Similarly, in flight, it’s better to employ the computation method of fuel consumption management than to solely rely on what the gauges display. So then, why do we even need fuel gauges? Well, simply put, the regulations require them.
As it relates to normal, utility, acrobatic, and commuter category airplanes, 14 CFR 23.1337 requires “…a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used.” Interestingly, the standards only require accuracy when the calibrated indicator reads “zero” in level flight. So, this begs the next question, do the gauges/indicators have to work, beyond just reading zero when empty? For this we turn to 14 CFR 91.205. It stipulates that for operations with a standard category airworthiness certificate, an operable fuel gauge is required. The term “operable” is somewhat open to interpretation, but generally means that instruments and equipment are working as intended by the manufacturer.
The most notable case I found was a 1998 decision incident to the FAA bringing action against a flight school and aircraft rental business for continuing to rent a Cessna 172 with a sticking or stuck fuel gauge. It seems the FAA discovered that a pilot had recorded the problem on the aircraft’s “squawk reporting sheet” and the company did not remedy the situation, at least not to the satisfaction of the FAA. The company initially stated that the gauges had been a constant problem and they had spent considerable effort and expense to try and fix them without success. They further maintained that the company had instituted a policy requiring pilots to use a dipstick and visually check each tank prior to flight. The FAA advised the operator that using a dipstick would not resolve the discrepancies and asserted that a fix was possible. Thereafter, the company continued to operate the aircraft relying on a CFI’s entry on the squawk reporting sheet essentially claiming that the fuel gauge checked out OK. This action or in-action by the operator prompted the FAA to issue a letter of investigation and subsequently pursue a civil penalty action for violation of 14 CFR 91.205(a) and (b)(9), 91.7(a), and 91.405(a). The case was decided at hearing and then on appeal in favor of the FAA. The operator was assessed a civil penalty of $7,500.
In the instant case, the FAA pursued action against the aircraft operator, an entity. Be advised, however, that similar circumstances could lead to FAA action against a pilot who flies with a faulty fuel gauge.