Aircraft Engine Fuel and Fuel Metering Systems

Aircraft fuel systems have two basic sections; airframe and powerplant. The airframe section consists of all the parts associated with storage and delivery, including the parts from the fuel tanks to the engine-driven pump. The powerplant section consists of the distribution parts, beginning after the engine-driven fuel pump and concluding at the point where air and fuel are mixed for combustion (inside a carburetor or in the combustion chamber of a cylinder or turbine engine). While many components in an aircraft fuel system are specific to the airframe, the sole purpose of the fuel system is to provide fuel to the engine(s).

The engine fuel system must supply fuel to the engine’s fuel metering device under all conditions of ground and air operation. It must function properly at constantly changing altitudes and in any climate. The most common fuels are AVGAS for reciprocating engines and Jet A for turbine engines. AVGAS is generally either 80 (red) or 100LL (blue) octane. The LL stands for low lead although it contains four times the lead of 80 octane AVGAS. Jet A is a kerosene-based fuel that is clear to straw in color.

For an aircraft to operate reliably in every phase of flight and in all expected environments, its fuel system must store and maintain a sufficient quantity of fuel and deliver it to the powerplant without interruption or contamination. The entire fuel system can be divided in three functional areas: storage, delivery, and distribution. The earliest systems were very simple, including a tank, fuel lines, a selector valve, and a carburetor. Over time, powerplant technology advanced, and with increased power came more complexity and more specific demands for distributing fuel. Fuel systems are designed to efficiently meet the design requirements of an aircraft powerplant with particular considerations for safety. Aviation maintenance technicians must be thoroughly familiar with the design, operation, and maintenance of the aircraft fuel system before inspecting or performing maintenance.

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