Fuel system
Encyclopedia
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
.
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 Cessna
s 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 is very similar to the one installed in cars.Large passenger jets use capacitors located at various stations in the fuel tanks. As fuel is burned more air enters the capacitors and the capacitance increases, this is then read by a computer and the fuel amount is calculated and displayed to the pilots.
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.
s 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.
Aerial refueling
Aerial refueling, also called air refueling, in-flight refueling , air-to-air refueling or tanking, is the process of transferring fuel from one aircraft to another during flight....
.
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 Cessna
Cessna
The Cessna Aircraft Company is an airplane manufacturing corporation headquartered in Wichita, Kansas, USA. Their main products are general aviation aircraft. Although they are the most well known for their small, piston-powered aircraft, they also produce business jets. The company is a subsidiary...
s 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
Moment (physics)
In physics, the term moment can refer to many different concepts:*Moment of force is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. A moment is valued mathematically as the product of the...
. 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
Gascolator
A Gascolator, also known as a main line strainer, sediment bowl or fuel strainer acts primarily as a fuel drain for water and small particles of sediment and is usually found at the lowest point of an aircraft's fuel system...
: 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
Fuel Starvation
Fuel starvation and fuel exhaustion are problems that can affect internal combustion engines fuelled by either diesel, kerosene, petroleum or any other combustible liquid or gas. If no fuel is available for an engine to burn, it cannot function...
. The vent also serves to allow for changes in atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
with altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
.
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 is very similar to the one installed in cars.Large passenger jets use capacitors located at various stations in the fuel tanks. As fuel is burned more air enters the capacitors and the capacitance increases, this is then read by a computer and the fuel amount is calculated and displayed to the pilots.
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
Bleed air
Bleed air in gas turbine engines is compressed air taken from within the engine, after the compressor stage and before the fuel is injected in the burners. While in theory bleed air could be drawn in any gas turbine engine, its usage is generally restricted to jet engines used in aircraft...
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
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as 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
Combustor
A combustor is a component or area of a gas turbine, ramjet, or scramjet engine where combustion takes place. It is also known as a burner, combustion chamber or flame holder. In a gas turbine engine, the combustor or combustion chamber is fed high pressure air by the compression system. The...
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 tankDrop tank
In aeronautics, a drop tank is used to describe auxiliary fuel tanks externally carried by aircraft. A drop tank is expendable and often jettisonable...
s 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.