Which of the following substances is NOT commonly used as an aircraft fuel?

The substance that is NOT commonly used as an aircraft fuel is wood. Aircraft fuels are chosen based on their energy density, burn characteristics, and availability to meet the performance requirements of aircraft engines.

Gasoline and kerosene are widely used in aviation. Gasoline is typically used in smaller, piston-engine aircraft, while kerosene is the primary fuel for jet engines due to its high energy density and efficiency at high speeds and altitudes. Propane, while not as common as gasoline or kerosene, can also be used in some specialized aircraft or in specific user applications.

Wood, however, is not a viable option for modern aircraft fuel due to its low energy density and the fact that it does not provide the necessary performance characteristics needed for flight. Additionally, burning wood would create too much smoke and residue, which would be detrimental to engine performance and reliability. Therefore, it is clear that wood is not a suitable fuel for aircraft.