Originally Posted by Kilgore Trout

Earlier posters have it correct.
Nothing adverse would happen to the aircraft in the situation described in the original post describing the keyed ignition switch being left in the "both" position with the electrical master switch off.

This comes with major caveats described above though.

Important points to take from this thread-

Most (all that I know of besides experimental and certified A/C with electronic ignition systems)(Porsche Mooney?) do not require an alternator or generator driven electrical system independent of magnetos to provide spark to their ignition systems. In other words- all tin cans you are gonna strap on run their spark for ignition from magnetos. Old school. Mechanical. They spin, and provide spark. Many reasons for this going back to early days, Lindbergh times, magnetic theory, and a whole bunch of science you might read up on. In effect you have been gifted with an independent, mechanically driven system which provides spark to those two spark plugs per cylinder which motivate your winged craft around.

Think about it. Your electrical system, ie- battery, alternator (generator), voltage regulator, they drive your starter, lights, radio, flaps (maybe), and a few other odds and ends you don't really need to get from A to B if you are able or wanting to aviate 1920's style.

Like a coupled GPS, or glass cockpit. Don't rely on those little electron driven bastards, okay? For too long anyway if your alternator fails.

Your magnetos, and the wires they run to their associated spark plugs, run your cylinders, your lifeblood over stormy Kansas or North Atlantic waters Charlie L. style.

Which would you rather fail? Alternator, Generator, or magnetos?

Good first flight magneto checks on run ups will save you much heartache. They do fail. They do go out of time. Spark plugs take a dive. FIX IT. OR GET IT FIXED BEFORE YOU VENTURE OUT.

Again, magnetos- physical- think of somebody's grampaw driving a tractor- did he need a radio, nav system, or even an electrical starter? Nope. Old school tractors were single magneto sparked. Crank that model A or International Harvester with a bar. Now we're plowing.

Magnetos. They spin- like your prop. They are physically geared, attached internally through the engine case to your prop. Your prop keeps turning, your magnetos keep burning. Fuel and air. Gap, timing, spark.

Things not to do- never ever turn a prop with important body parts or valued loved ones in the arc. Even with the switch in the off position. EVEN WITH THE SWITCH IN THE OFF POSITION. Remember magnetos are ALWAYS hot unless they are properly grounded. Do you know if they are properly grounded? No.

Never pose for photos with your hand on the prop. Learn about impulse couplings and ground leads. Teach your students, passengers, and yourself about the following thing. NEVER EVER TURN A PROP WITH SOMETHING IMPORTANT IN ITS ARC EVEN WITH THE IGNITION SWITCH IN THE OFF POSITION!

Ugh, I'm done, consult your local trusted A+P for a tour under the cowling.