Definition

An aircraft fuel system enables the fuel to be loaded, stored, managed, and delivered to the engine(s) of the aircraft.

General Description

Fuel systems differ greatly due to the varied size and complexity of different aircraft. In the most basic form, a fuel system will consist of a single gravity-feed fuel tank with an associated fuel line connecting it to the engine. In larger aircraft, the fuel system is likely to involve multiple fuel tanks, which may be located in the wing or the fuselage or both. Fuel systems normally use a primary engine-driven pump, which may be connected in parallel with an electrical pump. The electrical boost pump can serve as a backup in case of failure of the engine-driven pump. Electrical pumps and associated valves may also allow for refueling and defueling, and in some applications, fuel transfer to maintain proper aircraft centre of gravity. 

Fuel tanks include vents that allow air to enter the tanks as fuel is burned, which prevents a vacuum that could interfere with fuel flow. Other components include sensing elements that measure fuel quantity, which is displayed either on analog gauges or digital readouts in the cockpit. A fuel strainer or filter removes water and other impurities. A sump connected to the strainer allows pilots or mechanics to drain collected water and to check for fuel contamination.

Tanks on piston-driven aircraft are usually refueled directly from filler ports on each tank (as opposed to larger turbine-driven aircraft, which often have single-point refueling systems that feed all tanks from one port.)

Light Single-Engine GA Aircraft

Small piston-driven aircraft often have a single-tank fuel system. On newer aircraft, two fuel tanks, with one in each wing, are more common. A two-tank system requires additional components to allow controlled provision of fuel to the single engine. Electrical boost pumps may be incorporated, depending on the location of the tanks. The fuel is piped from the tanks through fuel lines to a fuel control valve, which is commonly referred to as a fuel selector valve. This valve serves several functions and will normally have Left, Right, Both, or Off selections. Left, Right, and Both allow for fuel to be fed from the engine from either the left tank or the right tank individually or from both at the same time. This function allows the pilot to balance the fuel tanks or to trim the aircraft laterally. The Off selection provides for fuel shutoff in the event of an engine fire, or to prevent undesired fuel migration when the aircraft is not in operation. In some installations, the shutoff function is provided by a separate valve located downstream from the fuel selector valve.

Light Twin-Engine GA Aircraft

Adding a second engine to an aircraft necessarily increases the complexity of the fuel system and its management. Additional features normally found in small multi-engine aircraft include in-tank boost pumps, a more robust fuel quantity indicating system, and a provision for fuel crossfeed. Refueling is still normally accomplished from filler ports on each tank.

Crossfeed allows for fuel from one wing tank to feed the engine on the other wing. In some cases, fuel is routed directly from the tank to the engine, while in others, fuel is transferred from one tank to the opposite tank before feeding it to the engine. The crossfeed capability allows the pilot to use all the fuel on board and to maintain lateral balance limitations in the event one of the engines fails.

System Description

Piston-driven aircraft engines are divided into two broad categories: Carbureted engines and fuel-injected engines.

Carburetors mix fuel with air and direct the fuel-air mixture into the engine's cylinders for combustion. Typically, a carbureted system will include a fuel primer in the cockpit, which is used manually to direct fuel into the engine on initial startup.

Carbureted engines can be subject to carburetor icing. Airflow into the carburetor's venturi causes a pressure drop with an associated temperature drop. If the temperature drops far enough, ice can form, even at outside air temperatures above 80 degrees F (26.7 degrees C). For this reason, carbureted engines have a carburetor heat system that draws hot air from the engine to melt the ice. Pilots are normally trained to apply carburetor heat if the engine begins running rough. Pilots also usually apply carburetor heat whenever they reduce power for descent.

Fuel-injected engines do not have carburetors. Instead, the injectors spray fuel directly into each cylinder. The mixture of fuel and air takes place in the cylinder itself rather than in a carburetor before reaching the cylinder. Fuel-injected engines are not subject to carburetor icing, and they are generally easier to start when cold. However, they can be more difficult to start when hot than carbureted engines.

Components of fuel-injected engines include a fuel servo that regulates the amount of fuel entering the engine in relation to air intake. Downstream of the fuel servo, a fuel manifold distributes the fuel evenly to each cylinder.

Typically, fuel-injected engines are found in higher-performance piston aircraft.

Threats

There are a number of fuel-related threats to safe aircraft operation. In addition to those described in the fuel management article, there are several threats related to the misuse or to the malfunction of an aircraft fuel system that must be considered. These include:

• Fuel leak: Fuel can leak at the engine, from the tank, or from anywhere in between due to a fuel tank or fuel line rupture.
• Fuel imbalance: Fuel imbalance can occur due to improper refueling techniques, poor fuel management, engine failure, or fuel leak.
• Mechanical failure of a fuel pump.
• Electrical failure: Electrical failure may limit the availability of fuel pumps and fuel system indications.
• Fuel contamination: Poor storage by a vendor can allow water or other contaminants to enter fuel in ground-storage tanks.

Effects

• A fuel leak from an engine can often be resolved by shutting down the affected engine (in multi-engine aircraft). A tank leak due to rupture in the tank will result in the loss of some or all of the fuel in that tank. If a fuel line is ruptured, it could result in some fuel becoming unusable.
• An uncorrected fuel imbalance can lead to difficulty in controlling the aircraft.
• A pump failure could result in the inability to use the fuel in the affected tank. This may be mitigated by a second (or even a third) pump in the same tank.
• Fuel freezing can lead to loss of power due to fuel starvation and potentially result in engine failure.
• In the event of electrical failure, some or all of the fuel boost pumps will be lost. In most aircraft, gravity fuel feeding is possible from at least some of the fuel tanks. Descent may be required to comply with the maximum fuel gravity feed altitude. Diversion to an alternate airport may be required due to unusable fuel.

Defences

• In all cases comply with the manufacturer's limitations and recommendations as published in the Aircraft Flight Manual (AFM).
• WARNING: The misidentification or mishandling of a fuel leak can potentially lead to depletion of all fuel on board the aircraft. Use the appropriate checklist to carefully identify and isolate the leaking component.
• Where possible, maintain the aircraft wing-to-wing fuel balance within limits by referring to the flight manual or appropriate checklist. As circumstances permit, it is advisable to keep multi-tasking at a minimum when crossfeeding or transferring fuel. Inattention at this stage can make the problem worse.
• Fuel pump circuit breakers should NOT be reset without consulting maintenance.
• In light aircraft with carbureted systems, use carburetor heat as appropriate. In larger aircraft at high altitude, if the fuel temperature approaches its freezing point, pilots can descent to warmer air, increase speed to increase the total air temperature, or transfer fuel to a tank containing warmer fuel. In aircraft without fuel temperature instrumentation, fuel temperature must be estimated by outside air temperature.