Aircraft Starting Magnetos

Dual magnetos are described here. They allow the driver to select whether the spark plugs are fed with HT from a magneto or from a battery/coil system. The magnetos involved have a second set of points with their operating cam for use with the battery/coil system and also arrangements to allow the selected HT source to use the same rotor arm and distributor cap to feed a single set of spark plugs.

The same type of arrangement for the shared rotor arm and distributor cap are also used on many aircraft magnetos. They don't have the second set of points and cam fitted. In place of the battery/coil system, the alternative HT source comes from a starting magneto which is described here.

An aircraft starting magneto is designed to be turned rapidly by hand to produce a continuous stream of sparks. The front of the magneto includes a spindle fitted with a small crank handle  There would be, typically, a 5:1 gear between the armature and the spindle so that the armature turns 5 times faster than the handle.

A starting magneto is needed because the mechanic trying to spin the prop, wouldn't be able to swing it fast enough to get a spark out of the main magneto. The handle on the starting mag is turned by a second mechanic and is used to feed a continuous stream of sparks to a dual magneto fitted to the aircraft engine. Once the engine starts the pilot would switch over to the normal magneto and the starting magneto is then redundant until it's time to start again. Some aircraft magnetos only have one HT output. This was fed into the starter terminal on the distributor section of the engine’s main magneto. So only one source of spark on the starting magneto but this was then distributed to however many cylinders/plugs the engine had by the main magneto's distributor. Some starting magnetos have two HT outputs. These were used on engines fitted with two main magnetos - one starting magneto output to each of the starting terminals on the two main magnetos.

Some aircraft dual magnetos make use of a special rotor arm similar to that shown in this picture. The rotor arm has two independent paths, one for the magneto HT coming from the inside of the magneto, the other for the starting magneto HT coming from the distributor cap. The two paths feed along two fingers on the rotor arm to independent spark gap terminals. The starting magneto HT is fed to the trailing terminal which results in some automatic retard to the ignition timing. This makes starting easier with less chance of kick back. When switched to the main magneto system, the HT is fed to the leading rotor arm terminal for normal advanced ignition timing. 
Note that there is a 'spare' finger on the rotor arm. That's used when the rotor arm is used in a magneto designed for running in the opposite direction as shown in this pair of Simms FST12RG-5SB magnetos. These magnetos are used on a twelve cylinder engine with two plugs per cylinder. It is just possible to make out an arrow at the top of the centre finger indicating the direction of rotation of each magneto. In addition, at the bottom of the rotor arm, the letters 'EX' and 'IN' are marked. This indicates which magneto is used for the plugs located next to the exhaust valve and which magneto is used for those next to the inlet valve.