Sylvie Willis
The weight of a plane is a critical factor in aviation, with the weight of fuel being a significant component. A plane's weight directly impacts its ability to achieve lift, and fuel weight is a variable that can be adjusted based on the mission's requirements. The weight of fuel is calculated using specific gravity, which is influenced by temperature. Airliners typically have a fuel fraction of less than half their takeoff weight, ranging from 26% for medium-haul flights to 45% for long-haul flights. However, the weight of the plane, including passengers, cargo, and fuel, must be carefully balanced to ensure a safe and efficient flight.
| Characteristics | Values |
|---|---|
| Fuel fraction for jet fighter aircraft | 29% and below for subcruisers, 33% for quasi-supercruiser, 35% and above for supercruising missions |
| Fuel fraction for specific aircraft | F-22 Raptor – 29%, Eurofighter – 31%, F-4 Phantom II – 29%, F-15 Eagle – 29%, Russian Mikoyan MiG-29 "Fulcrum" – 29%, Mikoyan MiG-31 "Foxhound" – 45%, Panavia Tornado – 26%, Rutan Voyager – 72%, Virgin Atlantic GlobalFlyer – 83% |
| Fuel consumption rate | Boeing 747 – 1 gallon/4 liters per second |
| Fuel cost | $5.70 per gallon, $6 per gallon |
| Fuel weight calculation | Based on specific gravity (SG) of the fuel, which varies with temperature; 1 litre of jet fuel at SG 0.780 is equal to 780 grams |
| Standard Empty Weight | Baseline for weight calculations, includes airframe, engines, fixed operating equipment, unusable fuel, and operating fluids |
| Basic Empty Weight (BEW) | Includes Standard Empty Weight components plus optional equipment and full engine oil |
| Licensed Empty Weight | Similar to BEW but excludes full engine oil, includes only undrainable oil |
| Gross Landing Weight | Takeoff weight minus fuel burned en route |
| Ramp Weight | Weight of the airplane loaded for flight before engine start |
| Zero Fuel Weight (ZFW) | Weight of the aircraft before the addition of fuel |
| Gross Takeoff Weight | Weight of the airplane just before brake release to begin the takeoff roll |
| Useful Load | Weight of crew, cargo, passengers, baggage, and usable fuel; varies from flight to flight |
| Maximum Ramp Weight (MRW) | Maximum weight for ground operations |
| Maximum Takeoff Weight (MTOW) | Maximum weight for takeoff |
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What You'll Learn
Fuel weight calculations
Standard Weights and Indices:
Aircraft manufacturers often use standard weights and indices to simplify the calculation of aircraft weight and balance. Standard weights are assigned to commonly carried items like fuel, oil, fluids, passengers, and baggage. These values are used in conjunction with the aircraft's weight indices, which represent the structural limits of the airplane during landing and takeoff.
Specific Gravity and Fuel Density:
Specific gravity (SG) is a crucial factor in determining fuel weight. SG is a dimensionless number that represents the ratio of the density of jet fuel to the density of water at a given temperature. The SG of jet fuel varies with temperature, typically ranging from 0.775 to 0.840. By multiplying the volume of fuel (in litres) by its SG, we can determine the weight of the fuel in kilograms.
Useful Load and Maximum Takeoff Weight:
The useful load of an aircraft includes everything that is added to the aircraft for a specific flight, such as crew, passengers, baggage, cargo, and usable fuel. It is calculated by subtracting the Basic Empty Weight (BEW) or Licensed Empty Weight (excluding fuel) from the Maximum Takeoff Weight (MTOW). The MTOW is the maximum weight allowed for takeoff and is crucial for determining the maximum fuel load an aircraft can carry.
Fuel Fractions and Aircraft Performance:
Fuel fraction refers to the percentage of an aircraft's weight that is fuel. Different types of aircraft have different fuel fraction requirements. Airliners typically operate with fuel fractions between 26% for medium-haul flights to 45% for long-haul flights. Higher fuel fractions are needed for extended missions or supercruising capabilities, as seen with the Rutan Voyager's record-breaking 72% fuel fraction for its around-the-world flight in 1986.
Thrust-to-Weight Ratio and Fuel Efficiency:
Aircraft require a specific thrust-to-weight ratio to achieve lift. A heavier payload, including fuel, decreases this ratio, necessitating higher velocity and increased fuel consumption to produce sufficient lift. Therefore, a fully loaded aircraft will be less fuel-efficient and have a shorter range. Additionally, the weight distribution and balance of the aircraft, influenced by fuel placement, play a significant role in its performance and stability.
In conclusion, fuel weight calculations in aviation involve a complex interplay between aircraft weight indices, fuel density, useful load considerations, and performance requirements. These calculations ensure safe and efficient flight operations, helping pilots and operators make informed decisions about fuel loading, range, and payload management.
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Fuel fractions
The weight of an aircraft is a critical factor in aviation. It is essential to know how much an airplane weighs to maintain level flight and avoid stalls. Aircraft weight is influenced by various factors, including the weight of the aircraft itself, passengers, cargo, and fuel.
Fuel fraction is a critical parameter in aircraft design and performance. It refers to the percentage of the aircraft's total weight that is attributed to fuel. Different aircraft have different fuel fraction requirements depending on their mission and operational needs. For example, jet fighter aircraft with fuel fractions of 29% and below typically yield subcruisers, while 33% provides quasi-supercruising capabilities, and 35% and above are required for efficient supercruising missions. Airliners have fuel fractions ranging from 26% for medium-haul flights to 45% for long-haul journeys.
The fuel fraction of an aircraft is influenced by several factors, including the aircraft's design, engine efficiency, and mission requirements. For instance, the Russian Mikoyan MiG-31 "Foxhound", a supersonic interceptor, has a fuel fraction of over 45%, while the Panavia Tornado, with a relatively low internal fuel fraction of 26%, often utilised drop tanks.
Calculating fuel weight is crucial for determining an aircraft's overall weight and balance. Fuel weight is typically measured in litres or gallons and then converted into kilograms or pounds. The specific gravity of the fuel, which can vary with temperature, is used to determine its weight. For example, jet fuel with a specific gravity of 0.780 weighs 780 grams per litre. By knowing the volume of fuel and its specific gravity, the total fuel weight can be calculated.
Optimising fuel fractions is essential for efficient aircraft operations. A heavier aircraft will be less fuel-efficient and have a shorter range. Therefore, pilots and airlines carefully manage the balance between passenger and cargo weight and the amount of fuel loaded to ensure safe and cost-effective flights.
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Weight and balance
The weight and balance of an aircraft must be carefully calculated to ensure safe flight. The center of gravity (CG) is a crucial factor in determining an aircraft's stability and controllability. The CG is the point at which the aircraft's total weight is assumed to be concentrated, and it must fall within specific limits set by the designer for safe flight. If the CG is too far forward, the aircraft becomes challenging to control, and the takeoff and landing procedures can be compromised. On the other hand, if the CG is too far aft, the aircraft becomes less stable, with overly sensitive and twitchy flight controls.
To calculate the weight and balance, various methods and considerations are employed. One important concept is the moment, which is the force that causes an object to rotate. The moment is calculated by multiplying the weight of an item by its distance from a reference datum, usually the airplane's nose or the leading edge of the wings. The total moment is the weight of the aircraft multiplied by the distance between the datum and the CG. Additionally, manufacturers often provide maximum gross weight, center of gravity limits, and tables with sample weight and balance numbers for different loading situations.
Federal Aviation Regulation 91.7, Civil Aircraft Airworthiness, places the responsibility on the pilot in command to determine the aircraft's safety for flight, including ensuring it is within its weight and balance limits and does not exceed its maximum gross weight. Proper weight and balance calculations are essential for the safety of the aircraft and its occupants, and countless accidents have occurred due to pilots neglecting these crucial preflight checks.
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Thrust-to-weight ratio
The weight of an aircraft is determined by the size and materials used in its construction, as well as the payload and fuel it carries. The weight of a plane is always directed towards the centre of the Earth. The thrust, on the other hand, is determined by the size and type of propulsion system used, as well as the throttle setting selected by the pilot. Thrust is typically directed forward along the centre line of the aircraft.
The thrust-to-weight ratio is a dimensionless ratio of thrust to weight of a reaction engine or a vehicle with such an engine. Reaction engines include jet engines, rocket engines, pump-jets, Hall-effect thrusters, and ion thrusters. These engines generate thrust by expelling mass (propellant) in the opposite direction of intended motion, in accordance with Newton's third law.
The thrust-to-weight ratio is calculated by dividing the thrust by the weight of the aircraft (not to be confused with mass). The formula is:
The thrust-to-weight ratio is an important indicator of an aircraft's performance, along with the lift-to-drag ratio. It is also crucial data when designing an aircraft, as it indicates the manoeuvrability of the aircraft. An aircraft with a high thrust-to-weight ratio has high acceleration and a high rate of climb.
The thrust-to-weight ratio of an aircraft varies continuously during a flight. Thrust varies with throttle setting, airspeed, altitude, air temperature, etc. Weight also varies with fuel burn and payload changes. The instantaneous thrust-to-weight ratio of a vehicle can vary during operation due to factors such as fuel consumption and changes in gravitational acceleration.
Aircraft weight is carefully calculated before takeoff, and the weight can vary depending on factors such as the number of passengers and bags, fuel load, cargo weight, and even the weight of the pilot. Therefore, the amount of fuel carried by a plane can significantly impact its thrust-to-weight ratio.
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Fuel cost
The weight of a plane's fuel is a significant factor in the overall weight of the aircraft, and it can vary depending on the type of aircraft and the length of the journey. Airliners typically have a fuel fraction of less than half their takeoff weight, with medium-haul flights having around 26% fuel fraction and long-haul flights up to 45%. For context, the Rutan Voyager's 1986 around-the-world flight had a fuel fraction of 72%, the highest at the time.
In terms of fuel cost, jet fuel prices can vary depending on various factors, including the region and the current market price. As of July 28, 2025, the national average price for jet fuel in the US was $6.29 per gallon. However, prices can fluctuate, and historical data shows that jet fuel prices have experienced overall rises compared to prices in the year 2000.
The cost of fuel for a specific aircraft depends on its fuel efficiency, the distance travelled, and the price of fuel at the time of refuelling. For example, a Boeing 747-300 can carry 52,410 US gallons of usable fuel, and with a fuel cost of around $6 per gallon, the total fuel cost for a journey can be substantial.
While passenger weight can impact fuel consumption, the effect on fuel costs for airlines is relatively minor. This is because aircraft are designed to operate within specific thrust-to-weight ratios, and minor increases in weight may not significantly affect overall fuel efficiency. Additionally, the rise in ticket prices is influenced more by factors such as extra security and the use of x-ray machines.
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Frequently asked questions
The amount of fuel as a fraction of a plane's weight is known as the fuel fraction. Airliners have a fuel fraction of less than half their takeoff weight, with medium-haul flights at 26% and long-haul flights at 45%.
A plane's fuel requirements depend on its useful load, which includes the weight of the crew, passengers, baggage, cargo, and usable fuel. The heavier the plane, the more fuel it will need to generate enough lift to take off.
The weight of a plane's fuel is calculated by taking the volume of fuel in litres and multiplying it by the fuel's specific gravity (SG). The SG of jet fuel varies with temperature, and it is typically around 0.780 in the tropics.
