How Fuel level is measured in aircraft?

What is level measurement?

Level measurement is a measurement of fluid level. In a process, level measurement is highly crucial and by doing so, we can ensure that the process is handled in a controlled manner safely.

Challenges in measuring the level of fuel in an aircraft

The most safety-critical aspects of flight trip planning are the calculation of fuel requirements for fuel trips and the reserve fuel on aircraft. The calculation of fuel in aircraft is complicated because of the following reasons

  • The rate of fuel varies based on the speed of the aircraft, aircraft altitude, ambient temperature. The above factors are somewhat impossible to predict before.
  • Fuel usage varies based on the weight of the aircraft

In jet aircraft, fuel is measured in pounds due to the fuel volume is changed based on the temperature. This is very critical when a jet flies in high altitudes where the temperature will be -40 to -50 Fahrenheit. Furthermore, fuel is consumed by mass rather than volume.

Aircraft Fuel Level Sensors

We can use the same level measuring mechanism as in automobiles for calculating smaller aircraft but we can’t use this in larger aircraft. To find fuel levels in big aircraft, we have to use multiple sensors due to the big size of the aircraft. Some level sensing devices show very small inaccuracy and we cannot use this type of sensing device. We need very high accuracy sensing devices because even the small inaccuracy may lead to very catastrophic consequences.

Sensors in aircraft must be able to cope with altitude changes.

So, ultrasonic or capacitance sensors are used in aircraft.

Ultrasonic sensors use ultrasonic waves to measure the height or level of the fuel tank.

While using capacitance type of sensors to measure the fuel level, the fuel is passed through particular vents and it causes the variance of capacitance in the sensors and it allows measuring the level of the fuel. The computer in the aircraft sends the information to the pilots subsequently.

Overview of Ultrasonic type sensors

An ultrasonic sensor is used to calculate the distance by emitting sound waves and converting the reflected signal into an electrical signal. The major components in ultrasonic sensors are

Transmitter- which uses piezoelectric crystals to generate sound

Receiver- which encounters the sound after it has traveled to and from the target.

Overview of Capacitance Type Sensors

The capacitive fuel meter uses a variable-capacity element to precisely change the AC voltage according to the amount of fuel in the fuel tank. An electrical signal is used to energize the fuel gauge. The fuel gauge varies with the amount of fuel in the tank and shows it into the pilot’s cabin.

The components capacitive fuel meter systems are

• Control Monitor or Signal Conditioner

• Indicator

• Tank Unit (Fuel Quantity Sender, Fuel Probe)

Aircraft’s Fuel System

In aircraft, fuel is stored in the wings which are rectangular in shape with a very detailed internal structure. A common fuel measurement system consists of a number of fuel sensors mounted in the tanks to directly sense the fuel level inside the tanks, which is proportional to the amount of fuel. In most cases, capacitive probes are used in commercial airplanes. The fuel volume V is calculated by measuring the height h of the fuel at various positions within the tank.


For a particular fuel type, K can be accurately modeled as a linear function of temperature, meaning that K can be inferred using temperature measurements. Due to the hydrocarbon’s chemical composition varies based on the airport, country, and suppliers. So the value of K varies from their normal linear fit.

To avoid this issue, we use the capacitive sensor for reference. This reference is called compensator. The compensator is always submerged in the fuel.

K is calculated as K = - Cc/ Cc 0

Fuel Quantity Indicating Systems

Every aircraft have some indicator to show fuel quantity. These devices vary with the aircraft’s fuel system complexity. Some reading indicators don’t need any electrical power but this type of sensor can be used only on light aircraft with fuel tanks that are close to the cockpit’s proximity.

Slight Glass: It’s a clear glass or plastic tube that connects to the fuel tank and holds the same amount of fuel as the tank.

This slight glass can be calibrated either in gallons or in fractions of a full tank. Other types of targets use floating-point with indicator bars attached. The float will move up and down based on the fuel level in the tank. The part of the connecting rod that passes through the fuel tank cap represents the amount of fuel in the tank. These two mechanisms are coupled to another simple fuel gauge with a float attached to a rod that moves up and down in a calibrated cylinder.

In modern aircraft, electric fuel quantity indicators are used than mechanical indicators. DC (Direct Current) is used to run these indicators and ratiometer-type indicators are energized by using the variable resistance in a circuit. A connecting arm adjusts a wiper on a variable resistor in the tank unit when float in the tank moves. One of the coils of the ratiometer-type fuel gauge in the instrument panel is wired in series with this resistor. The current flowing through one of the coils in the indicator changes as the current passing through the tank unit resistor changes.The magnetic field in which the indicating pointer pivot changes as a result of this. The fuel quantity is indicated by the calibrated dial.

A digital display that operates with the same variable resistance signal as the tank unit is available. The fluctuating resistance is converted to a digital display on the cockpit instrument panel. Fully digitized instrument systems, such as those found on glass cockpit aircraft, convert fluctuating resistance into digital signals, which are processed by a computer and displayed on a flat-screen.

Fuel Measurement

The following characteristics characterize modern fuel measuring and management system implementation solutions:

  • Density measuring by hand
  • Architectures that are fault-tolerant
  • Fault detection and isolation are automated to a large extent.
  • Correction of onboard altitude errors
  • Microprocessors and FPGAs of the current generation
  • Integrated circuits for specialized applications
  • Exhibits that are cutting-edge

Fuel metering and management systems are frequently custom-designed to match the requirements of each aircraft type. The system’s designers adhere to requirements for maturity at entry-into-service, the most up-to-date technologies, safety, dependability, correctness, and maintainability while reducing the customer’s total cost of ownership.