Aviation Gasoline (Avgas), commonly used in piston-engine aircraft, is a specialized fuel formulated to meet the high-performance demands of flight. Understanding the fuel’s weight per volume is a fundamental calculation in aviation, but the answer is not a single, fixed number. Avgas, such as the widely used 100LL (100 Low Lead), is distinct from Jet Fuel, which is kerosene-based and powers turbine engines, and the two have significantly different densities.
The Standard Weight and Specific Gravity
The nominal, industry-standard weight for Avgas is typically calculated at 6 pounds per U.S. gallon for general flight planning purposes. This figure represents an average value used by pilots and operators for quick estimations of fuel weight before a flight. The actual weight of a gallon of Avgas can vary slightly, which is why the scientific concept of Specific Gravity (SG) is employed for precise quality control. SG is a ratio comparing the fuel’s density to the density of water at a standard temperature.
The Specific Gravity of Avgas 100LL generally ranges from about 0.71 to 0.72 when measured at the standard temperature of 60°F (15.6°C). This means that Avgas is approximately 71% to 72% as dense as water, which weighs about 8.34 pounds per gallon. This SG range translates to an actual density of approximately 5.92 to 6.02 pounds per gallon at that standard temperature.
How Temperature Affects Fuel Density
The weight of Avgas is not a fixed constant because the fuel is subject to thermal expansion, a principle common to most liquids. As the temperature of the fuel increases, the liquid expands in volume, which causes a corresponding decrease in its density, thus reducing the weight per gallon. Conversely, when the fuel temperature drops, the Avgas contracts, becoming denser and heavier per gallon. This physical property means that a gallon of fuel loaded on a hot summer day will weigh less than a gallon loaded in freezing winter conditions.
Precise calculations require the use of density correction tables to determine the true volume and weight of the fuel. These tables provide a Volume Correction Factor (VCF) that adjusts the measured volume back to the standard reference temperature of 60°F. Aviation fuel suppliers and large commercial operators use these correction factors to ensure accurate billing and quality control during delivery and storage. In extreme climates, this thermal expansion is significant enough that failing to apply these corrections can result in a measurable difference in the actual fuel mass loaded onto the aircraft.
The Role in Aircraft Weight and Balance
The variability in fuel density, driven primarily by temperature, has direct and significant consequences for aircraft operation. Every pilot is required to perform an accurate Weight and Balance (W&B) calculation before a flight to ensure the total aircraft weight is within safe limits and the Center of Gravity (CG) falls within the approved envelope. Miscalculating the fuel weight can lead to an aircraft being unintentionally overweight, which degrades performance and increases takeoff and landing distances.
The nominal 6 pounds per gallon figure is a planning convenience, but it can be misleading when the actual fuel temperature is far from the 60°F standard. For instance, if a large volume of very cold, and therefore denser, fuel is loaded, the actual weight added to the aircraft will be greater than the weight calculated using the nominal figure. This difference can shift the aircraft’s CG, potentially moving it outside the safe limits, which affects stability and control. This is why sophisticated operations may require measuring the fuel by weight directly or applying the temperature-based density correction to the volume, rather than relying solely on the standard average value.