Chapter 31: Aircraft Fuel Systems

31.1 Aircraft Fuel

•  AVGAS (Aviation Gasoline): Used for piston engines. Graded by octane rating and color-
• coded.
•  AVTUR (Aviation Turbine Fuel / Jet Fuel): Kerosene-based fuel used for gas turbine
• engines. Its quality is defined by properties like flashpoint and freezing point. Additives are
• mixed in, including an icing inhibitor (FSII) and a static dissipator.

32.2 Fuel Tanks and System Components

•  Tank Types:
• Integral Tanks: The most common type on large aircraft, where a sealed-off section of the
• wing or fuselage structure (the "wing box") is used as the fuel tank.
• Rigid Tanks: A separate, self-contained tank fitted into a compartment.
• Flexible (Bladder) Tanks: A rubberized bag fitted into a structural cavity.
•  System Components:
• Booster Pumps: Electric, centrifugal pumps located inside the tanks. They supply fuel under
• low pressure from the tanks to the engine-driven pumps, preventing vapor lock at high
• altitude.
• Collector Tank (Feeder Box): A small compartment at the lowest point in the tank where
• the booster pumps are located. It is designed to remain full of fuel during maneuvers to
• prevent the pumps from running dry.
• Baffles: Internal walls with holes that reduce the sloshing (surging) of fuel in the tanks.
• Vent System: Allows air to enter and exit the tank as fuel is consumed or the aircraft
• climbs/descends, preventing pressure differences. A vent/surge tank at the wingtip collects
• any fuel that overflows into the vent lines.
• Cross-feed and Shut-off Valves: A system of valves that allows fuel from any tank to be fed
• to any engine, and allows engines/tanks to be isolated in case of a leak or fire.
• Fuel Quantity Measurement: Modern aircraft use a capacitive fuel gauging system. It
• measures the mass (weight) of the fuel, not the volume, which is much more accurate as fuel
• density changes with temperature.