Service and Repair Manual

     (3) Skin Repair Over Bulkhead, - Skin patch repair over a bulkhead is illustrated in figures 13, 22, and 14. Remove the skin to enable the damaged area of the bulkhead to be cut away and splice in the new section of the bulkhead. Place the reinforcing frame about the bulkhead as described in this section, paragraph 3.c.(2) and fit it so that it will clear the edge of the bulkhead approximately 1/32 inch. The existing countersunk holes in the bulkhead should be used as a pattern when attaching the flush patch which must be counterpunched. Avoid damaging the bulkhead when effecting repairs of this type.

     d, Damage Reparable by Insertion. - When effecting an extensive repair to the skin necessitating the insertion of all or a large part of the skin, the damaged portion of the fuselage should be supported so that the removal of the skin will not place a strain on the adjacent structure. Remove the necessary rivets taking care not to elongate or damage the rivet holes. (See section I, paragraphs 9. and 10.g.) Repair any damage to the internal structure and add splice plates where the skin sheet has been cut. Splice plates should be of the same gage and material as the damaged sheet and twice the width of the lap at the nearest parallel edge of this sheet. This type of repair is illustrated in figure 23. Form the insert and drill the rivet holes to match the original holes in the fuselage and fittings. Remove all burrs caused by drilling and rivet the insert in place. Attach each splice plate with the same rivet pattern and rivets on each side of the cut as originally used in attaching the nearest parallel edge of the sheet. When inserting a complete skin section remove the damaged section and attach the new insert using the original rivet pattern.

     4. Stringers,

     a. Negligible Damage. - Smooth isolated dents free from cracks, abrasions, and sharp corners which can be removed without excess sieve hammering, maybe considered negligible damage. Small cracks running longitudinally and not exceeding 1 inch should be stopped by drilling 1/8-inch diameter holes at their ends to prevent spreading. File all nicks in the stringer until smooth. Nicks on the legs of the stringer should not exceed 1/4 inch in depth after clean-up.

     b. Damage Reparable by Patching. - Use the methods shown in figure 24 provided the cross section of the damaged area does not exceed one-half of the total cross section of the stringer. Repair damage to the leg attaching the stringer to the skin, as shown in the upper section of figure 24. Use a filler of .068 inch to pick up the skin rivets in the damaged area and a formed channel of .040-inch 24ST of ample length to take four 1/8-inch rivets at 1/2-inch spacing in the free leg of the stringer on each side of the damaged portion in addition to the corresponding rivets in the skin. For damage to the free leg of the angle use .051-inch plate as shown in the lower section of figure 24 with four 1/8-inch rivets on each side of the damage.

A filler plate ts unnecessary unless damage after cleaning up exceeds 1-1/2 inches in length, if so it should be formed from .064-inch sheet and attached with 1/8-inch rivets at 1/2 -Inch spacing.

     c. Damage Reparable by Insertion -If the damage occurs at a bulkhead or destroys more than one-half of the stringer cross section, remove the rivets which attach the damaged section of the stringer to the skin. Remove the damaged portion of the stringer by cutting ft off at each end at a point midway between two of the existing rivet holes and proceed to splice the new section. (See figure 25.) The insert material should be cut from stock of the same cross-sectional area as the original stringer. The length should be sufficient to leave a maximum clearance at each end of the insert of 1/32 inch. Use the same stock and cut two splice pieces of ample length to take the necessary number of rivets. Attach the splice pleces to the original member and the insert with an equal length on each side of the cut and with a clearance of 1/32 inch between the splice piece and the skin. Rivet the splice piece in place with 5-5/32 inch or 8-1/8 inch rivets spaced at 1/2 inch on each slde of the insertion joint as illustrated fn figure 25. Use the existing skin rivet holes asa pattern todrlll the holes in the inserted stock. Attach the skln to the inserted stock using the existing skin rivet holes.

     5. Longerons,

     a. General, - The fuselage is reinforced between the fire wall and bulkhead No. 8 with upper and lower longerons. (See figures 5 and 12.) The upper longeron is a formed channel with flanged legs which is reinforced within added channel between the fire wall and bulkhead No. 3 and added angles which extend from bulkhead No. 3 to bulkhead No. 6. The cabin track running from bulkhead No. 3 to bulkhead No. 8 is attached to the back of the upper longeron. The lower longeron consists of an extruded angle and an extruded "T" section (figure 26) which form the wing-fuselage attachment plus the heavy gusset straps which extend from the fire wall completely around the edge of the lower fuselage opening. The skin on each side of the fuselage acts as a diagonal tension web between the longerons and the bulkheads in this region act as stiffeners to keep the longerons apart. The longerons are subjected to heavy axial and bending loads and should in general be replaced if damaged.

     b. Repair of Upper Longegron. - The upper longeron may be spliced between stations 2 and 3, 4 and 5, or 6 and 7, (figures 12, 15, and 27) by the methods shown in figures 28, 29, and 30. Minor damages to the legs or flanges may be reinforced by attaching .102-inch gage 24ST aluminum-alloy patches with 3/16-inch diameter rivets, providing there is room to use the required number of rivets. The number of rivets to be used at each end of the damage, between the fire wall and bulkhead No. 5, is one for each