Regan Brown
The fuel system is one of the most important systems in an aircraft. The fuel tank is an indispensable component of every aircraft, and there are various ways in which fuel is delivered to the aircraft's engine. In high-wing aircraft, gravity propels the fuel from the tank to the carburetor, and fuel pumps may not be necessary. However, low-wing aircraft and those with fuel-injected engines will need an engine-driven pump to move the fuel. The fuel system has one purpose: to deliver a steady flow of fuel from the tanks to the engine. The position, capacity, and configuration of fuel tanks depend on the aircraft's specific requirements. For example, smaller, single-engine aircraft typically feature fuel tanks positioned above the wing, utilizing a gravity feed system for fuel delivery to the engine. In most large aircraft, the fuel is stored in the wings, and the wing tank fuel is used last during the course of the flight.
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What You'll Learn
- Fuel tank types: integral, rigid, bladder, drop, wing, pylon, belly, etc
- Fuel tank locations: wings, fuselage, empennage, tail, or cargo hold
- Fuel delivery methods: gravity-fed, fuel pumps, fuel injection
- Fuel tank safety: preventing leaks, fires, and explosions
- Fuel tank maintenance: refuelling, defuelling, inspection, and repair
Fuel tank types: integral, rigid, bladder, drop, wing, pylon, belly, etc
Fuel tank types vary depending on the aircraft and its requirements. The three most common types of fuel tanks are integral, bladder, and rigid removable fuel tanks.
Integral tanks are areas inside the aircraft structure that have been sealed to allow fuel storage. They are called "wet wings" because they are commonly used in the wings of larger aircraft. These tanks cannot be removed for service or inspection, so inspection panels must be provided to allow internal inspection, repair, and overall servicing of the tank. Most large transport aircraft use this system to store fuel in the wings, fuselage, and empennage of the aircraft.
Rigid removable tanks are installed in a compartment designed to accommodate the tank. They are typically made of metal, plastic, or fibreglass construction and may be removed for inspection, replacement, or repair. These tanks are not relied on for the aircraft's structural integrity and are commonly found in smaller general aviation aircraft.
Bladder tanks, also known as bag tanks or fuel cells, are reinforced rubberized bags installed in a section of the aircraft structure designed to accommodate fuel. They are made of a flexible material and can be rolled up and put into a specially prepared structural bay or cavity through a small opening. Bladder tanks are used on aircraft of all sizes and have a long life.
Other types of fuel tanks include wing, pylon, and belly tanks. Wing tanks are the most popular location for fuel tanks, as they help prevent wing bending stresses. Pylon tanks are external fuel tanks that are attached to the aircraft's pylons, which are structures used to support the aircraft's wings. Belly tanks are typically used on military aircraft and are mounted on the aircraft's belly or underside.
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Fuel tank locations: wings, fuselage, empennage, tail, or cargo hold
The placement of fuel tanks in aircraft is a crucial aspect of their design, balancing structural integrity, safety, and functionality. The fuel tanks in most aircraft are located in the wings, but some aircraft also have tanks in the fuselage, empennage, tail, or cargo hold.
Wings
The most common location for aircraft fuel tanks is in the wings. This design improves overall structural efficiency by counter-balancing the wing's lift and the fuselage's weight. It also reduces aircraft fuel system complexity, particularly in high-wing single-engine aircraft, where the head pressure of the fuel being above the engine allows for simultaneous feeding from both wings. In addition, placing fuel in the wings improves safety by keeping it away from the passenger compartment, reducing the risk of cabin fires in the event of a crash landing. Furthermore, it helps prevent wing bending stresses and provides more room for passengers and cargo in the fuselage.
Fuselage
Some aircraft, particularly older designs, also have fuel tanks in the center fuselage, known as center tanks. The Concorde used forward and aft "Trim Tanks" to manage the center of gravity during supersonic flight. However, following the TWA 800 accident, the FAA increased certification standards for fuselage fuel tanks, leading to improved safety but greater engineering complexity. Increasing fuel capacity in the fuselage can also reduce available cargo space and add stress to the wings during flight.
Empennage, Tail, and Cargo Hold
Wide-body aircraft may have additional fuel tanks in the empennage, tail, or horizontal stabilizer to control the center of gravity during long-haul flights. These locations help balance the weight distribution of the aircraft. The tail tanks are usually used first, followed by the center tanks, and finally the wing tanks.
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Fuel delivery methods: gravity-fed, fuel pumps, fuel injection
Fuel delivery methods in aircraft can vary depending on the type of aircraft and the manufacturer. Gravity-fed fuel systems, fuel pumps, and fuel injection are three common methods of delivering fuel to aircraft engines.
Gravity-fed fuel systems are commonly used in high-wing aircraft, where the fuel tanks are located above the engines. In this design, gravity is used to deliver the fuel downwards towards the engine. Some aircraft also have surge tanks, which are part of the fuel vent system. During aircraft manoeuvres, any fuel that moves out of the tanks falls into the surge tank through a vent pipe. When the aircraft levels off, the fuel is gravity-fed back to the main tanks.
Fuel pumps are another critical component of aircraft fuel delivery. Each aircraft fuel pump typically consists of an electric pump and an engine-driven fuel pump arranged in parallel. These pumps draw fuel from the tanks and deliver it to the carburettor or fuel injectors. The engine-driven fuel pump acts as the primary pump, while the electric pump serves as a backup and helps prevent vapor lock during high-altitude flights.
Fuel injection is a method used in some high-performance single-engine aircraft and combines gravity flow with fuel pumps. Fuel injection systems spray pressurised fuel directly into the engine intake or cylinders. This provides a measured, continuous spray for smooth engine operation.
Regardless of the fuel delivery method, the basic requirement of an aircraft fuel system is to store and deliver clean fuel to the engine(s) at the required pressure and flow rate to sustain operations. The fuel is pumped through various valves, strainers, and pumps before reaching the engine. Before entering the engine, the fuel passes through a heat exchanger and a filter to maintain optimal temperature and remove any debris.
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Fuel tank safety: preventing leaks, fires, and explosions
Fuel tank safety is a critical aspect of aircraft design, maintenance, and operation. It involves taking several precautions to prevent fuel-related accidents, such as leaks, fires, or explosions. Here are some measures to ensure fuel tank safety and prevent leaks, fires, and explosions:
Preventing Leaks
To prevent leaks, aircraft fuel tanks are typically made from resilient materials such as aluminium alloys or composite materials. These materials are designed to resist corrosion and leakage. Proper sealing of the fuel tanks is crucial to prevent leaks and spills. Additionally, regular maintenance and inspections of the fuel system's components, including fuel lines, pumps, valves, and filters, are essential to ensure they are free from defects or damage that could lead to leaks or malfunctions.
Preventing Fires and Explosions
The risk of fire or explosion in aircraft fuel tanks is primarily due to the presence of flammable vapors or fuel/air mixtures. To mitigate this risk, inerting systems are sometimes installed in the fuel tanks. These systems replace oxygen with inert gases, such as nitrogen, to maintain a non-flammable atmosphere. Proper airflow management is also critical to prevent vapor buildup and pressure spikes, which could lead to explosions. Ventilation systems are carefully designed and maintained to ensure a constant flow of fresh air and pressure relief.
Grounding Procedures
Strict grounding procedures are implemented to prevent static electricity buildup, ensuring compatibility between fuel types and aircraft systems, and avoiding overfilling tanks. These procedures are crucial safety measures to prevent potential ignition sources and reduce the risk of fires and explosions.
Fuel Tank Design
The design of fuel tanks considers the stresses and forces encountered during flight, including changes in temperature, pressure, and altitude. For example, positive pressure is maintained in the tanks to prevent fuel evaporation and the development of a vacuum as the engines draw fuel. Additionally, surge tanks are used in the fuel system to capture and gravity-feed excess fuel back to the main tanks during aircraft maneuvering, helping to prevent wing bending stresses.
By following these safety measures and guidelines, the risks of leaks, fires, and explosions in aircraft fuel tanks can be significantly reduced, ensuring the safe operation of aircraft and protecting passengers, crew, and the aviation industry as a whole.
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Fuel tank maintenance: refuelling, defuelling, inspection, and repair
Fuel tank maintenance is a critical aspect of aircraft upkeep, encompassing various procedures such as refuelling, defuelling, inspection, and repair. Here is a detailed guide on these maintenance processes:
Refuelling
Refuelling an aircraft involves transferring fuel from a source to the aircraft's fuel tanks. The process can vary depending on the aircraft's design and fuel system. In high-wing aircraft, the fuel tanks are typically located inside the wings, with a filler cap on top allowing for refuelling. Low and mid-wing aircraft, on the other hand, often rely on a more complex fuel pump system to draw fuel from the tanks to the engine. These aircraft may require an engine-driven or electrically powered pump to facilitate the refuelling process. It's important to ensure that the refuelling area is clean, clear, and tidy to maintain safety and efficiency during the process.
Defuelling
Defuelling, or fuel drainage, is another essential aspect of fuel tank maintenance. Aircraft fuel systems are designed with drains at the bottom of the tanks to allow for the removal of fuel samples for inspection and the drainage of any excess or unused fuel. This process helps in moisture removal and can be crucial for troubleshooting and maintenance purposes.
Inspection
Regular inspections of aircraft fuel tanks are necessary to ensure optimal performance and prevent potential issues. Organisations should implement planned maintenance programmes that include fuel tank testing and inspections. During inspections, it is important to look for contamination, leaks, tank damages, and piping deteriorations. Connections, supports, and components such as screws, nuts, clamps, hoses, and valves should be checked and tightened or replaced if necessary. Sector valves should also be inspected for free operation, excessive backlash, and accurate pointer indication. The Environment Agency recommends annual inspections of fuel tanks by trained professionals to ensure compliance and avoid potential fines or prosecutions.
Repair
In the event of defects or damage, repair or replacement of parts may be necessary. If clamps, hoses, or valves are found to be defective during inspection, they should be repaired or replaced accordingly. This may involve tightening clamps with a hose-clamp torque wrench or replacing clamps, hoses, or both if they do not tighten properly. Organisations should work with experienced engineers to address any issues identified during inspections and ensure the proper functioning of the fuel system.
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Frequently asked questions
There are various types of aircraft fuel tank systems, and the method of fuel delivery depends on the type of aircraft and its specific requirements. Smaller, single-engine aircraft typically feature fuel tanks positioned above the wing, using a gravity feed system. Other aircraft may have tanks beneath the wing and use pumps or fuel injection.
The three main classifications for fuel tanks are integral, rigid, and bladder. Integral tanks are sealed areas inside the aircraft structure, such as "wet wings". Rigid tanks are built separately and installed in the wings or fuselage. Bladder tanks are reinforced rubber bags installed in a section designed to accommodate fuel.
The location of the fuel tank varies depending on the type of aircraft. In most large aircraft, the fuel is stored in the wings, but some aircraft also have tanks in the centre fuselage or the tail. Passenger aircraft typically have fuel tanks in the wings to reduce stress on the wings and keep fuel away from passengers and crew.
Once the fuel is pumped by the tank pumps, it is routed to the low-pressure fuel valve and then passes through engine-driven pumps. Before reaching the engine, the fuel goes through a heat exchanger and a filter to maintain optimal temperature and remove debris. Finally, it is pumped to the fuel nozzles in the combustion chamber.
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