Fuel sumping

That’s fine for a vehicle that never leaves the ground. For one that does, we put in more effort. We sump small samples of fuel out of each tank and examine them. We’re ensuring that the right grade of aviation fuel is in the tank; we’re also looking for the presence of contaminants such as dirt or water in the fuel. A piston aircraft fueled with Jet A, or with a lower octane level than is recommended by the manufacturer, will experience detonation, an explosive ignition of the fuel-air mixture that can cause serious damage to the engine.

The presence of water is not as dire as Jet A, but it still creates problems if not drained from the tanks. Water can accumulate from condensation inside the tank, or it can seep into the tanks if the fuel caps are not properly fastened, or fuel cap seals are cracked or brittle.