(c) Alloy steel parts. Alloy steel parts such as aircraft bolts, turnbuckle ends, axles and other heat-treated alloy steel parts, which have been heat-treated to improve their mechanical properties, should not be welded.

(2) Repair of tubular members-(1) Inspection. Prior to repairing tubular members, the structure surrounding any visible damage should be carefully examined to insure that no secondary damage remains undetected. Secondary damage may be produced in some structure remote from the location of the primary damage by the transmission of the damaging load along the tube. Damage of this nature usually occurs where the most abrupt change in direction of load travel is experienced. If this damage remains undetected, loads applied in the normal course of operation may cause failure of the part.

(ii) Location and alinement of welds. Unless otherwise noted welded steel tubing may be spliced or repaired at any joint along the length of the tube. Particular attention should be paid to proper fit and alinement to avoid eccentricities.

(iii) Members dented at a cluster Dents at a steel tube cluster joint may be repaired by welding a specially formed steel patch plate over the dented area and surrounding tubes, as shown in figure 4-1. To prepare the patch plate, cut a section of steel sheet of the same material and thickness as the heaviest tube damaged. Trim the reinforcing plate so that the fingers extend over the tubes a minimum of 1.5 times the respective tube diameter as shown in the figure. Remove all the existing finish on the damaged cluster joint area to be covered by the reinforcing plate. The reinforcing plate may be formed before any welding is attempted, or it may be cut and tack-welded to one or more of the tubes in the cluster joint, then heated and formed around the joint to produce a smooth contour. Apply sufficient heat to the plate while forming so that there is generally a gap of no more than onesixteenth inch from the contour of the joint to the plate. In this operation avoid unnecessary heating and exercise care to prevent damage at the apex of the angle formed by any two adjacent fingers of the plate. After the plate is formed and tack-welded to the cluster joint, weld all the plate edges to the cluster joint.

(iv) Members dented in a bay. Dented, bent, cracked or otherwise damaged tubular members may be repaired by using a split sleeve reinforcement, after first carefully straightening the damaged member, and in the case of cracks, drilling No. 40 (0.098) stop holes at the ends of the crack.

(a) Repair by welded sleeve. This repair is outlined in figure 4-2. Select a length of steel tube sleeve having an inside diameter approximately equal to the outside diameter of the damaged tube and of the same material and at least the same wall thickness. Diagonally cut the sleeve reinforcement at a 30° angle on both ends so that the minimum distance of the sleeve from the edge of the crack or dent is not less than 12 times the diameter of the damaged tube. Cut through the entire length of the reinforcing sleeve and separate the half sections of the sleeve. Clamp the two sleeve sections to the proper positions on the affected areas of the original tube. Weld the reinforcing sleeve along the length of the two sides, and weld both ends of the sleeve to the damaged tube as shown in the figure. The filling of dents or cracks with welding rod in place of reinforcing the member is not acceptable.

(b) Repair by bolted sleeve. Due to the large percentage of the tube area removed by the bolt holes, bolted sleeve repairs should not be used on welded steel structures without prior approval of the repair by the Civil Aeronautics Administration.

(v) Welded-patch repair. Dents or holes in tubing may be repaired by a welded patch of the same material and one gage thicker, as shown in figure 4-3 provided:

(a) Dented tubing. (1) Dents are not deeper than one-tenth of tube diameter, do not involve more than one-fourth of the tube circumference, and are not longer than tube diameter.

(2) Dents are free from cracks, abrasions and sharp corners.

(3) The dented tubing can be substantially re-formed without cracking before application of the patch.

(b) Punctured tubing. Holes are not longer than tube diameter and involve not more than one-fourth of tube circumference.

(c) Location of patch. No part of the patch is permitted in the middle third

of the tube. The patch should not overlap a tube joint.

(vi) Splicing by inner sleeve method. (a) If the damage to a structural tube is such that a partial replacement of the tube is necessary, the inner sleeve splice shown in figure 4-4 is recommended, especially where a smooth tube surface is desired. Diagonally cut out the damaged portion of the tube, and remove the burr from the edges of the cut by filing or similar means. Diagonally cut a replacement steel tube of the same material and diameter and at least the same wall thickness to match the length of the removed portion of the damaged tube. At each end of the replacement tube allow a one-eighth-inch gap from the diagonal cuts to the stubs of the original tube. Select a length of steel tubing of the same material and at least the same wall thickness and of an outside diameter approximately equal to the inside diameter of the damaged tube. This inner sleeve tube material should fit snugly within the original tube, with a maximum diameter difference of onesixteenth inch. From this inner sleeve tube material cut 2 sections of tubing, each of such a length that the ends of the inner sleeve will be a minimum distance of 11⁄2 tube diameters from the nearest end of the diagonal cut.

(b) If the inner sleeve fits very tightly in the replacement tube, chill the sleeve with dry ice or in cold water. If this is insufficient, polish down the diameter of the sleeve with emery cloth. Weld the inner sleeve to the tube stubs through the one-eighth-inch gap between the stubs, completely filling the one-eighthinch gap forming a weld bead over the gap.

(vii) Splicing by outer sleeve method. (a) If partial replacement of a tube is necessary, an outer sleeve splice using a replacement tube of the same diameter may be made. However, the outer sleeve splice requires the greatest amount of welding and, therefore, it should be used only where the other splicing methods are not suitable. Information on the replacement by use of the welded outside sleeve method is given in figures 4-5 and 4-6.

(b) Squarely cut out the damaged section of the tube. Cut a replacement steel tube of the same material and diameter and at least the same wall thickness to match the length of the removed portion of the damaged tube.

This re

placement tube must bear against the stubs of the original tube with a total tolerance not to exceed one-thirtysecond inch. Select a length of steel tubing of an inside diameter approximately equal to the outside diameter of the damaged tube, of the same material and at least the same wall thickness. This outer sleeve tube material should fit snugly about the original tube with a maximum diameter difference of onesixteenth inch. From this outer sleeve tube material, cut 2 sections of tubing diagonally or fishmouth, each of such a length that the nearest ends of the outer sleeve are a minimum distance of 11⁄2 tube diameters from the ends of the cut on the original tube. Use a fishmouthcut sleeve wherever possible. Remove the burr from all the edges of the sleeves, replacement tube, and original tube stubs. Slip the two sleeves over the replacement tube, line up the replacement tube with the original tube stubs, and slip the sleeves out over the center of each joint. Adjust the sleeves to suit the area and to provide maximum reinforcement. Tack-weld the 2 sleeves to the replacement tube in 2 places before welding. Apply a uniform weld around both ends of one of the reinforcing sleeves and allow the weld to cool. Then weld around both ends of the remaining reinforcing tube. Allow one sleeve weld to cool before welding the remaining tube, to prevent undue warping.

(viii) Splicing using larger diameter replacement tubes. (a) This method of splicing structural tubes shown in figure 4-7 requires the least amount of cutting and welding. However, this splicing method cannot be used where the damaged tube is cut too near the adjacent cluster joints or where bracket mounting provisions make it necessary to maintain the same replacement tube diameter as the original. As an aid in installing the replacement tube, squarely cut the original damaged tube, leaving a minimum short stub equal to 21⁄2 tube diameters on one end and a minimum long stub equal to 41⁄2 tube diameters on the other end.

(b) Select a length of steel tube of the same material and at least the same wall thickness, having an inside diameter approximately equal to the outside diameter of the damaged tube. This replacement tube material should fit snugly about the original tube with a maximum diameter difference of one

sixteenth inch. From this replacement tube material, cut a section of tubing diagonally or fishmouth of such a length that each end of the tube is a minimum distance of 11⁄2 tube diameters from the end of the cut on the original tube. Use a fishmouth-cut replacement tube wherever possible. Remove the burr from the edges of the replacement tube and the original tube studs. If a fishmouth cut is used, file out the sharp radius of the cut with a small, round file. Spring the long stub of the original tube from the normal position; slip the replacement tube over the long stub, then back over the short stub. Center the replacement tube between the stubs of the original tube. In several places tack-weld one end of the replacement tube; then weld completely around the end. In order to prevent distortion, allow the weld to cool completely; then weld the remaining end of the replacement tube to the original tube.

(3) Repairs at built-in fuselage fittings. Repairs of built-in fuselage fittings may be accomplished in a manner as shown in figure 4-8. Splices should be made in accordance with the methods described in the foregoing sections. The following sections outline the different methods as shown in the figure.

(i) Tube of larger diameter than original. A tube (sleeve) of larger diameter than original is used in the method shown in figure 4-9. This necessitates reaming the fitting holes (at longeron) to a larger diameter. The sleeve should extend approximately 6 inches forward (left of fitting) of the joint and 8 inches aft (right of fitting). The forward splice should be a 30° scarf splice. The rear longeron (right) should be cut off approximately 4 inches from the centerline of the joint and a spacer 1 inch long fitted over the longeron. This spacer and longeron should be edge welded. A tapered V-cut approximately 2 inches long should then be made in the aft end of the outer sleeve. The end of the outer sleeve should be swaged to fit the longeron and welded.

(ii) Tube of same diameter as original. In this method, shown in figure 4-9, the new section of tube is the same size as the longeron forward (left) of the fitting. The rear end (right) of the tube is cut at 30° and forms the outside sleeve of a scarf splice. A sleeve is centered over the forward joint as indicated.

(iii) Simple sleeve. The longeron is assumed the same size on each side of the fitting in this case, in figure 4-9, and is repaired by a simple sleeve of larger diameter than the longeron.

(iv) Large difference in longeron diameter each side of fitting. Figure 4-9 (D) assumes that there is a quarter of an inch difference in the diameter of the longeron on the two sides of the fitting. The section of longeron forward (left) of the fitting is cut at 30° and a section of tubing of the same size as this tube and of such length as to extend well to the rear (right) of the fitting is slipped through it. One end is cut at 30° to fit the 30° scarf at left and the other end fishmouthed as shown. This makes it possible to insert a tube of such diameter as to form an inside sleeve for the tube on the left of the fitting and an outside sleeve for the tube on the right of the fitting.

All

(4) Engine mounts—(1) General. welding on an engine mount should be of the highest quality, since vibration tends to accentuate any minor defect present. Engine mount members should preferably be repaired by using a larger diameter replacement tube telescoped over the stub of the original member and using fishmouth and rosette welds. However, 30° scarf welds in place of the fishmouth welds will be considered acceptable for engine mount repair work.

(ii) Check of alinement. Repairs to engine mounts should be governed by accurate means of checking alinement. When tubes are used to replace bent or damaged ones, the original alinement of the structure must be maintained. This can be done by measuring the distance between points of corresponding members that have not been distorted, and by reference to the manufacturer's drawings.

(iii) Cause for rejection. If all members are out of alinement, the engine mount should be replaced by one supplied by the manufacturer or one which has been built to conform to the manufacturer's drawings. The method of checking the alinement of the fuselage or nacelle points should be requested from the manufacturer.

(iv) Engine mount ring damage. Minor damage such as a crack adjacent to an engine attachment lug may be repaired by rewelding the ring and extending a gusset or a mounting lug past the damaged area. Engine mount rings

which have been extensively damaged should not be repaired but should be replaced unless the method of repair is specifically approved by an authorized representative of the Federal Aviation Agency.

(5) Landing gears-(i) Round tube construction. Landing gears made of round tubing may be repaired using standard repairs and splices, as shown in figures 4-2 and 4-8.

(ii) Streamline tube construction. Landing gears made of streamlined tubing may be repaired by any one of the methods shown in figures 4-9 and 4-12.

(iii) Axle assemblies. (a) Representative types of repairable and nonrepairable landing gear axle assemblies are shown in figure 4-13. The types as shown in A, B, and C of this figure are formed from steel tubing and may be repaired by any applicable method shown in figures 4-2 to 4-12 in this manual. However, it will always be necessary to ascertain whether or not the members are heat-treated.

(b) The axle assembly as shown in figure 4-13 D is, in general, of a nonrepairable type for the following reasons:

(1) The axle stub is usually made from a highly heat-treated nickel alloy steel and carefully machined to close tolerances. These stubs are usually replaceable and should be replaced if damaged.

(2) The oleo portion of the structure is generally heat-treated after welding and is perfectly machined to assure proper functioning of the shock absorber. These parts would be distorted by welding after machining.

(iv) Ski pedestals. Damaged pedestals made of steel tubing may be repaired by using any applicable method shown in figures 4-2 through 4-12.

(6) Built-up tubular wing or tail surface spars. Built-up tubular wing or tail surface spars may be repaired by using any of the applicable splices and methods of repair shown in figures 4-2 to 4-12 provided the spars are not heattreated. In the case of heat-treated spars, the entire spar assembly would have to be reheat-treated to the manufacturer's specifications after completion of the repair. In general, this will be found less practicable than replacing the spar with one furnished by the manufacturer.

(7) Wing and tail surface brace struts. In general it will be found advantageous

to replace damaged wing brace struts made either from round or streamlined tubing by new members purchased from the original manufacturer. However, there is no objection from an airworthiness point of view to repairing such members in a proper manner. An acceptable method in case streamlined tubing is used will be found in figure 410. Similar members made of round tubes may be repaired using a standard splice, as shown in figures 4-2, 4-4, or 4-5.

(i) Location of splices. Steel brace struts may be spliced at any point along the length of the strut provided the splice does not overlap any part of an end fitting. The jury strut attachment is not considered an end fitting; therefore, a splice may be made at this point. The repair procedure and workmanship should be such as to minimize distortion due to welding and the necessity for subsequent straightening operations. Every repaired strut should be carefully observed during initial flights to ascertain that the vibration characteristics of the strut and attaching components have not been adversely affected by the repair. The check should cover a wide range of speed and engine power combinations.

(ii) Fit and alinement. When making repairs to wing and tail surface brace members, particular attention should be paid to proper fit and alinement to avoid eccentricities.

(8) Repairs to welded parts. Repairs to welded assemblies may be made by either of the following methods:

(1) Replacing welded joints. Cutting out the welded joint and replacing it with one properly gusseted.

(ii) Replacing weld deposit. Chipping out the metal deposited by the weld process and rewelding after properly reinforcing the joint by means of inserts or external gussets.

(c) Stainless steel structures—(1) General. Structural components made from stainless steel, particularly the "18-8" variety (18 percent chromium, 8 percent nickel), joined by spot welding, should be repaired only at the factory of origin or by a repair station designated by the manufacturer and rated by the Federal Aviation Agency to perform this type of work, unless the repair is made using bolted or riveted connections which are specifically approved by an

authorized representative of the Federal Aviation Agency.

(2) Secondary structural and nonstructural elements. Elements such as tip bows or leading and trailing edge tip strips of wing and control surfaces may be repaired by soldering with a 50-50 lead-tin solder or a 60-40 alloy of these metals. For best results a flux of phosphoric acid (syrup) should be used. Since the purpose of a flux is to attack the metal so that the soldering will be effective, any excess flux should be removed by washing the joint. Due to the high heat conductivity of stainless steel, a soldering iron large enough to do the work properly must be used. Leaky spot welded seams in boat hulls, fuel tanks, etc., should be repaired in a similar

manner.

(d) Riveted or bolted steel truss type structures. Repairs to riveted or bolted steel truss type structures should be made employing the general principles outlined in the following sections on aluminum alloy structures. Methods of repair of vital members should specifically be approved by a representative of the Federal Aviation Agency.

(e) Aluminum alloy structures—(1) General. Extensive repairs to damaged stressed skin on monocoque types of aluminum alloy structures should be made at the factory of origin or by a repair station rated for this type of work. In any event such work should be undertaken only by a certificated mechanic thoroughly experienced in this type of work. The repairs should preferably be made in accordance with specific recommendations of the manufacturer of the aircraft. In many cases repair parts, joints, or reinforcements can be designed and proof of adequate strength shown, without the calculation of the design loads and stresses, by properly considering the material and dimensions of the original parts and the riveted attachments. Examples illustrating the principles of this method as applied to typical repairs are given in this manual or may be found in textbooks on metal structures. An important point to bear in mind making repairs on monocoque structures is that a repaired part must be as strong as the original with respect to all types of loads and general rigidity.

(i) Use of annealed alloys for structural parts. The use of annealed 17S or 24S alloys for any structural repair of an aircraft is not considered satisfac

tory on account of their poor corrosion resisting properties.

(ii) Hygroscopic materials improperly moisture-proofed. The use of hygroscopic materials improperly moistureproofed such as impregnated fabrics, leather and the like, in attempting to effect watertightness of joints and seams is not considered acceptable practice.

(iii) Drilling oversized holes. Great care should be exercised to avoid drilling over-size holes or otherwise decreasing the effective tensile area of wing spar capstrips, wing, fuselage, or fin longitudinal stringers, or other highly stressed tensile members. All repairs or reinforcements to such members should be done in accordance with factory recommendations or with the specific approval of a representative of the Civil Aeronautics Administration.

(iv) Disassembly prior to repairing. If the parts to be removed are essential to the rigidity of the complete structure, the remaining structure should be adequately supported prior to disassembly, in such a manner as to prevent distortion and permanent damage to the remainder of the structure. Rivets may be removed by using special tools developed for the purpose or by center-punching the heads, drilling not quite through with a drill of the same size as the rivets, and shearing the heads off by a sharp blow with a small cold chisel. Rivet joints adjacent to the damaged parts should be inspected for partial failure (slippage) by removing one or more rivets to see if the holes are elongated or the rivets have started to shear.

(2) Selection of material for replacement parts. In selecting the alloy, it is usually satisfactory to use 24S in place of 17S since the former is stronger. Hence, it will not be permissible to replace 24S by 178 unless the deficiency in strength of the latter material has been compensated by an increase in material thickness or the structural strength has been substantiated by tests or analyses. Information on the comparative strength properties of these alloys as well as 14S, R-301, 61S, 75S, etc., is contained in ANC-5, "Strength of Metal Aircraft Elements." The choice of temper depends upon the severity of the subsequent forming operations. Parts having single curvature and straight bend lines with a large bend radius may be advantageously formed from heat-treated material, while a part such as a fuselage