The weight of a gallon of airplane fuel is not a single, fixed number like water because aviation fuel is a petroleum product whose weight per unit of volume constantly changes. This variability is governed by the fuel’s density, which is the true measure for safety and performance in aviation.
The Standard Weight of Aviation Fuel
Aviation professionals use an industry standard weight for fuel at a reference temperature, typically 60 degrees Fahrenheit (15.6 degrees Celsius). The two main types of fuel have different standard weights due to their distinct chemical compositions. Jet fuel (primarily Jet A or Jet A-1) is kerosene-based, used in turbine engines, and weighs approximately 6.7 pounds per US gallon at the standard reference temperature.
Aviation Gasoline (Avgas) is used in piston-engine aircraft and is lighter than jet fuel. The most common type, Avgas 100LL, has a standard weight of approximately 6.0 to 6.02 pounds per US gallon. While these standard figures are used for initial flight planning, precision requires referencing specific density charts or using the actual measured density.
Factors Influencing Fuel Density
The weight of a gallon of fuel is not constant because density is highly sensitive to temperature. This is due to thermal expansion, where a liquid expands when heated and contracts when cooled. Consequently, a gallon of warmer fuel is less dense and weighs less than a gallon of colder fuel.
Fuel density changes inversely with temperature. For example, Jet A fuel pumped on a hot day will be less dense than the same volume pumped during a cold morning. Refueling operations must account for this by using temperature correction charts or density meters to accurately convert the volume of fuel delivered into a precise mass measurement.
Fuel composition is another factor, as the different hydrocarbon mixtures in kerosene-based jet fuel versus gasoline-based Avgas result in different densities. Even within jet fuel, variations exist that affect the energy content per volume. Higher density fuel offers greater energy content for a fixed volume, which benefits aircraft with limited tank space.
Why Mass is Crucial for Aircraft Operations
Although fuel is measured and sold by volume (gallons or liters), the only reliable measure for flight safety and performance is the fuel’s mass, expressed in pounds or kilograms. Mass is constant regardless of the fuel’s temperature, unlike volume, which fluctuates due to thermal expansion. The energy content required for a flight is directly proportional to the fuel’s mass, not its volume.
Aircraft performance is based on strictly defined limits, including the maximum takeoff weight (MTOW), which is a mass-based figure. Therefore, the total weight of the aircraft, including the fuel, must be calculated using mass. Accurate weight and balance calculations, such as center of gravity (CG) determination, depend on knowing the precise mass of the fuel load.
For large commercial aircraft, the fuel quantity indicating system measures and displays the fuel load in mass units (pounds or kilograms) to provide the flight crew with a consistent, reliable figure. While smaller general aviation aircraft often use volume gauges, large jets utilize advanced sensors that measure both volume and temperature to compute the true fuel mass.