How an airplane’s fuel system works

The fuel system is one of the most important systems in an aircraft. It allows proper fuel delivery to the engines and proper fuel distribution and balance, which is essential for safe flight.


Fuel storage tanks and ventilation system

In most large aircraft, fuel is stored in the wings. Some aircraft also have tanks in the center body or in the center fuselage called center tanks. Wide body aircraft also have tanks in the tail or the horizontal stabilizer which are used to control the aircraft’s center of gravity during long haul flights.

Fuel storage in the wings helps avoid wing bending stresses. And for this reason, fuel from the wing tank is used last during flight. For example, if an aircraft has a center tank, the fuel from the center tank is used first before the fuel is drained from the wings. Also, in larger aircraft, the wing tank is split into an outer tank and an inner tank. In this case, the fuel from the inner tank is used before the fuel from the outer tank. This again helps relieve stress on the wing.

SIMPLEFLYING VIDEO OF THE DAY
Wing view-DY

Photo: Tom Boon – Single Flight

In addition to storage tanks, there are tanks present in the fuel system called buffer tanks. These tanks are also part of the fuel ventilation system. All main aircraft fuel tanks are connected to the surge tank by a vent pipe. During aircraft maneuvers, any fuel exiting the tanks falls into the buffer tank through the vent pipe. And when the plane stabilizes, the fuel from the buffer tank is reinjected by gravity into the main tanks.

Screenshot 2022-11-06 at 16.27.18

Photo: Airbus

The surge tank is also vented to atmosphere to release fuel in the event of a fuel overflow. It is, at the same time, fitted with ram air which helps to pressurize the main fuel tanks. This keeps the tanks at a slight positive pressure. This, in turn, prevents excessive fuel evaporation. As the aircraft climbs higher and higher, the reduced atmospheric pressure lowers the boiling point of the fuel. This causes the fuel to evaporate. When the tanks are fed under overpressure, the fuel does not experience reduced pressure.

The positive pressure also helps prevent a vacuum from forming in the tanks when the engines draw fuel from the tanks.

Screenshot 2022-11-06 at 16.30.24

Photo: Airbus A380 FCOM

The inner workings of the fuel system

Fuel tanks consist of tank pumps or booster pumps that can be controlled by the pilot. In most cases, each tank has two tank pumps. These pumps are powered by the aircraft’s main electrical system. The job of these pumps is to pump 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, tank pumps are a necessity because the reduced pressure at altitude can cause the fuel to boil, causing vapor locks which can prevent fuel from entering the engine-driven pump.

A380 fuel control panel

Photo: Airbus A380 FCOM

The fuel tank also has suction valves that allow fuel to be sucked up by the engines in the event of a fuel tank pump failure. This forces pilots to descend to a lower altitude which prevents fuel boiling at low pressure.

After the fuel is pumped out by the tank pumps, it is then routed to the low pressure (LP) fuel valve, sometimes referred to as the spar valve. From there, the fuel passes through the engine-driven pumps. Some aircraft are equipped with both a LP low pressure pump and an HP high pressure pump, which is driven by the engine’s high pressure compressor.

Before fuel is routed to major engine components, it passes through the fuel/oil heat exchanger and fuel filter. The heat exchanger keeps the fuel at an optimum temperature, while the filter traps any debris in the fuel. After passing through the intercooler and filter, the fuel is pumped by the HP pump to the fuel nozzles in the combustion chamber.

Fuel is also used to operate system actuators such as variable stator vanes inside engines using hydraulic fuel signals. In some aircraft, fuel is also used to cool electrical generators.

HMU A380

Photo: Airbus A380 FCOM

During normal operation, the left tank supplies fuel to the left engine and the right tank supplies fuel to the right engine. In the event of engine flameout, the remaining engine can be supplied with fuel from the other side using a cross feed valve. For example, if the right engine failed, fuel from the left wing tank could be routed to the right engine when the crossover valve is opened.

Cross-talk can also be used to balance fuel in the air between tanks. To perform this procedure, pilots can turn off the wing tank pumps on the lighter side and open the cross feed valve. This allows the fuller tank to power both motors. Once balance between the tanks is achieved, the wing tank pumps can be restarted and the cross feed valve closed.

A319 top panel

Auxiliary power unit (APU) fuel is usually fed from one of the wing tanks. It has its own pump which turns on automatically when the APU start sequence is initiated. In the event of a malfunction of the APU pump, the supply tank pumps can be turned on.

Supplies

The refueling points of most large aircraft are under the wings. However, in some aircraft it is in the side belly. This point is called a refueling fitting, and is where the hose from the fuel tank connects.

This type of refueling is known as pressure refueling because the fuel is routed to the tanks at high pressure.

Southwestern plane being refueled

Photo: Getty Images

To control refueling, a control panel is available. In this panel, the operator can dial in or preset the amount of fuel needed. Once the hose is connected, the refueling valves open and refueling begins. This whole process is automatic. When refueling, the outer tanks are filled first. When full, the fuel overflows into the inner tank and the center tank. When the fuel level reaches the selected value, the refueling valves are closed and refueling stops.

A320 refueling panel

Photo: Anas Maaz

Most manufacturers also offer a way to refuel the aircraft manually using gravity. For this, manual refueling points are located on the wings. In manual refueling, the tanker controls the refueling, and it is recommended to fill the wing tanks before filling the center tanks.

The main disadvantage of this type of refueling is that the refueling process can take a long time.

How is the amount of fuel measured?

To measure the amount of fuel, capacitors are used. The capacitor consists of two plates which are supplied with alternating current.

Boeing 737 fuel gauge

Photo: Boeing 737 FCOM

The current flow in such a circuit depends on four factors. They are:

  • The voltage level applied
  • The frequency of 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. Indeed, at a given instant, the dielectric constant can be either air, or fuel, or a mixture of air and fuel. When the capacitor is soaked in fuel, there is an increase in current, which is compared to a reference capacitor with air as the dielectric. The difference between these two measurements can then be used to get a very accurate fuel indication.

The main problem with this system is that it cannot compensate for temperature. The specific gravity (SG) or density of fuel is inversely proportional to temperature. Thus, during a drop in temperature, the volume of fuel decreases and causes fuel indication errors. Similarly, when there is an increase in temperature, the volume of fuel increases.

To solve this problem, compensators are used. Compensators are probes that are placed deep inside fuel tanks to ensure that they are always covered with fuel. If there is a temperature reduction that causes the SG to rise, the compensator increases the current flow to the fuel meter circuit to correct the erroneous measurement by the fuel metering capacitors.

74

Comments are closed.