Originally Posted by
RandomPerson8008
I did not thing the magnetos were capable of power anything other than the spark plugs.
This is correct. I think you may be misunderstanding the purpose of the wires going from the starter switch to the magnetos. They are in actual fact
grounding wires. When that wire leading to the mag is pulled to ground (at the switch), then the mag shuts off. This is useful so that a) you can check each mag is functioning individually and b) that if the engine is turned over with the switch in the OFF position (ie. both mags are grounded) the engine won't fire (more of carry-over from when the O-540 lives in a plane).
As such, in the event of both the alternator and battery going out you are very much out of luck regarding any and all electrical systems, and AC's post above regarding what instruments you do still have and how to use them to get on the ground is pretty well spot on.
An extra bit of mag wiring trivia - position R in the starter switch actually connects via a wire to the L mag to shut if off, therefore testing the R mag as it'll still be running, and vice-versa. The general electrical diagram in the RFM is really only a basic guide to the 'interconnectedness' of the systems, and for a true understanding of how it works and where the electrons actually go, you need the wiring diagrams in the Maintenance Manual - but they're not as easy to read!
And one final aside, if your instructors barely have time to answer good technical questions like these, then I'd be considering looking for another school, but that's just my 2c.