A commercial jet flying against a sunset.Most of us who drive know the frustration and inconvenience that comes with running out of gas while on the road, even if we learned our lesson the first time. But while running out of gas in an automobile can lead to a ruined afternoon or a long walk to the nearest pump, an empty fuel tank in an aircraft is a genuine disaster. Fuel quantity indication accuracy is more important for the aviation industry than perhaps any other, with millions of lives and millions of dollars in property on the line every day. That means finding the right liquid level sensors for the job and taking into account the unique needs of mid-air level sensing.

The Challenges of Aircraft Fuel Gauges

The most obvious factor that distinguishes airplane fuel gauge sensing from that of other vehicles is the devastating effect of inaccurate readings. The rigors of flight, however, pose other practical challenges as well.

Changes in altitude during flight can affect volume, density, and permittivity of liquid fuels, all of which are useful metrics for fuel level sensing using floats or capacitors. Turbulence can also agitate fuel, leading to similar difficulties. These problems are doubled in larger aircraft with two engines and two or more fuel tanks.

Because of these variables, pilots are often trained to gauge remaining fuel themselves, independent of aircraft fuel gauges by taking into account the amount of fuel at takeoff alongside the average rate of engine consumption. This gives pilots an accurate enough picture of how long their plane can fly before needing to be refueled under ideal circumstances but fails to take into account issues such as fuel leaks, blockages of the fuel line, or excess fuel consumption as a result of engine malfunction. Both pilots’ own calculations and working fuel level sensors are therefore complementary, rather than competing systems.

How An Airplane Fuel Gauge WorksA jet being refueled on the runway.

Much smaller aircrafts utilize simple float level gauges that feed information on fuel level to magnetic couplings and potentiometers which then relay the information to the pilot.

Larger craft with more complicated fuel systems often uses a sophisticated system based around capacitance probes. Several capacitors are mounted in the fuel tank; as fuel is consumed and enters the tank through special vents, the tank’s capacitance alters, allowing tank levels to be calculated by the on-board computer. Multi-level sensors may additionally be employed to indicate dangerously low tank levels to the pilot.

Aircraft fuel systems also rely on pressure and temperature sensors to ensure proper rates of fuel injection and to prevent harmful condensation. Ultimately, aircraft rely on multiple methods of level detection and other technologies to keep a close eye on fuel levels and ensure the safe travel of millions of people every day.

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