What counts as fuel
Immersive Aircraft engines run on furnace-type fuel — the same burnable items you would put in a furnace, such as coal, charcoal, or logs. Different engines accept different fuels and burn them at different rates, so a basic 
Engine will happily drink coal while an 
Eco Engine is more frugal. You do not craft a special "aviation fuel" item; you feed the engine directly.
Refueling step by step
Dismount your aircraft and right-click the engine block with your fuel item in hand. The engine has an internal fuel slot, so you can also open it like a container and drop fuel inside. Watch the flame indicator or fuel gauge — when it is empty the engine cuts out mid-flight. Carry a stack of coal in your inventory on every trip so you can top up at a landing spot. If you are flying a 
Boiler or 
Steel Boiler setup, remember those use a different heat mechanic and may need water as well as fuel, so check the boiler's own interface rather than the engine's.