14 C.F.R. § 27.959 Unusable fuel supply.
Title 14 - Aeronautics and Space
The unusable fuel supply for each tank must be established as not less than the quantity at which the first evidence of malfunction occurs under the most adverse fuel feed condition occurring under any intended operations and flight maneuvers involving that tank.
Title 14: Aeronautics and Space
PART 27—AIRWORTHINESS STANDARDS: NORMAL CATEGORY ROTORCRAFT
Subpart E—Powerplant
Fuel System
§ 27.959 Unusable fuel supply.
