Summary of Aircraft Fuel Systems: Components and Safety

## Introduction Aircraft fuel systems deliver, store, and control fuel so engines run reliably and safely. This material explains common components, their functions, safety procedures, and practical considerations for both reciprocating and turbine-powered aircraft. > **Definition:** A fuel system is the collection of tanks, pumps, valves, lines, and instruments that store and deliver fuel to an aircraft's engines and accessories. ## System Components and Functions ### Fuel Tanks - Tanks may be **integral fuel cells** (sealed portions of structure) or **bladder-type cells** (removable flexible tanks). - Tanks are often divided into compartments and fitted with **baffles** to prevent fuel surge during attitude changes. > **Definition:** An integral fuel cell is a sealed-off section of the airframe structure that acts as the fuel tank. Practical example: Large transport wings commonly use integral tanks where the wing skin and ribs form the tank walls. ### Fuel Quantity Indication - Four general types: **sight glass**, **mechanical**, **electrical**, **electronic (capacitance)**. - Electronic capacitance gauges are typically more accurate because they measure by **weight** rather than volume. ### Strainers and Filters - Strainers remove particles and water. Common locations: at the tank outlet and in the line between tank outlet and fuel metering device. - Filters on turbine aircraft also protect fuel control units from contaminants and ice. ### Pumps - **Engine-driven pump**: primary pump driven by the engine; may include a bypass valve to allow flow if pump is blocked. - **Booster (centrifugal) pump**: installed in tanks for: starting pressure, preventing vapor lock at high altitude, and transferring fuel between tanks. > **Definition:** A booster pump is a small centrifugal pump inside or near a tank used to assure positive fuel pressure for start, transfer, and vapor-lock prevention. ### Flowmeters and Fuel Measurement - On turbine aircraft, mass-flow fuel flowmeters measure **fuel density** (used to compute mass flow). This is important because turbine engines respond to mass of fuel, not just volume. - On fuel-injected horizontally opposed piston engines, fuel flow can be indicated by a **pressure gauge measuring pressure drop across injector nozzles**. ### Valves and Selectors - **Selector valves** direct fuel from chosen tanks to engines. They often include a **detent**. > **Definition:** A detent is a tactile stop in a valve mechanism that provides a positive feel when the selector is in ON or OFF positions. - **Cross-feed systems** allow fuel from any tank to supply any engine; used to balance loads or supply an inoperative pump. ### Single-point and Pressure Fueling - **Single-point fueling**: pressure fueling through an underwing port into a manifold that distributes fuel to selected tanks at the fueling station. - Markings near filler openings: on reciprocating-engine aircraft, label with “AVGAS” and the minimum grade; on turbine aircraft, label with “JET FUEL,” permissible designations, maximum fueling pressure, and maximum defueling pressure. ### Fuel Jettison Systems - Some aircraft certified with takeoff weight higher than landing weight must be able to **jettison fuel** to reduce landing weight in an emergency. ### Anti-microbial and Additives - **Prist** is added to turbojet fuel for two reasons: (1) it is biocidal and kills scum-forming bacteria in tanks, and (2) it lowers the freezing point of entrained water, acting as an antifreeze. ### Fuel Temperature and Icing - Turbine aircraft have fuel temperature indicators because cold ambient conditions can cause water to precipitate from fuel and freeze on filters, potentially blocking fuel flow. Fuel heaters may be used to keep fuel above freezing. ## Safety, Maintenance, and Procedures ### Purging and Entering Tanks - Before welding a fuel tank, all fuel vapors must be purged (for example, by flowing live steam through the tank or using an ap