Here's a circuit of what I think is wired up in the plane.
Or at least it's what I think it should be. Even if the diode shorts out, you would still only have 12V on both sides of the solenoid coil. In that case, the engine might not start at all and you'd likely get a little spark in the start switch every time you tried to start the engine.
The back EMF across the coil of the solenoid can become quite high as one end is open circuit when the start switch is released. That's why you should use a diode with a high PIV rating. With the diode in place, that back EMF across the diode should only get up to about 0.7V (the forward voltage drop across a silicon diode), but the coil of the solenoid could generate quite a bit of current through the diode depending on the number of turns and the voltage it tries to generate, so a diode capable of withstanding a high current is best for this application.
Even then, being periodically zapped with a high current pulse, and living in a high temperature environment, it's not unreasonable to expect occasional failures of the diode.
In a pinch if the diode fails at a remote place where it can't be replaced, it would be OK to just remove it temporarily to get you home. Just don't have any sensitive electronics switched on while starting the engine (if you can avoid it). In fact, this last point is important with or without the diode.