FAA Federal Aviation Regulations (FARS, 14 CFR)

A reversible propeller must be adaptable for use with a reversing system in an airplane so that no single failure or malfunction in that system during normal or emergency operation will result in unwanted travel of the propeller blades to a position substantially below the normal flight low-pitch stop. Failure of structural elements need not be considered if the occurrence of such a failure is expected to be extremely remote. For the purposes of this section the term "reversing system" means that part of the complete reversing system that is in the propeller itself and those other parts that are supplied by the applicant for installation in the aircraft.