FAA Federal Aviation Regulations (FARS, 14 CFR)

(a) The special factors prescribed in §§27.621 through 27.625 apply to each part of the structure whose strength is --

(1) Uncertain;

(2) Likely to deteriorate in service before normal replacement; or

(3) Subject to appreciable variability due to --

(i) Uncertainties in manufacturing processes; or

(ii) Uncertainties in inspection methods.

(b) For each part to which §§27.621 through 27.625 apply, the factor of safety prescribed in §27.303 must be multiplied by a special factor equal to --

(1) The applicable special factors prescribed in §§27.621 through 27.625; or

(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.