Aircraft fuel system

Aircraft fuel system
Diagrammatic representation of the Boeing 737-300 fuel system

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, while a tanker (such as the KC-135) in addition of managing its own fuel can also provide fuel to other aircraft.

General aviation single piston engine aircraft fuel system

Small piston-engined aircraft often have two fuel tanks, one in each wing, and they use additional components as a means of providing fuel to the single engine. Single fuel tank systems are commonplace.

The fuel is piped through fuel lines to a fuel control valve ( usually known as the fuel selector). This valve serves several functions. The first function is to act as a fuel shut-off valve. This is required to provide the crew with a means to prevent fuel reaching 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 and reduce the banking moment. Occasionally the shut-off function is in a different valve located after the fuel selector valve.

After the selector valve there usually is a gascolator: a fuel filter that can be drained. Drainage points are usually in each tank (often more than one per tank) at the fuel filter and at the injection pump.

Each tank needs to be vented to allow air in the tank to take the place of burned fuel otherwise the tank would be in negative pressure which in the long run would result in engine fuel starvation. The vent also serves to allow for changes in atmospheric pressure with altitude.

Part of the fuel system is also the fuel level indication system, which in the simplest form is a transparent window on the tank side and in its usual application a float driven potentiometer installed in the tank. Due to the TWA 800 disaster, a revision was made to aircraft fuel systems to address the potential explosion hazard of electrical components located in the fuel tank. Single piston aircraft fuel level systems moved to utilize float level gauges from the CNG and LPG industries which had the float drive a magnetic coupling and relocated the potentiometer outside the fuel tank proper.

Some single engine aircraft use capacitive probes in the fuel tanks. As fuel is burned more air enters the tank and the capacitance increases, this is then read by a computer and the fuel amount is calculated and displayed to the pilots. The systems installed in small single engine aircraft do not address the explosion hazard requirement now common for this class of aircraft.

Recent advances in magnetoresistive technology have evolved new fuel level sensors for general aviation applications. . Additionally this system will be unaffected by any additive or fuel combination to replace 100 LL for piston powered aircraft.

Magneto Resistive Level Sensor

Multi-engine aircraft fuel system

Adding tanks and engines increases the complexity of the fuel system and its management. Additional features found in multi-engine aircraft are:

• Each wing tank will often have its own electric boost pump as well as each engine will have its own mechanical one replicating the fuel system described above for the single engine
• In case of single engine operations, there is often a method incorporated to "cross feed" the engine (left Tank feeding right engine and vice versa)
• To balance asymmetric weight, flow valves and pumps will often be used to feed both engines from one tank or simply transfer fuel between tanks.

Turbine fuel systems

All of the considerations made for the twin piston are applicable to turbine fuel systems. Additional consideration apply because of the higher altitudes, different fuel, lower temperatures, and longer flights.

To avoid humidity or the fuel itself to solidify at the low temperatures (-55 °C), fuel tanks have thermometers and heating systems. Additionally many are pressurized with engine bleed air to keep moist air out and ensure positive pressure feed to the pumps. In larger aircraft fuel tanks are also in the fuselage and their load might affect the position of the center of gravity of the aircraft. This will impose limitations on the amount of fuel carried and the order in which fuel has to be used. Turbine engine fuel burn rates are higher than reciprocating engines. In addition, fuel needs to be injected in to a combustor therefore the injection system of a turbine aircraft will need to provide fuel at higher pressures and rates than a piston-engined aircraft.

The refueling system of larger aircraft will include a single positive pressure refueling point from which all tanks can be fueled. How much fuel and which tanks are fed during refueling operations is determined by the controls in the refueling panel, usually installed nearby and accessible to ground crews.

External tanks

External tanks are used to extend the range of an aircraft, drop tanks are used by combat aircraft that need to discard them after use for performance reasons. To transfer fuel from the tip tank to the main tank on each side, there must be a fuel pump in the tip tank.

References

Bibliography

• Taylor, John W.R. The Lore of Flight, London: Universal Books Ltd., 1990. ISBN 0-9509620-1-5.