What Are Those Fuel Gauges Telling You?

FARs reference to ‘only when empty’ is just a tale handed down through the decades.

Wednesday, May 31, 2023 at 11:05 AM ET

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You won't find a reference in the FARs that says fuel gauges are only required to read accurately when the tank is empty. [Credit: Shutterstock]

Key Takeaways:

The common belief that airplane fuel gauges are only required to be accurate when empty is a widespread myth.
Federal Aviation Regulations (FAR 23.1337(b)) mandate that fuel quantity indicators must display the amount of usable fuel available during flight.
Fuel gauges are specifically calibrated to read zero when the remaining fuel in the tank is equal to the "unusable fuel supply," which is the fuel that cannot reach the engine.
This calibration point, where the gauge reads zero while some unusable fuel is still present, likely contributed to the misconception about accuracy only at empty.

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Question: I have heard that the regulation regarding the accuracy of fuel gauges in airplanes states they are only required to read accurately when the fuel tank is empty. This doesn’t make much sense to me, and I couldn’t find it in the FARs. Is it true?

Answer: You won’t find a reference in the federal aviation regulations (FARs) that says fuel gauges are only required to read accurately when the tank is empty. That statement “only when empty” is a myth that has been handed down through the decades.

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What you will find in Part 23 (which covers the certification of aircraft) under FAR 23.1337(b) powerplant instrument installation is “Fuel quantity indication. There must be a means to indicate to the flight crew members the quantity of usable fuel in each tank during flight.” The reg continues: “Each fuel quantity indicator must be calibrated to read zero during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply.”