14 CFR §29.619

1. (a)The special factors prescribed in §§ 29.621 through 29.625 apply to each part of the structure whose strength is—
   1. (1)Uncertain;
   2. (2)Likely to deteriorate in service before normal replacement; or
   3. (3)Subject to appreciable variability due to—
      1. (i)Uncertainties in manufacturing processes; or
      2. (ii)Uncertainties in inspection methods.
2. (b)For each part of the rotorcraft to which §§ 29.621 through 29.625 apply, the factor of safety prescribed in § 29.303 must be multiplied by a special factor equal to—
   1. (1)The applicable special factors prescribed in §§ 29.621 through 29.625; or
   2. (2)Any other factor great enough to ensure that the probability of the part being understrength because of the uncertainties specified in paragraph (a) of this section is extremely remote.