The fuel system is one of the most important systems in an aircraft. It allows for proper fuel supply to the engines and proper fuel distribution and balance which is essential for a safe flight.
The fuel storage tanks and venting system
In most large aircraft, the fuel is stored in the wings. Some aircraft also have tanks in the center body, or the center fuselage called center tanks. Widebody aircraft also have tanks in the tail or the horizontal stabilizer which are used to control the center of gravity of the aircraft during long-haul flights.
The storage of fuel in the wings helps to prevent wing bending stresses. And due to this reason, wing tank fuel is used last during the course of the flight. For example, if an aircraft has a center tank, the center tank fuel is used first before the fuel is drained from the wings. Also, in larger aircraft, the wing tank is divided into an outer and inner tank. In this case, the inner tank fuel is used before the fuel from the outer tank is used. This again helps to relieve stresses on the wing.
Photo: Tom Boon – Simple Flying
In addition to the storage tanks, there are tanks present in the fuel system known as surge tanks. These tanks are also a part of the fuel vent system. All the main fuel tanks in the aircraft are connected to the surge tank through a vent pipe. During aircraft maneuvering, any fuel that moves out of the tanks falls into the surge tank through the vent pipe. And 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 if there is a fuel overflow. It is, at the same time, provided with ram air which helps to pressurize the main fuel tanks. This keeps the tanks at a slight positive pressure. This, for one, prevents excessive fuel evaporation. As the aircraft climbs higher and higher, the reduced atmospheric pressure decreases the boiling point of the fuel. This causes fuel to evaporate. When the tanks are fed with positive pressure, the fuel is kept from experiencing reduced pressure.
The positive pressure also helps to prevent a vacuum from developing in the tanks as the engines draw fuel from the tanks.
Photo: Airbus A380 FCOM
The inner workings of the fuel system
The fuel tanks consist of tank pumps or fuel booster pumps which can be controlled by the pilot. In most cases, each tank has two tank pumps. These pumps are powered by the main electrical system of the aircraft. The job of these pumps is to pump the fuel from the fuel tanks to the main engine-driven fuel pump, which then pumps fuel to the engine itself.
In aircraft capable of flying at high altitudes, the tank pumps are a necessity because the reduced pressure at altitudes can cause fuel to boil, causing vapor locks that can prevent fuel from entering the engine-driven pump.
Photo: Airbus A380 FCOM
The fuel tank also consists of suction valves that allow fuel to be drawn by the engines in the event of tank pump failure. This requires the pilots to descend to a lower altitude which prevents low-pressure fuel boiling.
Once the fuel is pumped by the tank pumps, it is then routed to the Low Pressure (LP) fuel valve, sometimes called the spar valve. From there, the fuel passes through the engine-driven pumps. Some aircraft have both a Low Pressure, LP pump and a High Pressure, HP pump, which is driven by the high-pressure compressor of the engine.
Before the fuel is routed to the main engine components, it goes through the fuel/oil heat exchanger and the fuel filter. The heat exchanger keeps the fuel at an optimum temperature, while the filter blocks any debris in the fuel. Once passed through the exchanger and the filter, the fuel is pumped by the HP pump to the fuel nozzles in the combustion chamber.
The fuel is also used to run the actuators of systems such as the variable stator vanes inside the engines using fuel hydraulic signals. In some aircraft, the fuel is also used to cool the electrical generators.
Photo: Airbus A380 FCOM
In normal operations, the left-wing tank supplies fuel to the left engine, and the right-wing tank supplies fuel to the right engine. In an engine flameout event, the remaining engine can be supplied with fuel from the other side using a crossfeed valve. For example, if the right engine were to fail, fuel from the left-wing tank could be routed to the right engine when the crossfeed valve is opened.
The crossfeed can also be used to balance the fuel in the air between the tanks. To perform this procedure, the pilots can turn off the wing tank pumps of the lighter side and open the crossfeed valve. This allows the fuller tank to supply both engines. Once the balance between the tanks is achieved, the wing tank pumps can be turned back on and the crossfeed valve closed.
The fuel for the Auxiliary Power Unit (APU) is typically fed from one of the wing tanks. It has a pump of its own which automatically comes on when the APU start sequence is initiated. If the APU pump were to malfunction, the supplying tank pumps can be turned on.
The refueling points in most large aircraft can be found under the wings. However, in some aircraft, it is in the side belly. This point is called a refuel coupling, and this is where the fuel bowser hose is connected.
This type of fueling is known as pressure fueling because the fuel is delivered to the tanks at high pressure.
Photo: Getty Images
To control the refueling, a control panel is available. In this panel, the operator can dial in or pre-set the amount of fuel that is required. Once the hose is connected, the refuel valves open, and the fueling begins. This entire process is automatic. During refueling, the outer tanks are filled up first. When it is full, fuel overflows into the inner tank and the center tank. When the fuel level reaches the selected value, the refuel valves are closed, and the fueling stops.
Photo: Anas Maaz
Most manufacturers also provide a means to fuel the aircraft manually using gravity. For this, manual refuel points are located on the wings. In manual refueling, the fueler controls the refueling, and it is recommended to fill up the wing tanks before filling up the center tanks.
The main disadvantage of this type of refueling is that it can take a lot of time to complete the fueling process.
How is fuel quantity measured?
To measure the fuel quantity, capacitors are used. The capacitor consists of two plates that are supplied with AC electrical current.
Photo: Boeing 737 FCOM
The current flow in such a circuit depends on four factors. They are:
- The level of voltage applied
- The frequency of the supply
- The size of the capacitor plates
- The dielectric constant.
The first three of these factors (voltage, frequency, and plate size) remain fixed, and the only factor that changes is the dielectric constant. This is because, at a given time, the dielectric constant can either be air, fuel, or a mixture of air and fuel. As the capacitor is drenched in fuel, there is an increase in current, which is compared to a reference capacitor with air as a dielectric. The difference in these two measurements can be then used to get a very accurate indication of fuel.
The main problem with this system is that it cannot compensate for the temperature. The Specific Gravity (SG) or density of the fuel is inversely proportional to the temperature. Thus, when there is a drop in temperature, the fuel volume reduces and causes errors in fuel indication. Similarly, when there is an increase in the temperature, the fuel volume increases.
To solve this problem, compensators are used. The compensators are probes that are placed deep inside the fuel tanks to ensure that they are always covered in fuel. If there is a reduction in temperature that causes the SG to go up, the compensator increases the current flow to the fuel indicator circuit to correct the erroneous measurement by the fuel-measuring capacitors.