Fuel quantity indicator.

CFR · 2024-01-01 · US · Title 14

If the unusable fuel supply for any tank exceeds one gallon, or five percent of the tank capacity, whichever is greater, a red arc must be marked on its indicator extending from the calibrated zero reading to the lowest reading obtainable in level flight.

official document · source cfr-2024 · id 14:§ 25.1553