51-01-01 DOUGLAS: Applies to All DC-6 Aircraft. \n\n\tTo be accomplished as indicated below. \n\n\t(a)\tAll airplanes at each 300 hours of flight time-Inspect for broken studs at the front spar lower and upper fittings at wing joint Station 421. Also inspect lower forward front spar fittings, P/N 4332884 and -1, and upper forward fittings, P/N 4332885 and -1, for cracks in area approximately 2 inches outboard of fitting face. \n\n\t(b)\tIf broken stud is found, replace both studs at spar fittings with parts having threads rolled after heat treat, install bolt with threads rolled after heat treat at center spar, and replace lower fitting P/N 4332884 and -1 and upper fitting P/N 4332885 and -1 with newly designed heavier fittings P/N 4397872 and -1 and P/N 4397873 and -1, respectively. Return of an airplane to service prior to the availability of the newly designed fittings is permissible by installation of new fittings of the old design (P/N 4332884 and 4332885), providing replacementis made at or before 8,000 hours of service on those parts. Spotface new fittings after installation to align with adjacent fittings. \n\n\tIf records show previous case of broken parts at front spar fitting, and only corrective action taken was the replacement of studs with threads rolled after heat treat, inspect fittings at each 300 hours of operation until same replacement action outlined above is taken at next engine change period. \n\n\t(c)\tContinue inspections outlined in (a), until following replacement action is accomplished: \n\n\t\t1.\tIf not previously incorporated, install studs at the front spar and bolts at the center spar which have threads rolled after heat treat. (See AD 49-06-02.) \n\n\t\t2.\tReplace the lower forward front spar fittings P/N 4332884 and -1 with new heavier P/N 4397872 and -1 and replace the upper forward front spar fitting P/N 4332885 and -1 with new heavier P/N 4397873 and -1 spotfacing fittings after installation. \n\n\t(d)\tReplacement action outlined initem (c) 1 and 2, to be accomplished not later than January 1, 1952. \n\n\t(Douglas General Service Letter DC-6 No. 61 covers this same subject. Refer to Douglas Service Letter No. 66, Supplement No. 2 dated July 18, 1951, for wing alignment at Station 421.)