Text in force at 25-153: from 25-0, effective 1 Feb 1965.
Turbopropeller powered airplane propeller-drag limiting systems must be designed so that no single failure or malfunction of any of the systems during normal or emergency operation results in propeller drag in excess of that for which the airplane was designed under Sec. 25.367. Failure of structural elements of the drag limiting systems need not be considered if the probability of this kind of failure is extremely remote.
29 FR 18291 · Source