attachments are given in columns 6 and 7 of table I, figure 57, while the patterns for the span wise attachments consist of one bolt for every two rivets in the nearest attachment of the skin to a spar cap strip. Openings which do not extend beyond two adjacent stringers may be closed with semistructural doors requiring approximately one-half the number of bolts for their attachment. The frame for a semistructural door should be made from stock one gage heavier than the sheet being repaired and should be cut to the shape illustrated in figure 57. The width of the frame on each side of the opening should equal one-half the width of the opening,and the length should be such as to take the required number of rivets given in column 5 of table I, figure 57, at the spacing given in column 4, all of these required end rivets to be placed in the two stringers and in the skin between these stringers. The washer head type of bolt No. 525-10 which is not countersunk may be used on semistructural doors except in areas near the leading edge where their drag would he detrimental. The nut plates No. 659-D contain a fibre washer which automatically locks the screws into place. Attachment of the nut plates to stock of gage .040 Inch or lighter, where the material must be dimpled, or press counter sunk to obtain flush rivet heads on the outer surface requires the use of 9. countersunk shim plate No. 1066-D-1 or a .064-inch gage strap laid along the line of bolt holes between the nut plates and the frame. This shim strap is drilled and machine countersunk for each rivet and bolt, the frame is drilled and dimpled to match, and the door is countersunk for the bolts.    b. Negligible Damage.      (1) Dents. - Skin dents, free from cracks , abrasions, and sharp corners may be neglected. These dents should be restored to shape wherever possible to prevent their developing into cracks, using care, however, not to stretch or crack the skin in the process. Inspect all rivets near the damage to see that they have not been loosened or sheared.      (2) Holes and Cracks. - Holes and cracks which can be cleaned up to a 1-inch circle are considered negligible if in .040 inch or lighter skin and if the edge of the cleaned-up hole is at least 1 inch from the nearest rivet or cut-out and at least 3 inches measured along the span to the nearest skin splice rivets. Two such holes within 10 inches of each other measured chord wise and 5 inches measured span wise, however, may not be neglected. Negligible holes must be cleaned up to prevent the spread of cracks and should be plugged as in figure 58.    c. Damage Reparable by Patching. - Minor damage not considered negligible may in general be cleaned up and patched by attaching a 24ST sheet of the same gage under the skin using the required number of rivets and rivet spacing as given in figure 57. It is preferable to place patches under the skin so that the damaged area may be inspected for the formation of cracks. For aerodynamic reasons the damaged material which is cleaned away

should be replaced with a filler sheet. The original contour may be maintained by making patches discontinuous at stringers and bulkheads; care must be taken to use the total required number of rivets per inch on chord wise damage. On the other hand, continuous patches which pass over and are attached to the stringers require less rivets and are structurally superior. Existing rivets which interfere with the attachment of a patch should be carefully drilled out and the resulting holes used in arranging the new required rivet pattern.      Damage near reinforced sections of the skin, particularly in the area between stations 27 and 65 of the lower surface, may be patched only when sufficient attachment can be obtained by using the original rivet or bolt holes. Do not drill additional holes in any reinforced area because these areas have only the minimum required margins of safety.      Damage to the skin of the gas tank doors may be patched by using a sheet of the same gage 2431* stock which is large enough to take four rows of 3/16-inch rivets at 3/4-inch spacing along the inboard and outboard ends of the damaged area plus one row of 5/32 inch rivets at 1-inch spacing on the other two sides. Clean up the damaged area giving all corners a minimum radius of half an inch to prevent the formation of cracks and remove enough rivets to permit placing the patch between the skin and the stiffeners. Patches or splice plates must be made continuous across the stringers and should take 3/16-inch rivets in both legs of each stiffener. A filler sheet should be added to preserve the contour if the damage is located in an unfired section of the door. The filler should be attached to the patch with a single row of 1/8-inch rivets at 1-1/4 inch spacing.      d. Damage Reparable by Insertion. - Damage to areas which cannot be effectively patched due either to their location or to their extent may be repaired by the use of inserts. The required insert should be cut from 24ST stock of the same gage as the damaged skin. The insert should be sufficiently large to extend to the edges of the damaged sheet or to areas in which the necessary splice plates or frames may be used (figures 58 and 59) and its corners should be rounded off to a half inch or larger radius. Use the insert as a template in cutting away the damaged material. Makeup the necessary splice plates or frames and rivet these to the skin using the data in figure 57.      e. Damage Necessitating Replacement. - Damaged reinforcement plates and extensively damaged sheets of skin should be replaced with equal gage 24ST stock cut and drilled to match the damaged part. Use care in drilling out the existing rivets. (Refer to section I, paragraph 9.) 4. Ribs.      a. Negligible Damage. Cracks or holes which after clean-up have not reduced the length of the ma