Aircraft Fuel System

The fuel system is designed to provide an uninterrupted flow of clean fuel from the fuel tanks to the engine. The fuel must be available to the engine under all conditions of engine power, altitude, attitude, and during all approved flight maneuvers . Fuel systems differ greatly from aircraft to aircraft due to the relative size and complexity of the aircraft in which they are installed.

1. Aircraft Fuel System

2. Content Description of Fuel System Different types of Fuel System Fuel System Components Types of fuel used

3. Description of fuel system The fuel system is designed to provide an uninterrupted flow of clean fuel from the fuel tanks to the engine. The fuel must be available to the engine under all conditions of engine power, altitude, attitude, and during all approved flight maneuvers . Fuel systems differ greatly from aircraft to aircraft due to the relative size and complexity of the aircraft in which they are installed. In the most basic form. a fuel system will consist of a single, gravity feed fuel tank with the associated fuel line connecting it to the aircraft engine.

4. Different types of fuel System Small Single-Engine Aircraft Fuel Systems Gravity-Feed System - The gravity-feed system utilizes the force of gravity to transfer the fuel from the tanks to the engine. For example, on high-wing airplanes, the fuel tanks are installed in the wings. This places the fuel tanks above the carburettor, and the fuel is gravity fed through the system and into the carburettor. If the design of the aircraft is such that gravity cannot be used to transfer fuel, fuel pumps are installed. For example, on low-wing airplanes, the fuel tanks in the wings are located below the carburettor.

5. Fuel-Pump System - Aircraft with fuel-pump systems have two fuel pumps. The main pump system is engine driven with an electrically driven auxiliary pump provided for use in engine starting and in the event the engine pump fails. The auxiliary pump, also known as a boost pump, provides added reliability to the fuel system. The electrically-driven auxiliary pump is controlled by a switch in the flight deck.

6. Large Reciprocating-Engine Aircraft Fuel Systems Large, multiengine transport aircraft powered by reciprocating radial engines are no longer produced. However, many are still in operation. A selector valve for each engine allows an engine-driven pump to pull fuel from the main tank or an auxiliary tank. The fuel passes through a strainer before reaching the pump where it is delivered to the engine. The outlet of the pump can feed either engine through the use of a crossfeed line with valves controlled in the cockpit. A hand-operated wobble pump located upstream of the strainer is used to prime the system for starting. Fuel vapor lines run from the pressure carburetor to the vent space in the main and auxiliary tanks. Fuel pressure gauges are tapped off of the carburetor for power indication. The hand-operated wobble pumps were replaced by electric pumps on later model aircraft. A fuel pressure warning light tapped in downstream of the engine-driven fuel pump alerts the crew should fuel pressure decline.

7. Small Multiengine (Reciprocating) Aircraft Fuel Systems Low-Wing Twin The fuel system on a small, multiengine aircraft is more complicated than a single-engine aircraft but contains many of the same elements. An example system used on a lowing aircraft is illustrated . It features the main fuel tanks in the wing tips and auxiliary tanks in the wing structure. A boost pump is located at the outlet of each main tank. This pressurizes the entire fuel system from the tank to the injectors eliminating the possibility of vapor lock. An engine can operate with just its boost pump running in the event the engine-driven injection pump fails. Typically, the boost pumps are used to prime and start the engine.

8. High-Wing Twin A simplified system on a high-wing, twin-engine aircraft that combines gravity feed with an electric fuel pump is illustrated . Directly downstream of the selector valves are the fuel strainers and then an electric fuel pump for each engine. This pump draws fuel from the selected tank and sends it under pressure to the inlet side of the fuel injection metering unit. The metering unit for each engine provides the proper flow of fuel to the distribution manifold which feeds the injectors.

9. Fuel System Components Fuel Tanks The fuel tanks, normally located inside the wings of an airplane, have a filler opening on top of the wing through which they can be filled. A filler cap covers this openin g . The tanks are vented to the outside to maintain atmospheric pressure inside the tank. They may be vented through the filler cap or through a tube extending through the surface of the wing. Fuel tanks also include an overflow drain that may stand alone or be collocated with the fuel tank vent. This allows fuel to expand with increases in temperature without damage to the tank itself. If the tanks have been filled on a hot day, it is not unusual to see fuel coming from the overflow drain

10. Fuel Gauges The fuel quantity gauges indicate the amount of fuel measured by a sensing unit in each fuel tank and is displayed in gallons or pounds. Aircraft certification rules require accuracy in fuel gauges only when they read “empty.” Any reading other than “empty” should be verified. Do not depend solely on the accuracy of the fuel quantity gauges. Always visually check the fuel level in each tank during the pre-flight inspection, and then compare it with the corresponding fuel quantity indication. If a fuel pump is installed in the fuel system, a fuel pressure gauge is also included. This gauge indicates the pressure in the fuel lines. The normal operating pressure can be found in the AFM/POH or on the gauge by colour coding

11. Fuel Primer Both gravity-feed and fuel-pump systems may incorporate a fuel primer into the system. The fuel primer is used to draw fuel from the tanks to vaporize fuel directly into the cylinders prior to starting the engine. During cold weather, when engines are difficult to start, the fuel primer helps because there is not enough heat available to vaporize the fuel in the carburetor . It is important to lock the primer in place when it is not in use. If the knob is free to move, it may vibrate out of position during flight which may cause an excessively rich fuel-air mixture. To avoid over priming, read the priming instructions for the aircraft.

12. Fuel Selectors The fuel selector valve allows selection of fuel from various tanks. A common type of selector valve contains four positions: LEFT, RIGHT, BOTH, and OFF. Selecting the LEFT or RIGHT position allows fuel to feed only from the respective tank, while selecting the BOTH position feeds fuel from both tanks. The LEFT or RIGHT position may be used to balance the amount of fuel remaining in each wing tank.

13. Fuel Valves There are many fuel valve uses in aircraft fuel systems. They are used to shut off fuel flow or to route the fuel to a desired location. Other than sump drain valves, light aircraft fuel systems may include only one valve, the selector valve. It incorporates the shutoff and selection features into a single valve. Large aircraft fuel systems have numerous valves. Most simply open and close and are know by different names related to their location and function in the fuel system (e.g., shutoff valve, transfer valve, crossfeed valve). Fuel valves can be manually operated, solenoid operated, or operated by electric motor.

14. Fuel Pumps Fuel pumps are part of most aircraft fuel systems. Standards exist for main pumps and emergency pumps. Operation of any fuel pump may not affect engine operation by creating a hazard, regardless of the engine power or thrust setting or the functional status of any other fuel pump. On reciprocating engines, one main fuel pump must be engine-driven and there must be at least one for each engine. Turbine engines also require dedicated fuel pumps for each engine. Any pump required for operation is considered a main fuel pump. The power supply for the main pump for each engine must be independent of the power supply for each main pump for any other engine. There must also be a bypass feature for each positive displacement pump.

15. Fuel Strainers. All aircraft fuel systems have filters and strainers to ensure that the fuel delivered to the engine(s) is free from contaminants. The first of these is encountered at the outlet of the fuel tank. A sump is used to encourage the collection of debris in the lowest part of the tank, which can then be drained off before flight. The actual tank outlet for the fuel is positioned above this sump. Some type of screen is used to trap contaminants attempting to flow out of the tank into the fuel system. Finger screens are common on light aircraft

16. Types of fuel used Conventional aviation fuels : 1. J et fuel :is a clear to straw-colored fuel, based on either an unleaded kerosene (Jet A-1), or a naphtha-kerosene blend (Jet B). Similar to diesel fuel, it can be used in either compression ignition engines or turbine engines. 2 .Avgas (aviation gasoline :is used by small aircraft, light helicopters and vintage piston- engined aircraft. Its formulation is distinct from the conventional gasoline (petrol U.K.) used in motor vehicles which is commonly called mogas or autogas in aviation context. Emerging aviation fuels : Biofuels :Alternatives to conventional fossil-based aviation fuels, new fuels made via the biomass to liquid method (like sustainable aviation fuel) and certain straight vegetable oils can also be used. Compressed natural gas (CNG) : are fuel feedstocks that aircraft may use in the future.

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