Second Alternator question

Post by chili4way »

Yes, they are both for 24V systems but are completely independent circuits. Unlike the Diamond twins where the engine alternators load balance/back each other up, the voltage regulators are oblivious to each other. Adding the air conditioner to a twin adds a third autonomous alternator.
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Re: Second Alternator question

Post by Karl »

If the alternator drive belt fails, you will need to land almost immediately.

We had one fail a few years ago on a Thielert engine during climb-out, in the time it took to fly a circuit to land the temperatures were in the red and oil pressures virtually zero, with fluctuating rpm.

The alternator drive belt also drives the water pump.
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