56-13-01 DOUGLAS: Applies to All DC-6, DC-6A and DC-6B Aircraft. \n\n\tCompliance required as indicated. \n\n\tNumerous instances have been reported on wing skin and stringer 22 cracking aft of the main landing gear fittings at the center spar, Stations 122 and 175, on DC-6 aircraft having high flight time. The following must be accomplished as indicated on all DC-6 Series aircraft having in excess of 10,000 hours total flight time. \n\n\t1. Accomplish inspection of the wing skin in area aft of the center spar at approximately Stations 122 and 175, especially for cracks emanating from the radii in the skin cutouts, as soon as practicable but not later than next periodic inspection nearest 300 hours. In case complete radii cannot be inspected directly by visual means from underneath the lower wing surface, then a reliable alternate method of inspection must be employed. X-ray may fall into this category. If a skin crack is detected, stringer 22 must also be inspected for cracks. Any skin crack or stringer 22 crack found during the inspection must be repaired prior to further passenger flight. \n\n\t2. Skin cracks less than 3/4 inch in length are to be repaired as outlined on Service Rework Drawing 5593157. \n\n\t3. Skin cracks in excess of 3/4 inch but less than 1 1/2 inches in length are to be repaired as outlined on Service Rework Drawing 5613739. \n\n\t4. Skin cracks in excess of 1 1/2 inches in length at Station 175 are to be repaired as outlined in Service Rework Drawing 5610926 together with the preventive rework shown on 5593157 and 5610629. \n\n\t5. Skin cracks in excess of 1 1/2 inches in length at Station 122 will require entire skin replacement between center and rear bottom spar caps, Wing Station 60 to Station 149, together with the preventive rework shown on 5593157 and 5610629. \n\n\t6. Stringer cracks (in horizontal leg only) are to be repaired as outlined on Service Rework Drawing 5610629. Cracks found in stringer 22 which are more extensive than through one horizontal leg of the stringer will require a complete stringer replacement. \n\n\t7. All aircraft which do not require repairs are to be reinspected at each periodic inspection period nearest 2,500 hours until preventive repairs per 5593157 and 5610629 are installed. \n\n\t8. After rework per paragraphs 1 through 6 has been accomplished or preventive rework per Drawings 5593157 and 5610629 is installed, no further special inspection periods are required. Also, further special inspections are not required on DC-6 aircraft upon which Kits A and B of Douglas Service Bulletin DC-6 No. 569 have been incorporated. \n\n\tUpon request of the operator, an FAA maintenance inspector, subject to prior approval of the Chief, Engineering and Manufacturing Branch, FAA Western Region, may adjust the repetitive inspection intervals specified in this AD to permit compliance at an established inspection period of the operator if the request contains substantiating data to justify the increase for such operator. \n\n\t(Douglas Alert Service Bulletin No. A-673 revised May 8, 1956, covers the above subject.) \n\n\tThis supersedes AD 56-04-03. \n\n\tRevised July 7, 1962.