There are currently two types of aviation fuel that are widely available: 100LL and Jet A. Most piston engine aircraft utilize 100LL avgas, which is dyed blue to distinguish it from straw-colored Jet A fuel (used in turbine and diesel engines). Previous grades of avgas, such as leaded 100 (dyed green), 80 (dyed red), and 82 UL (dyed purple) are no longer available in most parts of the world. Even 100LL avgas can be difficult to find in many countries and is slated to be phased out in the United States within the next 10 years. In the meantime, it is important for pilots of piston engine aircraft to be vigilant when fueling their planes to avoid putting Jet A in the tanks. Although reciprocating engines may run briefly on jet fuel, detonation and overheating will soon cause a catastrophic failure. Likewise, turbine engines will run for a while on avgas, but eventually the lead content and different combustion ranges of the fuel will cause severe damage. If the wrong fuel type is suspected after a visual check, a simple test can be performed by placing a few drops of fuel on a white piece of paper. 100LL avgas will evaporate cleanly but Jet A fuel will leave an oily residue. Automotive gasoline should never be used in aircraft engines designed for 100LL avgas. It can cause preignition, detonation, vapor lock, and sticking or burned valves.

If water or other contaminants are discovered during the preflight, successive samples should be taken until the fuel is completely free—a slug of water drawn into the engine during takeoff can cause it to quit. Water (which is heaver than fuel) will sink to the bottom of the sampling jar and have the appearance of a clear glass bead. Gently rocking the wings can help any water in the tanks to move toward the low point where the drains are located. Fuel caps should also be carefully checked during preflight. Since they are located on the upper surface of the wing (a low pressure area in flight), fuel can be siphoned out very quickly if the caps are not secured.

When switching tanks during flight on low-wing aircraft, the electric boost pump should turned on to ensure an uninterrupted flow of fuel to the engine. The fuel pressure gauge should be monitored during this process as well as after turning the boost pump off to make sure that the engine-driven fuel pump is functioning properly.