5.2.2: Fuel system

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The main purpose of an aircraft fuel system is to provide a reliable supply of fuel to the power plant. Given that an aircraft with no fuel (or with no properly supplied fuel) can not fly (unless gliding), this system is key to ensure safe operations. The commonly used fuel is high octane index gasoline for piston aircraft, and some type of kerosene for jet aircraft. Even though fuel systems differ greatly due to the type of fuel and the type of mission, one can distinguish the following needs: refuel and defuel; storage; fuel pressurization; fuel transfer; engine feed; etc. Thus, the system is fundamentally composed by:

tanks;
fuel hydrants;
feeding pumps;
pipes and conducts;
valves and filters;
sensors, indicators, and control elements.

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Figure 5.16: Diagrammatic representation of the Boeing 737-300 fuel system. © User:RosarioVanTulpe / Wikimedia Commons / Public Domain.

Tanks are used to storage fuel. Three main types can be distinguished: independent tanks; integrated tanks; interchangeable tanks. The independent tanks (concept similar to car tanks) are nowadays obsolete, just present in regional aircraft. The most extended in commercial aviation are integrated tanks, meaning that the tank is also part of the structure of (typically) the wing. The integral tanks are painted internally with a anti-corrosion substance and sealing all union and holes. The interchangeable tanks are those installed for determined missions.

The filling up and emptying process is centralized in a unique point, the fuel hydrant, which supplies fuel to all tanks thanks to feeding pumps which pump fuel throughout the pipes and conducts conforming the distribution network of the system. To be more precise, there are two fundamental types of pumps: the fuel transfer pumps, which perform the task of transferring fuel between the aircraft tanks, and the fuel booster pumps (also referred to as engine feed pumps), which are used to boost (preventing from flameouts and other inconveniences) the fuel flow form the fuel system to the engine.

The system is completed with valves, filters, sensors, indicators, and control elements. Valves can be simply transfer valves or non-return valves (to preserve the logic direction of fuel flow) or vent valves (to eliminate air during refueling). Filters are used to remove contaminants in the system. Last, different sensors are located within the system to measure different performance parameters (fuel quantity, fuel properties, fuel level, etc). The measurements are displayed in several indicators, some of them shown directly to the pilot, some others analyzed in a control unit. Both pilot and control unit (the later automatically) might actuate on the system to modify some of the performances. Notice that the subsystems that encompass the indicators, displays, and control unit might be also seen as part of an electronic or avionics system.⁷

Figure 5.16 shows a diagrammatic representation of the Boeing 737-300 fuel system.

7. 1 Engine Driven Fuel Pump - Left Engine; 2 Engine Driven Fuel Pump - Right Engine; 3 Crossfeed Valve; 4 Left Engine Fuel Shutoff Valve; 5 Right Engine Fuel Shutoff Valve; 6 Manual Defuling Valve; 7 Fueling Station; 8 Tank No. 2 (Right); 9 Forward Fuel Pump (Tank No. 2); 10 Aft Fuel Pump (Tank No. 2); 11 Left Fuel Pump (Center Tank); 12 Right Fuel Pump (Center Tank); 13 Center Tank; 14 Bypass Valve; 15 Aft Fuel Pump (Tank No. 1); 16 Forward Fuel Pump (Tank No. 1); 17 Tank No. 1 (Left); 18 Fuel Scavenge Shutoff Valve; 20 APU Fuel Shutoff Valve; 21 APU; 22 Fuel Temperature Sensor; 23-36 Indicators.