Rajan Wilcox
Aircraft fuel systems are designed to store and deliver clean fuel to the engine(s) at a pressure and flow rate that can sustain operations. Fuel tanks don't need to be pressurised, but they can be. Pressurised fuel tanks are not open to the atmosphere, so fuel has nowhere to escape. The pressure helps to decrease fuel evaporation and assists the fuel boost pumps by providing a small positive pressure head on the fuel. Some aircraft have a small header tank between the normal fuel tank and the engine, to facilitate reliable fuel flow to the engine.
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What You'll Learn
Fuel tank design and function
The design of the fuel system depends on the type of aircraft and the number of engines. Single-engine light aircraft fuel tanks are usually in the wings, but some aircraft have a "header tank" between the normal tank and the engine to ensure a reliable fuel flow. In multi-engine aircraft, each wing tank often has its own electric boost pump, and each engine has a mechanical pump. The wing tanks are also connected to each other to ensure equal pressure when both tanks feed the engine.
Fuel tanks do not need to be pressurised, but some are. Pressurised tanks are not open to the atmosphere, so the fuel has nowhere to escape. This means that the tank must be reinforced to withstand the pressure difference. Pressurisation can be achieved through engine bleed air, which keeps moist air out and ensures a positive pressure feed to the pumps. It also helps decrease fuel evaporation. Nitrogen is also used to pressurise fuel tanks for fire suppression and to prevent auto-ignition.
External tanks are used to extend the range of an aircraft, and some combat aircraft use drop tanks that are discarded after use. The addition of tanks and engines increases the complexity of the fuel system. The management of the fuel system is crucial, especially in larger aircraft where the load in the fuselage tanks affects the centre of gravity, imposing limitations on the amount of fuel carried and the order in which it is used.
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Fuel tank pressure and aircraft performance
Aircraft fuel systems are designed to store and deliver clean fuel to the engine(s) at a pressure and flow rate that can sustain operations, regardless of the aircraft's operating conditions. The performance of an aircraft is directly linked to its fuel system and its ability to manage and deliver fuel. Fuel tanks do not need to be pressurised, but they can be. Unpressurised tanks are vented to the atmosphere, so a large pressure difference never occurs. Pressurised fuel tanks are not open to the atmosphere, so fuel cannot escape. If a pressurised tank is kept at a significant pressure difference to the outside air, it needs to be reinforced.
Some aircraft have a small "header tank" between the normal fuel tank and the engine to ensure a reliable fuel flow to the engine. On small or older aircraft, header tanks are often mounted above and/or immediately behind the engine. The addition of tanks and engines increases the complexity of the fuel system and its management. For example, multi-engine aircraft will often have a method to "cross-feed" the engine, with the left tank feeding the right engine, or vice versa.
The pressure in a fuel tank is provided by bleed air from the engines. This pressurisation helps to decrease fuel evaporation and assists the fuel boost pumps by providing a small positive pressure head on the fuel. It also keeps moist air out and ensures a positive pressure feed to the pumps. The XB-70 aircraft, for example, had a maximum speed of Mach 3, which could heat the airframe to 250°C. Nitrogen was injected into the fuel during refuelling to prevent auto-ignition, and the fuel pressurisation and inerting system maintained tank pressure.
The performance of an aircraft is also affected by the location of the fuel tanks. Low- and mid-wing single reciprocating engine aircraft cannot use gravity-feed fuel systems because the fuel tanks are not located above the engine. Instead, pumps are used to move the fuel from the tanks to the engine. The weight of the fuel in the fuselage of larger aircraft affects the centre of gravity of the aircraft, imposing limitations on the amount of fuel carried and the order in which fuel must be used.
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Fuel tank pressure safety considerations
Materials and Design
The choice of materials for fuel tanks is critical to withstanding pressure, temperature, and chemical exposure. Steel, known for its strength and durability, is commonly used in fuel tanks to withstand high pressures. Aluminum is often selected for aircraft fuel tanks due to its lightweight and corrosion-resistant properties. Composite materials offer a balance between strength and lightweight characteristics. Explosion-proof designs are also incorporated to minimize the risk of explosions in high-pressure or volatile environments.
Pressure Testing and Maintenance
Regular fuel tank pressure testing is essential for maintaining the integrity of the fuel system. Pressure tests help identify potential leaks, faulty components, and overall system health. Mechanics and vehicle owners can pressurize the tank with compressed air and monitor for leaks or pressure loss. A failing pressure test indicates a leak or damage, requiring prompt diagnosis and repair to ensure safety and efficiency.
Venting Systems
Venting systems are crucial for maintaining atmospheric pressure within fuel tanks. Unpressurized tanks are typically vented to the atmosphere to prevent significant pressure differences. Some aircraft fuel tanks use nitrogen inerting systems to maintain tank pressure and reduce the likelihood of autoignition. The B-52's wet wings, for example, are vented to the outside air.
Safety Features and Monitoring
Fuel tanks are equipped with safety valves that automatically vent excess pressure to maintain pressure below a safe upper limit. Modern monitoring systems, utilizing sensors and IoT technology, provide real-time data on fuel levels, temperature, and pressure. These systems enable proactive management of tank conditions and offer significant improvements in routine operation and crisis management.
Fuel Transfer and Engine Considerations
The design of the fuel system also impacts pressure safety. In some aircraft, fuel is transferred from outboard tanks to the main tank under pressure provided by bleed air from the engines. High-wing, high-performance, single-engine aircraft may use fuel injection systems that spray pressurized fuel into the engine intake or cylinders, requiring precise metering for smooth engine operation.
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Fuel system management
For single-engine piston aircraft, the fuel system is relatively simple. Fuel is stored in tanks, typically located in the wings, and gravity is utilised to feed fuel to the engine. A fuel control valve, or fuel selector, plays a crucial role by acting as a shut-off mechanism in case of emergencies and allowing the pilot to choose the active tank. Additionally, a strainer is employed to remove any sediment or water from the fuel before it reaches the carburetor or primer pump.
In multi-engine aircraft, the fuel system becomes more intricate. Each wing tank may have its own electric boost pump, and each engine has a dedicated mechanical pump. This setup enables cross-feeding, where one engine can draw fuel from the tank on the opposite side. The use of multiple tanks and engines increases the complexity of the system and necessitates careful management to ensure reliable fuel flow.
To maintain fuel quality and prevent issues caused by low temperatures, fuel tanks are equipped with thermometers and heating systems. Additionally, some tanks are pressurised using engine bleed air to prevent moisture ingress and ensure a positive pressure feed to the pumps. This pressurisation also helps decrease fuel evaporation during flight.
The management of external tanks, such as drop tanks used by combat aircraft, is another aspect of fuel system management. These external tanks extend the aircraft's range, and a fuel pump is necessary to transfer fuel from these auxiliary tanks to the main tank. Overall, effective fuel system management involves a combination of mechanical components, careful monitoring, and operational procedures to ensure the safe and efficient utilisation of fuel in various aircraft configurations and operating conditions.
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Fuel tank heating and condensation prevention
Aircraft fuel tanks are pressurised to prevent water condensation and the freezing of fuel or condensation at low temperatures. Pressurisation increases the freezing point of the fuel and any condensation that has entered the tank.
To prevent condensation and freezing, fuel tanks are equipped with thermometers and heating systems. These heating systems are designed to warm jet fuel and any condensation that has entered the tank. They are controlled by a thermostat, which senses the temperature of the fuel and only activates the heating system when the fuel is cold.
Pressurisation is also used to prevent condensation. Fuel tanks are pressurised with engine bleed air to keep moist air out. Normally vented tanks also help to reduce the water influx rate, although they have a breathing mode that can pump moist air during temperature and pressure swings.
The rate at which condensation forms in a fuel tank depends on factors such as temperature, humidity, and the volume of the tank and the air inside it. For example, a nearly empty 25-gallon tank with an air volume of 20 gallons at 120 °F and 100% relative humidity will generate about 0.25 oz (7.5 ml) of water per day. Keeping the tank 90% full would reduce the water generation rate by 90%, and being in 30% humidity air would reduce the rate by 70%.
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Frequently asked questions
No, they don't need to be pressurised, but they can be. Unpressurised tanks are vented to the atmosphere, so a large pressure difference never develops.
Pressurised fuel tanks are not open to the atmosphere, so fuel has nowhere to escape to. Pressurisation also helps to decrease fuel evaporation and assists fuel boost pumps by providing a small positive pressure head on the fuel.
Fuel tanks can be pressurised with engine bleed air to keep moist air out and ensure a positive pressure feed to the pumps. Nitrogen can also be injected into the fuel during refuelling to pressurise the tank and prevent autoignition.
A single positive pressure refuelling point is used to fuel all tanks in larger aircraft. The refuelling pipe has a fixed orifice to prevent over-pressurisation. The refuelling valve shuts off the supply when the tanks are full.
It is important to ensure that the fuel caps on the tip tanks are secure after refuelling. If a cap comes off when the tank is full, the pressurised air can force the fuel out of the tank, resulting in an out-of-balance condition and loss of lateral control.
