14 C.F.R. § 25.1553 Fuel quantity indicator.
Title 14 - Aeronautics and Space
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.
Title 14: Aeronautics and Space
PART 25—AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES
Subpart G—Operating Limitations and Information
Markings and Placards
§ 25.1553 Fuel quantity indicator.
