I have loaded a photo that describes the simple circuit used for starting most of our aircraft engines. When you press the starter button, electricity flows from the positive terminal of the battery (using conventional flow not electron flow here) through the starter switch, through the solenoid coil and back to the negative terminal of the battery. This causes the core in the solenoid to pull the big heavy contacts closed allowing the heavy starting current to flow from the positive terminal through the contacts, through the starter motor and back to the negative terminal, via the chassis in my drawing. When you release the starter button, the spring in the solenoid returns the contacts to the open position. If the contacts in the solenoid weld shut (can happen, google it) then your in trouble because, unlike a car, it is pretty hard to pop the bonnet and disconnect the battery, so you watch your enigne cranking over until the battery goes flat or the starter motor burns out. Now, when the starter button is released the magnetic field in the solenoid coil collapses and thus generates a high voltage (opposite to the applied battery voltage) and that voltage spike, which can be a few hundred volts can wreck things. I cannot see why a diode across the coil, wired the right way is not used to 'short out' the back EMF. Anyway, that's my lot, hope I have been of some use.