Aircraft and Aviation Fuel Quantity Indicating System

As you prepare to go on a long car journey, you will likely need to check the fuel levels of your car. Luckily for you, there is a handy dial in the dashboard of your car which indicates your fuel supply. A pilot has a similar routine inside the cockpit, however, the miles covered are significantly more and the implications far more costly. Additionally, the fuel system of an aircraft is far more complex than in a car. Luckily for the pilot, a variety of fuel system indicators provide a comprehensive view of the system’s health.

Fuel Quantity Indicators

There are various types of fuel quantity indicators include simple, direct reading indicators, mechanical indicators, electric indicators, digital and electronic. The basic principle of most quantity indicators is the use of a float in the fuel tank. As the fuel levels change, the float moves up or down. In a float-type indicator, a carbon rod is attached to the float and extends through the fuel cap to indicate the fuel level. Mechanical gauges use the float system but feature a mechanical element that moves a pointer on a dial face located in the cockpit. A system of magnetic coupling is often found within mechanical fuel quantity indicators.

Electric fuel gauges operate using direct current (DC). These systems operate using variable resistance within an electric circuit to drive a ratiometer-type indicator. The movement of the float in the tank moves the indicator. Changes in the resistor change the electric current flowing in the indicator, which in turn. shows up on a calibrated dial corresponding to fuel quantity.

Digital indicators work in the same manner as electric fuel gauges however they convert the signal to digital display on a cockpit instrument head or digital flatscreen inside the cockpit.

More modern electronic fuel quantity systems, also known as fuel probes, do not have any moving parts inside the fuel tank. Instead, variable capacitance transmitters are installed at the bottom of the fuel tank. As the level of the fuel changes, the capacitance of each unit changes. Each of the readings is taken and an average fuel quantity measurement is computed.

Fuel Pressure Gauges

These gauges are a helpful verification that the fuel system is actually delivering fuel to the fuel metering device. The amount of fuel that is flowing through the fuel injectors directly corresponds to the pressure inside the emptying fuel injectors. Monitoring the fuel pressure at the indicators is a reliable measure of fuel flow. If a blockage occurs in the injector however, the readings will be off as the fuel flow is reduced, but the pressure level reading will be high. Pressure warning lights in the cockpit help to alert of any unusual activity within the fuel system.

Valve In-Transit Indicator Lights

More complex aircraft may have multiple fuel tanks that are all interconnected by a system of tubes and valves. To avoid pressure build up or leaks, it is important that each valve functions correctly. The essential movement of valves is opening and closing. System lights turn on and off in relation to the opening and closing of the valves.


At NSN Fulfillment, owned and operated by ASAP Semiconductor, we can help you find all the fuel indicator and flight instruments for the aerospace, civil aviation, and defense industries. A dedicated account manager is available 24/7 to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, email us at sales@nsnfulfillment.com or call us at +1-914-359-2001.


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