I know for a fact that aeroplanes were used at the last stages of the first world war to capture images of the enemy trench system and supply lines. They were also used to alert the artillery how far off they were to an enemy trench. However, I have also read some sources stating that aeroplanes in World War 1 were both unstable and sometimes didn’t have enough fuel to stay on the air for long. It was apparently a very dangerous job as they could suddenly drop at any second. But, in terms of figures, how often would the engines stall/ fail causing the pilot to crash in mid-air? And approximately how long could a pilot stay on the air?
Here are some rough specs for common planes (as I said in the comment, the endurance depends on various factors).
Sopwith Camel (BR)
SPAD S.XIII (FR)
Albatross D.III (GER)
Fokker Dr.I (GER)
Siemens-Shuckert D.III (GER)
Bristol F2.B (BR)
So we see that our average endurance for a combat mission was 1:30-2:00. Cruise times ranged between 3 and 5 hours on average.
Mid-air engine failures were around 5-10% per plane (not per flight). I've calculated this from two numbers: number of aircraft built and non-combat related deaths. For example, 5734 Sopwith Camels were built during WWI and 385 Sopwith Camel pilots died from non-combat related causes while flying. This is about 7%. Most other aircraft had the same ratio. Although these deaths are not necessarily from engine failures, that was a leading cause.
All depends on the aircraft, development went very quickly during WW1. In the early days it was indeed a very precarious business, aircraft were very new and nobody really knew what they were doing.