Aircraft Fuel Systems
Aircraft fuel systems vary in complexity from the extremely simple systems found in small, single-engine air-planes to the complex systems in large jet transports. Regardless of the type of aircraft, all fuel systems share many of the same common components. Every system has one or more fuel tanks, tubing to carry the fuel from the tank(s) to the engine(s), valves to control the flow of fuel, provisions for trapping water and contaminants, and a method for indicating the fuel quantity. Although fuel systems in modern aircraft are relatively simple, the safety and reliability of these systems is dependent upon proper inspection and maintenance.
All powered aircraft, whether rotary or fixed wing, depend upon a continuous, uninterrupted flow of uncontaminated fuel under all operating conditions. The weight of the fuel constitutes a large percentage of the aircraft's total weight. This may range from about 10% of the gross weight of small personal airplanes, to more than 40% for jet aircraft used on long overseas flights.
The weight of the fuel requires that the structure be strong enough to carry it in all flight conditions. The aircraft designer locates the fuel tanks so that the decreasing weight from fuel consumption will not cause balance problems. To reduce stresses on the airframe and improve structural life, many jet transports have fuel management procedures that specify how the fuel is to be used from the various tanks. For example, a Boeing 747 will first use the fuel in the center wing tank, followed by fuel in the inboard tanks until their quantities are equal to the out-board tanks.
Improper management of the fuel system has caused more aircraft accidents than failures of any other single system. Engine failure will occur if all of the fuel in the tanks has been burned, but engines will also stop if an empty tank is selected, even though there is fuel in the other tanks.
Contamination in the fuel may clog strainers or filters and shut off the flow of fuel to the engines. Contamination may take many forms, including solid particles, water, ice and bacterial growth. Water that condenses in partially filled tanks will stop the engine when it flows into the metering system. Water in turbine-powered aircraft is a special problem, as the more viscous jet fuel will hold water entrained in such tiny particles that it does not easily settle out. When the fuel temperature drops at high altitude, the water may form ice crystals that can freeze on the fuel filters and shut off the flow of fuel. Many jet engines have fuel heaters to prevent ice formation on the fuel filters and fuel metering system components.
The type or grade of aircraft fuel must be carefully matched to the engine. It is the responsibility of the pilot in command to verify before a flight is started that the aircraft is adequately supplied with the proper fuel. The per-son doing the refueling can assist by being vigilant for problems with fuel quality and type. A significant problem is the introduction of jet fuel, which is designed for turbine engines, into the fuel tanks of turbocharged pis-ton engine aircraft. These engines are designed to operate on high-octane gasoline. At high power settings, jet fuel contamination causes severe detonation, which can lead to catastrophic engine failure, usually on takeoff. The potential for a disastrous accident is obvious.