52-28-05 DOUGLAS: Applies to All Model DC-6 Series Aircraft (Fuselage Numbers 1 to 213 Inclusive). \n\n\tCompliance required as indicated. \n\n\tThe following inspections and rework pertain to the center wing lower surface access hole structure at Station 149. \n\n\t1.\tInspection. \n\n\t\t(a)\tConduct following inspection as soon as practical but not later than the next 50 hours operation unless already accomplished and continuing thereafter at regular periodic inspection intervals nearest to 500 hours from the time of initial inspection until permanent repair is made as outlines in 2(b). Using at least an 8-power magnifying glass and/or dye check method or equivalent, make inspections for cracks in the lower wing skin and doubler at the aft access hole paying particular attention to the corner areas. Alternate inspection procedures which will provide equivalent safety may be approved. If cracks are found, make repairs as indicated in item 2 before the next schedules flight. \n\n\t\t(b)Periodic visual inspection must be continued at the most frequently established inspection period between 15 and 35 flying hours for airplanes reworked, as per item 2(a) until the rework of item 2(b) is accomplished. If a crack is found beyond the stop drill hole prior to the replacement period as indicated in item 2(c) make repair as per item 2(b) before the next scheduled flight. \n\n\t2.\tRepair. \n\n\t\t(a)\tIf cracks are found in either the lower wing skin or doubler less than 1-inch long, stop drill using a 1/4-inch drill or 3/8-inch drill hole if space permits. The combined length of the crack and drill hole should not exceed 1 1/4 inches in total length. \n\n\t\t(b)\tIf cracks are found in either the lower wing skin or doubler greater than 1-inch long, or if the cracks extend under the adjacent angle which cannot be visually inspected, incorporate the rework on Douglas Drawing No. 5400661 before the next scheduled flight. In cases where only one corner of the access hole is cracked, Douglas approved interim repair may be used subject to replacement with permanent rework, per Drawing No. 5400661, within a period not to exceed 1,500 hours from time interim repair is made. \n\n\t\t(c)\tThe rework of item 2(a) must be replaced with the rework reinforcement of item 2(b) within 3,000 flying hours from time rework of item 2(a) is accomplished. \n\n\t(Douglas Service Letter No. 130, dated July 10, 1952, also covers this same subject.)