Text in force at 25-153: from 25-40, effective 2 May 1977.
The unusable fuel quantity for each fuel tank and its fuel system components must be established at not less than the quantity at which the first evidence of engine malfunction occurs under the most adverse fuel feed condition for all intended operations and flight maneuvers involving fuel feeding from that tank. [Fuel system component failure need not be considered.]
42 FR 15043 · Source