Ameer Randolph
The fuel system is one of the most important systems in an aircraft. The fuel tank is an indispensable component of every aircraft, and it must be optimally positioned to store and deliver clean fuel to the engine at the correct pressure and flow rate. Fuel systems vary notably from plane to plane, with some aircraft having tanks in the wings, the centre body, the tail, the belly of the aircraft, or a combination of these locations. In high-wing aircraft, gravity propels the fuel from the tank to the carburetor, while low-wing aircraft require an engine-driven pump to move the fuel. The weight of the tanks and fuel also counteract wing bending loads during manoeuvres and reduce fatigue on the spar structure.
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What You'll Learn
Fuel tank types: integral, rigid, bladder, drop, and wing
Fuel tanks are typically classified into three types: integral, rigid, and bladder.
Integral tanks are constructed within sealed regions of the wings, known as "wet wings". They consist of non-corrosive aluminium frames that can withstand immersion in fuel. These tanks are designed to bend under aerodynamic stress and accommodate expansion or contraction due to temperature changes. Integral tanks are usually lighter than rigid metallic tanks but are more challenging to maintain and repair as they cannot be removed or replaced individually. They are often used in high-wing aircraft with gravity feed systems, where fuel flows from the tank to the engine using gravity.
Rigid tanks, on the other hand, are built separately from the aircraft and installed in the wings or fuselage. They are typically made from materials like aluminium alloy or stainless steel and are designed to prevent leaks through riveting and seam welding. Rigid tanks simplify maintenance and repair as they can be easily removed, repaired, and reinstalled, or replaced if needed. These tanks are pressure-tested to ensure they do not leak or collapse during flight.
Bladder tanks, also known as fuel cells, are made of reinforced flexible materials such as synthetic rubber. They are similar to rigid tanks but do not require large cuts into aircraft structures for installation. Bladder tanks can be installed through small openings, such as inspection holes, and then unfurled to their full size. They are used on aircraft of all sizes and offer strength and longevity.
In addition to these main types, there are also drop tanks and wing tanks. Drop tanks are external fuel tanks that can be attached to the wings or fuselage of an aircraft to increase its fuel capacity. Wing tanks refer to the fuel tanks located within the wings of an aircraft. Most light aircraft have their fuel tanks located inside the wings, with filler caps on top for refuelling and drains at the bottom for fuel sampling and moisture removal.
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Fuel tank locations: wings, fuselage, belly, and tail
Fuel tank locations vary depending on the type of aircraft. Most aircraft store fuel in the wings, although some also have tanks in the centre body or fuselage, and wide-body aircraft have extra tanks in the tail or horizontal stabilizer.
Wings
The wings are the most common location for aircraft fuel tanks. Most light aircraft and single-engine high-wing aircraft have their fuel tanks located inside the wings. This is because there is a lot of empty space in the wings, and creating space elsewhere for fuel would make the aircraft larger and heavier. In multi-engine aircraft, wing fuel allows aircraft designers to direct fuel from a wing directly to its respective engine. Storing fuel in the wings also improves the overall structural efficiency of the aircraft by counter-balancing the wing’s lift and the fuselage’s weight, and reduces aircraft fuel system complexity. It also reduces the loads where the wings meet the fuselage, preventing wing bending stresses. In the event of a crash landing, having the fuel in the wings keeps it away from the cabin and the occupants, reducing the risks of a cabin fire.
Fuselage
Some aircraft have fuel tanks in the centre body or fuselage, called centre tanks. However, adding weight to the fuselage increases bending stress on the wing root structures and can reduce the practical cargo capacity.
Belly
Some aircraft have a belly tank. However, if the belly tank is filled first, the weight is supported by the fuselage/wing joint before being transferred to the landing gear. This can cause the wing structure to become overloaded and fail prematurely.
Tail
Wide-body aircraft have extra tanks in the tail or the horizontal stabilizer, which are used to control the centre of gravity of the aircraft during long-haul flights.
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Fuel transfer: gravity-fed or pumped
The fuel transfer system in an aircraft can either be gravity-fed or pumped. The former is the simplest, least expensive, and most reliable system, as it has no moving parts and is yet to see any known failures. In this system, gravity is used to deliver the fuel, which means that the fuel tanks must be located above the engine. This is why high-wing aircraft commonly use gravity-fed systems.
The gravity-fed system is also advantageous because fuel will continue to flow even when the tank outlet is no longer submerged in fuel. However, it can only flow downhill and at low pressure. This means that all aircraft with gravity-fed fuel systems have carbureted engines, as gravity cannot create enough pressure to run an injected engine.
Low- and mid-wing aircraft cannot utilize gravity-fed fuel systems because their fuel tanks are not located above the engine. Instead, they use one or more pumps to move the fuel from the tanks to the engine. These pumps are either mechanically or electrically powered. Engine-driven pumps are found in older aircraft models, while electric fuel pumps are more common in modern aircraft due to their reliability and efficiency.
Pumped systems are able to provide sufficient pressure to move fuel to an engine that is higher than the tank, with the pressure required for a fuel-injection system. However, a disadvantage of this system is that no fuel will move if the outlet of the tank is exposed to air.
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Fuel overflow: surge tanks and venting
Fuel overflow is prevented by the use of surge tanks and venting. All the main fuel tanks in an aircraft are connected to the surge tank through a vent pipe. During aircraft manoeuvring, any fuel that moves out of the tanks falls into the surge tank through the vent pipe. When the aircraft levels off, the fuel from the surge tank is gravity-fed back to the main tanks.
The surge tank is also vented to the atmosphere to release fuel in the event of a fuel overflow. It is provided with ram air, which helps to pressurize the main fuel tanks, keeping them at a slight positive pressure. This prevents excessive evaporation, which can occur when the aircraft climbs higher and the reduced atmospheric pressure causes the fuel's boiling point to decrease.
The positive pressure also helps to prevent a vacuum from developing in the tanks as the engines draw fuel. The fuel tank also consists of suction valves that allow fuel to be drawn by the engines in the event of tank pump failure.
Vents in the fuel system allow internal pressures to equalize. A sensing unit in each tank measures the fuel quantity, which is displayed on the fuel quantity gauges in the cockpit.
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Fuel safety: preventing leaks, explosions, and fires
Fuel tank safety is a critical aspect of aircraft design, maintenance, and operation. It aims to prevent fuel-related accidents, such as leaks, fires, or explosions. Fuel tank leaks can lead to fuel loss, compromising the aircraft's range and performance, and creating potential fire hazards. Therefore, it is crucial to implement preventive measures to minimise the occurrence of fuel tank leaks and mitigate safety risks.
To prevent leaks, regular inspection and maintenance of the fuel system are essential. Aircraft maintenance personnel follow stringent protocols, including visual inspections, non-destructive testing, and repairs or replacements of fuel tanks. Advanced techniques such as ultrasound, X-ray, and thermal imaging help detect even the smallest leaks. Proper sealing of fuel tanks is also crucial to prevent unwanted leaks and spills. Additionally, environmental factors such as temperature and pressure changes should be considered when designing fuel tanks to ensure they can withstand varying conditions without developing leaks.
Fuel tank explosions and fires are often caused by the ignition of flammable fuel/air vapour mixtures in the tanks. To prevent this, fuel tank inerting systems can be used to replace oxygen with inert gases like nitrogen, creating a non-flammable atmosphere. Proper airflow management is also critical to avoiding vapour buildup and pressure spikes that could lead to explosions. Ventilation systems are meticulously designed to ensure a constant flow of fresh air and pressure relief.
Furthermore, aircraft fuel tanks are typically made from resilient materials such as aluminium alloys or composite materials, engineered to resist corrosion and leakage. Regular maintenance and inspections of these tanks and their components, including fuel lines, pumps, valves, and filters, are necessary to ensure they are free from defects or damage that could lead to fuel leaks or malfunctions.
By following these safety measures and guidelines, the risks of fuel leaks, explosions, and fires can be significantly reduced, ensuring the safety of aircraft and their occupants.
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Frequently asked questions
There are various ways that fuel can get to the aircraft fuel tank. In most large aircraft, the fuel is stored in the wings, but some aircraft also have tanks in the fuselage, or centre body. The specific requirements of the aircraft determine the position, capacity and configuration of the fuel tanks. Some aircraft have gravity-fed systems, while others use pumps or fuel injection.
In high-wing aircraft, gravity propels the fuel from the tank to the carburetor, and fuel pumps might not be necessary. However, low-wing aircraft will need an engine-driven pump to move the fuel.
Tank pumps or fuel booster pumps, controlled by the pilot, are used to pump fuel from the fuel tanks to the main engine-driven fuel pump. Each tank has two pumps, powered by the aircraft's main electrical system.
