Not knowing much about aviation history, I'm defining "early airplanes" very broadly, as being any wooden-framed propeller plane. Historians, please correct my definition, in your replies.
What altitudes and speeds were these planes used at? What for? What were the most common causes of crashes? How often did the plane survive? How often did the pilot survive? What did the crash sites look like?
It's a broad topic, I know - please feel free to narrow it, based on your expertise.
I have no statistics and I'm no specialist (just an aviation hobbyist), but AFAIK most crashes were caused by stalling the plane while climbing. The stalling speed (i.e. the lowest speed a plane is capable of flight) was very close to the maximum speed of the plane. That means a very narrow margin of error. The engines were unreliable, too. If the engine stopped while climbing, the speed was gone in a blink of the eye, because of the very poor aerodynamics. Another problem was the rotary engine used by many airplanes. The engine itself actually rotated with the propeller for some engineering reasons, but this was essentially a gyroscope which opposed changes of direction (when running) but caused unexpected changes of direction when you changed the RPM. I hope someone more informed than me can continue.