An aircraft fuel system allows the crew to store, manage, and deliver fuel to the propulsion system of an aircraft. Fuel systems differ greatly due to different performance of the aircraft in which they are installed. A single engine piston aircraft will have a very simple fuel system.
The fuel is piped through fuel lines to a Fuel Control valve (Fuel selector). This valve serves several functions. The first one is to act as a fuel shutoff valve. This is required to provide the crew with a mean to prevent fuel to be sent to the engine in case of an engine fire. The second role is to allow the pilot to choose which tank feeds the engine. Many aircraft have the Left Tank, Right Tanks selection available to the pilot. Some Cessnas have only the “Both tanks” feeding position, and many have the “Both tanks” position in addition to the Left and right. The reason to have the Left and Right Tank option is to allow pilots to balance fuel load
Most modern aircraft are equipped with 2 or more fuel tanks (or cells). In high wing aircraft, the cells are housed in the wings. Since they are higher than the engine, the fuel flows down to the engine by the force of gravity.
On low wing aircraft fuel pumps are required. To initially get fuel to the engine for starting, an electrical “boost pump” is turned on to pump fuel to the engine. After the engine is started, a mechanical fuel pump driven by the engine feeds fuel to the engine. The electric boost pump can now be turned off.
Each fuel tank is equipped with a drain valve located at the lowest point in the tank. This drain allows the pilot during preflight walk-around to check for and drain off any water which may have accumulated in the fuel tank. There is usually another drain located at the lowest part of the fuel piping system. This valve must also be drained during pre-flight to eliminate any water which may have accumulated in the fuel lines. Associated with this drain is a fuel strainer which filters out foreign matter which may be in the fuel system.
A vent line allows air to enter the tank as fuel is used. During hot weather, fuel may expand and overflow through the vent when tanks are full.
A fuel selector valve located inside the cockpit allows the pilot to select which tank(s) are to be in use during flight. Most small aircraft operate with the selector set on Both, such that both the left and right fuel tanks are simultaneous feeding fuel to the engine. The pilot may set the selector on Left or Right tank as a means of equalizing the loading of the aircraft. Usually, the selector should be set to both for take-off and landing. Pilots of low wing aircraft should exercise caution in their fuel management if tank selection is other than both. Running a tank dry can cause the engine to quit and vapour lock to occur in the fuel lines. It may be impossible to restart the engine under these conditions.
There is a fuel gauge in the cockpit for each fuel tank. The lower 1/4 of the fuel gauge indication is marked with a red line as a caution to the pilot of a low fuel condition. The pilot should never rely on the fuel gauge as the sole measure of fuel remaining. The gauges on aircraft are subject to a variety of indicator errors. The pilot should therefore double check the fuel remaining based on the power setting of the engine in flight and time in flight.
Inside the cockpit a fuel mixture control and a fuel primer pump are located on the instrument panel. The mixture control is used to adjust the air/fuel mixture for the altitude being flown. It allows the pilot to adjust the fuel/air ratio entering the engine. As altitude is gained, the intake air becomes less dense. Less fuel must be fed through the carburettor to permit the fuel/air mixture to remain correct proportion. If leaning is not accomplished by the pilot, a rich mixture (too much fuel) results. This is not only wasteful of fuel, but can result in fouled spark plugs due to carbon and soot buildup on the spark plugs. A rough running engine results. An additional gauge called an Exhaust Gas Temperature Gauge can be installed in the aircraft as an aid in achieving the proper “leaning” of the engine.
The fuel primer is a plunger that can be used in cold weather to inject fuel directly into the carburettor as an assist in starting the engine in cold conditions.
Three different grades of fuel are used in reciprocating engine aircraft. These grades are designated by octane rating and are colour coded so the pilot can insure the proper grade of fuel is being pumped into the tanks.