Jet fuel, or aviation turbine fuel (ATF), is a kerosene-based fuel refined for use in aircraft powered by gas-turbine engines. The weight of this fuel is not a fixed number because it constantly changes with temperature and composition. The weight of a single gallon is determined by its density (mass per unit volume). Because density varies, a standardized measurement is required.
Calculating the Standard Weight
The aviation industry uses a standard reference point to establish a baseline for jet fuel density. For Jet A and Jet A-1, the most common commercial jet fuels, the standard weight is measured at a reference temperature of \(60^{\circ}\text{F}\). This standard density is approximately \(6.7\) pounds per US gallon.
Many operational calculations rely on a specific standard figure, such as \(6.75\) pounds per US gallon, for planning purposes. However, the exact weight can vary depending on the crude oil source and the refining process. Industry specifications, like those set by the American Society for Testing and Materials (ASTM), allow for a density range for Jet A fuel at \(60^{\circ}\text{F}\) that generally falls between \(6.46\) and \(6.99\) pounds per US gallon.
This difference represents a significant variance in the actual weight of the fuel delivered. A tanker truck carrying \(10,000\) gallons of jet fuel could be carrying a total weight that differs by over \(5,000\) pounds across the allowed density range. The standard weight figure serves as a common average for general estimation.
Variables That Change Fuel Density
The primary reason a gallon of jet fuel does not weigh a constant amount is the effect of temperature on volume. Liquids expand when heated and contract when cooled, a process known as thermal expansion. Because mass remains constant, a heated gallon of fuel occupies a larger volume, meaning its density, and thus its weight per gallon, decreases.
For example, \(1,000\) gallons of fuel loaded at \(0^{\circ}\text{C}\) could expand to \(1,040\) gallons if the temperature rose to \(40^{\circ}\text{C}\). The fuel remains the same mass, but the increased volume means each gallon weighs less.
Another factor contributing to density variations is the fuel’s specific gravity (SG), which indicates its composition. Jet A fuel is kerosene-based and has a higher SG, typically around \(0.804\) kilograms per liter. Jet B, a blend of kerosene and naphtha used in extremely cold climates, has a lower SG, often closer to \(0.780\) kilograms per liter. This difference in composition means a gallon of Jet B naturally weighs less than a gallon of Jet A, even at the same temperature.
Why Aviation Measures Fuel by Weight
The aviation industry measures fuel in pounds or kilograms because aircraft performance is dictated by mass, not volume. The energy released when jet fuel is burned is directly proportional to its mass. Therefore, using weight provides a consistent, accurate measure of the energy content available to the engines.
Weight is also the constant measurement required for calculating the aircraft’s weight and balance, a fundamental aspect of flight safety. Total aircraft mass, including the fuel, determines takeoff and landing distance requirements, climb performance, and structural load on the wings.
Using a volume measurement would require constant recalculation of the fuel’s weight as it cools in the tanks during flight or heats up on a hot tarmac. By measuring fuel in pounds, aviation eliminates the variable introduced by temperature-induced volume changes. This ensures the pilot’s flight plan reflects the true mass of the fuel, which is the only reliable way to calculate range and endurance.