The fuel system supplies fuel to the engine. Typically, fuel tanks are located inside the wings. Fuel is pressurized by an engine driven fuel pump, and many airplanes are also equipped with a backup electric fuel pump.

On a high wing airplane fuel can be fed to the engine using gravity, whereas it must be pumped up to the engine on a low wing airplane.

Fuel Vents

Fuel vents allow air to enter the fuel tanks as fuel is consumed. If air were not allowed into the fuel tanks to replace fuel as it was burned, suction would be created inside the fuel tanks. As the fuel burned, this suction would become greater and greater, counteracting the fuel pumps. The flow of fuel to the engine would become restricted by this suction, resulting in power loss and eventual engine failure.

Overflow Drains

One or more overflow drains will also be provided for the fuel system. They prevent fuel tank structural damage by allowing fuel to escape the tank overboard in the event the pressure inside the tank becomes too great. This commonly occurs when the airplane is fully fulled and allowed to sit in warm air or in the sun. The fuel expands when heated and this expansion would damage the fuel tanks if it were not for the overflow drain, which allows it to simply drip overboard.

Fuel Selector Valve

From the fuel tanks, fuel lines carry the fuel to a fuel selector valve in the cockpit. The pilot uses the fuel selector from which tank the fuel is drawn. The fuel selector valve also has an off position, used to stop the flow of fuel to the engine. Alternatively, an airplane may be equipped with a fuel shutoff valve, which simply has an on or off selection.

Fuel Sumps

Several fuel sumps will be installed at low points in the fuel system, such as the bottom of each fuel tank and fuel line. Fuel is a light weight liquid. Any water or foreign material in the fuel will find its way to these low points. As part of the preflight inspection, the pilot examines fuel samples from these sumps to ensure the fuel is not contaminated.

Fuel Filters

To ensure fuel is clear of any contaminants, one or more fuel filters are used. Typically, the fuel is filtered just before entering the engine. Since allowing contaminated fuel to the engine is better than allowing no fuel to the engine, fuel filters often allow fuel to bypass the filter, should the filter become clogged with contaminants.


A fuel primer is used to inject fuel from the fuel tanks directly into the engine. This is commonly used to aid engine starting in cold weather. Fuel primers are used on low and high wing airplanes alike. The most important thing to know about fuel primers is to make sure they are locked closed when not being used. A fuel primer that is not locked after use may allow extra fuel into the engine, potentially flooding and killing it.

Fuel Gauges

Fuel gauges allow the pilot to verify the fuel on board. Airplane fuel gauges are often not very accurate and are just there to give you a rough idea of fuel on board. Before each flight, the pilot determines how much fuel is required and visually verifies the required fuel is on board by physically checking the fuel tank. Airplane fuel gauges are not used by pilots for flight planning purposes.

Fuel Grades

Aviation fuel is provided in a variety of grades. Only certain fuel grades are approved for use in each airplane. Often times, the Pilot's Operating Handbook for the airplane will list the minimum acceptable fuel grade. Higher grade fuels can be used, but you should never use a fuel grade lower than the minimum grade approved for your airplane. Using a lower than approved grade of fuel may damage the engine and result in loss of engine power or engine failure.

To aid pilots in identifying fuel grades, dye is added to many aviation fuel grades. The most common fuel grade available is 100 low lead, which is dyed blue. Grade 100 Avgas is dyed green, and 80 is dyed red. Many airports offer 87 octane aviation fuel, which is not dyed and has the appearance of automotive gasoline. Jet fuel is also not dyed. Finally, if multiple grades of fuel are mixed inside the fuel tank, the dyes interact to produce no color.

You should never use a lower than approved fuel grade.

"The only time you have too much fuel is when you're on fire."

Some say the glass is half full. Some say it is half empty. A pilot says that fuel sample is really contaminated with water.