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.
source: Docket No. 5084, 29 FR 16150, Dec. 3, 1964, unless otherwise noted.
cite as: 14 CFR 29.1553