Text in force at CS25-28: from CS25-0, effective 17 Oct 2003.
Turbo-propeller powered aeroplane 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 aeroplane was designed under CS 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.
ED Decision 2003/002/RM · Source