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Fuel systems (aircraft)

Fuel the [Aircraft system|Aircraft system] that allows the crew to store, manage and deliver fuel to the propulsion system.
Fuel system 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 (like the KC-141 the B707 Tanker Transport) in addition of managing own fuel will also have to have the capability of delivering fuel to the receiving aircraft by means of hoses (drogue and probe system) or Boom.

As a simplification of fuel systems, in the following the single engine and the twin engine piston general aviation aircraft fuel systems will be discussed. These will provide an understanding of the requirements and possible ways to satisfy them. Afterward a brief illustration of what it is required in a larger turbine aircraft will be given.

General Aviation single piston engine aircraft fuel system

Generally these aircraft have two tanks one in each wing and they need a mean of providing fuel to the single engine.

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 and reduce the banking moment.
Sometime 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 fule filter and at the injection pump.

Except in very simple aircraft, most systems have an electric and a mechanical fuel boost pump. The mechanical pump is engine driven and always working. The electrical has two function help prevent gas locks (the evaporation of fuel in the lines preventing effective suction from the mechanical pump) and backup of the mechanical pump in case of failure. The mechanically driven fuel pump provides positive fuel pressure to the Carburator or the [Fuel injection] pump (which should be considered part of the engine).

In such a simple fuel system the refueling of the tanks is by gravity from a fuel cap in each. There is little to manage except feeding the fuel from either both tanks at the same time or either one to ensure the load remains balanced.

Last but not least, each tank needs to be vented to allow air in the tank to take the place of the fuel burned up otherwise the tank would be in negative pressure which in the long run would result in engine fuel starvation.

Part of the fuel system is also the fuel level indication system, which in the simplest form is a tranparent window on the tank side and in its usual application is very similar to the one installed in cars.

General Aviation twin piston engine aircraft fuel system

To discuss a slightly more complex system let's assume the aircraft has 4 tanks: 2 wing tanks feeding the respective engine and two wing tip tanks unable to feed the engines but only capable to feed the respective wing tank.

This configuration adds several layer of complexity:

  • To transfer fuel from the tip tank to the main tank on each side, there must be a fuel pump in the tip tank;
  • Each wing tank will have its own electric boost pump as well as each engine will have its own mechanical one replicating the fuel system seen above for the single engine
  • In case of single engine operations, there must be a way to "cross feed" the engine (Left Tank feeding right engine and vice versa)
  • To balance the weight, there must be a way to feed both engines from one tank

So adding tanks and engines increases the complexity of the fuel management, dictating when to transfer tip tank fuel (need to have space in the main tanks otherwise it would vent overboard) and multiple position of two or more fuel selector.

Consideration must be given to the length of the fuel lines and capacity of fuel boost pumps (one pump electrical pump might be unable to feed two engines in a cross feed situation if one of the mechanical pumps is inoperative).

A good graphics on how a twin engine fuel system capable of cross feeding is available for normal operations and cross feed operation on the National Transportation Safety Board Website. Roll rate or load factor limits might be different if the tip tanks are full or empty.

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 limitation on the amount of fuel and the order in which fuel has to be used. Turbine engine fuel burn rate are higher than reciprocating engines. In addition to that, fuel needs to be injected in a high pressure chamber therefore the injection system of a turbine aircraft will need to provide fuel to higher pressure and rates. The whole system will therefore be engineered to supply the proper amount of fuel.

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

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