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As it was explained to me by Bill at B&C the solenoid on the starter has two
functions. It actuates the bendix drive AND it closes the contacts for the
starter motor. Given the mass of the bendix and the need to close the
contacts quickly and firmly, this solenoid necessarily has a very high
inrush current. It's average current is a couple of amps but its peak inrush
current may be as high as 30 amps. This is why aircraft starter switches fry
regularly.

Why some people use key starter switches is a mystery to me. They are
expensive and unreliable and trivially easy to hot wire. You never see them
in larger aircraft. "Excuse me ladies and gentlemen but the captain informed
me that there will be a delay in our departure until he can find the keys to
our airplane." I replaced the ignition switch with two toggle switches and a
hidden switch. If you are worried about security, someone tried to break
into my IV-P with absolutely no success. The bank vault cabin door resisted
all efforts to pry it open. Two tiny paint chips were the only damage. It
took longer to clean off the finger printing dust the sheriff used than to
touch up the door jam.

The better way to do things is to jumper the solenoid coil back to the
solenoid battery terminal (the big one) and then use a separate solenoid on
the airframe to run the starter. In practice the airframe starter solenoid
is activated by the ignition switch which in turn activates the starter's
solenoid. This keeps the inrush current to the ignition switch down to
around 10 amps instead of 30. Diodes or transorbs across the solenoid coil
provide a current path for the flyback current when the ignition switch is
opened (thus reducing arcing).

My airplane has the master solenoid and the starter solenoid in series so
that if I ever weld the starter solenoid I can interrupt the current to the
starter by turning off the master.

Regards
Brent Regan

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