ASA Aircraft Electrical Systems

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[@aborashed5803]

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[@pccaviationscience6769]

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[@Saltlick11]

Very good briefing

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[@ryans.5998]

So, the alternator requires a little electricity in order to "make" enough electricity to send back to the battery? I'm a little confused about the field pathway that you talked about. It seems like electricity from the main bus (which came from the battery) is being sent to the alternator, which then generates enough additional energy to replenish the battery. I was always under the impression that if your battery was totally dead, you could hand-prop the airplane and get it to start, which would then spin the alternator to recharge the battery. Where does that field current come from if the battery is totally drained? I know that the airplane would start because the magnetos are independent of the electrical system, but would this imply that the alternator just wouldn't be able to charge the battery even if the engine was turning?

[@pccaviationscience6769]

Good question. It depends on what you mean by "totally dead" battery. If the battery his truly totally dead, then the alternator would be useless. As you said, with no electricity for the field, you can't make more electricity. Not only that, but when you turn on the master switch the master relay couldn't work, so there would be no way to get the system running. Having said that, if your battery is so low that it won't turn the starter it PROBABLY still has SOME juice left. If you can get the engine running by prop-starting it - and you are correct about the mags being independent - then there is probably enough left in the battery to energize the master relay and provide enough magnetism to get the alternator producing. Once that happens, you have all you need. Now, I've never actually run the experiment with the totally dead (or disconnected) battery scenario. There MAY be some residual permanent magnetism left in the field core (rotor), and it MAY be enough to get things started. The old generators relied on this at startup, in fact. Bottom line from a practical / safety point of view is that if you battery is truly totally dead, you should swap it out for a charged one. If it's low enough that the starter doesn't work, hook-up some external power rather than prop-starting. There are just too many things that can go wrong prop-starting aircraft that isn't designed for it, in my opinion. Unless you've had instruction on the finer points of propping, and unless there is just no other option, it's just not worth it.