50-38-01\tNORTH AMERICAN: Applies to All Model AT-6 Series Aircraft. \n\n\tTo be accomplished at the next annual inspection and at each succeeding annual inspection thereafter.\n \n\tSeveral recent incidents have indicated that the inspections presently required are not sufficiently comprehensive to reveal all areas of the airplane which may have been adversely affected by intergranular corrosion, and that the required inspections should be repeated periodically. Accordingly, in order to minimize the possibility of structural failure due to such corrosion, the following must be accomplished: \n\n\tInspect all accessible structural aluminum alloy components for evidence of intergranular corrosion particularly in the following locations: At the upper and lower deck and the most forward and two aft bulkheads in the monocoque fuselage; frame around the baggage door; inboard end of horizontal stabilizer spars; fuel cell doors in the wing center section; wing attach angles; two inboard ribs oneach outer wing; trailing edge ribs above flaps; and the outboard rib of the wings, especially at the trailing edge. Full use should be made of all access provisions to accomplish as thorough an inspection as possible. \n\n\tIn conducting these inspections, full reliance cannot be placed on visual examination alone. A screwdriver or other instrument should be used to explore for dull sounding areas and for material which may be penetrated easily by pressure applied to the screwdriver tip or similar sharp point. Areas adjacent to joints and sheared edges should be examined thoroughly. \n\n\tFormed material in particular has been found to be subject to rapid intergranular corrosion, because of poor heat treatment of parts, which were formed in the annealed condition, and later heat treated. \n\n\tAll corroded parts must be replaced. \n\n\tThis supersedes AD 47-41-01.