frame would have to be formed from soft annealed sheet and heat-treated after forming. Sheet metal parts which are to be left unpainted should be made of clad (aluminum coated) material. All sheet material and finished parts should be free from cracks, scratches, kinks, tool marks, corrosion pits, and other defects which may be factors in subsequent failure.

(i) Forming sheet metal parts. Bend lines should preferably be made to lie at an angle to the grain of the metal (preferably 90°). Before bending, all rough edges should be smoothed, burrs removed and relief holes drilled at the ends of bend lines and at corners to prevent cracks from starting. For material in the heat-treated condition, the bend radius should be large. See table 4-3 for recommended bend radii.

(3) Heat treatment (i) General. All structural aluminum alloy parts should be heat treated in accordance with the heat treatment instructions issued by the manufacturers of the materials. If the heat treatment produces warping, the parts should be strengthened immediately after quenching. Riveted parts should be heat-treated before riveting, to preclude warping and corrosion. When riveted assemblies are heated in a salt bath, the salt cannot be entirely washed out of the crevices, thus causing corrosion.

(ii) Quenching in hot water or air. The quenching of 17S or 24S alloys in water above 100° F., or air at any temperature after heat treatment will not be satisfactory. For clad material, when the use of cold water will result in too great a distortion of the finished part, the use of oil, hot water, water spray or forced air draft is satisfactory, provided the parts will not be subject to severe corrosion in service. Quenching in still air is not satisfactory.

(iii) Transferring too slowly from heat treatment medium to quench tank. Transfer of 17S or 24S alloys from the heat treatment medium to the quench tank should be accomplished as quickly as possible. An elapsed time of 10 to 15 seconds will, in many cases, result in noticeably impaired corrosion resistance.

(iv) Reheating at temperatures above boiling water. Reheating at temperatures above that of boiling water of 178 or 24S alloys after heat treatment, and the baking of primers at temperatures above that of boiling water, will not be

considered acceptable without subsequent complete and correct heat treatment, as such practice tends to impair the original heat treatment.

(4) Riveting—(i) Identification of rivet material. Identification of rivet material is contained in § 18.30-6.

(ii) Replacement of aluminum alloy rivets. All protruding head rivets, (roundhead, flathead, and brazier-head) may be replaced by rivets of the same type or by AN-470 Universal-head rivets. Flushhead rivets should be used to replace flushhead rivets.

(a) Replacement rivet size and strength. Replacements should be made with rivets of the same size and strength wherever possible. If the rivet hole has become enlarged, deformed, or otherwise damaged, the hole should be drilled or reamed for the next larger size rivet, care being taken, however, that the edge distances and spacings are not less than minimums listed in (b) of this subdivision. Rivets may not be replaced by a type of lower strength properties, unless the lower strength is adequately compensated for by an increase in size or a greater number of rivets.

(b) Replacement rivet edge distances and spacings for sheet joints. Rivet edge distance is defined as the distance from the center of the rivet hole to the nearest edge of the sheet. Rivet spacing is! the distance from the center of the rivet! hole to the center of the adjacent rivet hole. Edge distances and spacings should not be less than the following:

(1) Single row. Edge distances not less than 2 times the diameter of the rivet and spacing not less than 3 times the diameter of the rivet.

(2) Double row. Edge distance and spacing not less than the minimums shown in figure 4-14.

(3) Triple or multiple rows. Edge distance and spacing not less than the minimums shown in figure 4-14.

(iii) Use of A17S-T3 aluminum alloy replacement rivets. It will be considered acceptable to replace all 17S-T3 rivets of three-sixteenths-inch diameter or less, and also all 24S-T4 rivets of five-thirty-seconds-inch diameter or less with A17S-T3 rivets for general repairs, provided the replacement rivets are onethirty-second-inch greater in diameter than the rivets they replace, and provided the edge distances and spacings are not less than the minimums listed

in subdivision (ii) (b) of this subparagraph.

(iv) Driving of rivets. A17S rivets may be driven in the condition received, but 17S rivets above three-sixteenths inch in diameter, and all 24S rivets should either be kept refrigerated in the "as quenched" condition until driven or be reheat-treated just prior to driving as they would otherwise be too hard for satisfactory riveting. Dimensions for formed flat rivet heads are shown in figure 4-15, together with commonly found rivet imperfections, which should be guarded against.

(v) Blind-type and hollow rivets. (a) Hollow rivets should not be substituted for solid rivets in load-carrying members without specific approval of the application by a representative of the Civil Aeronautics Administration.

(b) Blind rivets may be used in blind locations in accordance with the conditions listed in § 18.30-6, provided the edge distances and spacings are not less than the minimums listed in subdivision (ii) (b) of this subparagraph.

(vi) New and revised rivet patterns. (a) A new or revised rivet pattern should be designed for the strength required in accordance with the specific instructions in subparagraph (5) (vi) and (viii) (b) (4) of this paragraph.

(b) A general rule for the diameter of rivets used to join dural sheets is to use a diameter approximately three times the thickness of the sheet, or somewhat larger for thin sheet. Rivets should not be used where they would be placed in tension tending to pull the heads off. A lap joint of thin sheets should be "backed up" by a stiffening section.

(5) Repair methods-(i) Precautions. (a) When adding or replacing rivets adjacent or near to 175 or 24S rivets which have been installed previously, great care should be exercised or the older rivets will be loosened or may fail due to sharp vibrations in the structures caused by the action of the rivet gun and bucking bar. In every case all adjacent rivets should be carefully examined after the repair or alteration is finished to ascertain that they have not been harmed by operations in adjacent areas.

(b) Rivet holes should be drilled, round, straight, and free from cracks. The snap used in driving the rivets should be cupped slightly flatter than the

rivet heads shown in figure 4-15. Rivets should be driven straight and tight, but not overdriven or driven while too hard, since the finished rivet must be free from cracks. Information on special methods of riveting, such as flush riveting, usually may be obtained from manufacturer's service manuals.

(ii) Splicing of tubes. (a) Round or streamline tubular members may be repaired by splicing as shown in figure 4-16. Splices in struts should not overlap the fittings.

(b) When solid rivets go completely through hollow tubes, their diameter should be at least one-eighth of the outside diameter of the outer tube. Rivets which are loaded in shear should be hammered only enough to form a small head, and no attempt should be made to form the standard round head. The amount of hammering required to form the standard round head often causes the rivet to buckle inside the tube. Satisfactory rivet heads may be produced in such installations by spinning, if the proper equipment is available. Correct and incorrect examples of this type of rivet application are incorporated in figure 4-16.

(iii) Repairs to aluminum alloy members. Repairs to aluminum alloy members should be made with the same material or with suitable material of higher strength. The 75S alloy has greater tensile strength than other commonly used aluminum alloys such as 14S and 24S but it is subject to somewhat greater notch sensitivity. In order to take advantage of its higher strength characteristics, particular attention should be paid in design of parts to avoid notches, small radii, large or rapid changes in cross-sectional area. In fabrication, care should be taken to avoid processing and handling defects, such as machine marks, nicks, dents, burrs, scratches and forming cracks. Cold straightening or forming of 75S-T6 can cause cracking; hence, it may be advisable to limit this processing to minor cold straightening.

(iv) Wing and tail surface ribs. Damaged aluminum alloy ribs either of the stamped sheet-metal type or the built-up type employing special sections, square or round tubing, may be repaired by the addition of suitable reinforcement. Acceptable methods of repair are shown in figures 4-17 and 4-18. These examples deal with types of ribs commonly

(v) Repair of damaged skin-(a) Replacement of skin panels. In case metal skin is damaged extensively, repairs should be made by replacing an entire sheet panel from one structural member to the next. The repair seams should be made to lie along stiffening members, bulkheads, etc., and each seam should be made exactly the same in regard to rivet size, spacing, and rivet pattern as the parallel manufactured seams at the edges of the original sheet. If the two manufactured seams are different, the stronger one should be copied. See figure 4-20 for typical acceptable methods of repairs.

(b) Patching of small holes. (1) Small holes in skin panels which do not involve damage to the stiffening members may be patched by covering the hole with a patch plate in the manner shown in figure 4-20.

(2) Flush type patches also can be installed in stressed skin type of construction. An acceptable and easy flush patch can be made by trimming out the damaged area and then installing a conventional patch on the underneath side or back of the sheet being repaired. A plug patch plate of the same size as the opening can then be inserted and riveted to the patch plate installed as above. This will complete an acceptable flush type patch. Other types of flush patches similar to those used for patching plywood, reference figure 2-16, also can be used. The riveting pattern used, however, should follow standard practice so as to maintain satisfactory strength in the sheet. (See subdivision (i) of this subparagraph.)

(3) In general, patches in metal skin are not restricted as to size or shape; however, those of rectangular, circular, square, oval, and rectangular with round ends usually are more desirable as to appearance and ease of installation.

(vi) Splicing of sheets. In some cases the method of copying the seams at the

edges of a sheet may not be satisfactory; for example, when the sheet has cutouts, or doubler plates at an edge seam, or when other members transmit loads into the sheet. In these cases, the splice should be designed as illustrated in the following example:

Material: Clad 178 sheet, 0.032 inch thickness. Width of sheet (1. e. length at splice) ="W"=10 inches.

To determine rivet size and pattern for a single-lap joint, similar to figure 4-14:

(a) Use rivet diameter of approximately three times the sheet thickness 3X0.032= 0.096 inch. Use one-eighth A17S-T3 rivets (2 A17S-T3 would also be satisfactory).

(b) Determine the number of rivets re- ! quired per inch of width, "W", from table 4-5. Number per inch=4.9.75=3.7. Total number of rivets rivets.

required=10x3.7=37

(c) Lay out rivet pattern with spacing not less than those shown in figure 4-14. Referring to figure 4-14A, it is seen that a double-row pattern with the minimum spacing will give a total of 40 rivets. However, as only 37 rivets are required, 2 rows of 19 rivets each, equally spaced over the 10 inches will result in a satisfactory splice.

are

(vii) Straightening of stringers or intermediate frames-(a) Members which slightly bent. Members slightly bent may be straightened cold and examined with a magnifying glass for injury to the material. The straightened parts should then be reinforced to an extent depending upon the condition of the material and the magnitude of any remaining kinks or buckles. If any strain cracks are apparent, complete reinforcements should be added by following the manufacturer's recommendations and the attachment of the reinforcements should be made in sound metal beyond the damaged portion.

(b) Local heating. Local heating should never be applied to facilitate bending, swaging, flattening, or expanding operations on heat-treated aluminum alloy members, as it is difficult to control the temperatures closely enough to prevent possible damage to the metal and it may impair its corrosion resist

ance.

(viii) Splicing of stringers and flanges. (a) Splices should be made in accordance with the manufacturer's recommendations, which are usually contained in a repair manual.

(b) Typical splices for various shapes of sections are shown in figures 4-21 and 4-23. Splices should be designed to carry both tension and compression and

the splice shown in figure 4-22 will be used as an example illustrating the following general principles:

(1) Statement of principles. (1) To avoid eccentric loading and consequent buckling in compression, splicing or reinforcing parts should be placed as symmetrically as possible about the centerline of the member and attachment made to as many elements as necessary to prevent bending in any direction.

(ii) To avoid reducing the strength in tension of the original bulb angle, the rivet holes at the ends of the splice are made small (no larger than the original skin attaching rivets), and the second row of holes (those through the bulbed leg) are staggered back from the ends. In general the rivets should be arranged in the splice so that the design tensile load for the member and splice plate can be carried into the splice without failing the member at the outermost rivet holes.

(iii) To avoid concentration of load on the end rivet and consequent tendency toward progressive rivet failure, the splice is tapered off at the ends, in this case by tapering the backing angle and by making it shorter than the splice bar (see fig. 4-22).

The preceding principles are especially important in splicing stringers on the lower surface of stressed skin wings, where high tension stresses may exist. When several adjacent stringers are spliced, the splices should be staggered if possible.

(2) Size of splicing members. When the same material is used for the splicing member as for the original member, the net cross section area (i. e., the shaded areas in fig. 4-21) of the splicing member should be greater than the area of the section element which it splices. The area of a section element (e. g. each leg of an angle or channel) is equal to the width multiplied by the thickness. For example, in figure 4-22, the bar, "B," is assumed to splice the upper leg of the stringer, and the angle, "A," to splice the bulbed leg of the stringer. Since the splice bar, "B," is not as wide as the adjacent leg, and since the rivet diameter is also subtracted from the width, the bar is made twice as thick in order to obtain sufficient net area.

(3) The diameter of rivets in stringers. The diameter of rivets in stringers should preferably be between 2 and 3 times the

thickness, "t," of the leg, but should not be more than one-fourth the width, "W," of the leg. Thus, one-eighth-inch rivets are chosen in the example, figure 4-22. If this splice were in the lower surface of a wing, the end rivets would be made the same size as the skin attaching rivets, say three thirty-seconds.

(4) The number of rivets. (i) The number of rivets required on each side of the cut in a stringer or flange may be determined from standard textbooks on aircraft structures, or may be found from tables 4-4, 4-5, or 4-6. In determining the number of rivets required in the example, figure 4-22, for attaching the splice bar, "B," to the upper leg, the thickness "t" of the element of area being spliced is one-sixteenth inch (use 0.064), the rivet size is one-eighth inch, and table 4-5 shows that 9.9 rivets are required per inch of width. Since the width, "W," is one-half inch, the actual number of rivets required to attach the splice bar to the upper leg, on each side of the cut, is 9.9 (rivets per inch) X0.5 (inch width) =4.95; use 5 rivets.

(ii) For the bulbed leg of the stringer, "t"=6 inch (use 0.064), AN-3 bolts are chosen and the number of bolts required per inch of width=3.3. The width, "W," for this leg, however, is 1 inch, and the actual number of bolts required on each side of the cut is 1X3.3-3.3; use 4 bolts. When both rivets and bolts are used in the same splice, the bolt holes should be accurately reamed to size. It is preferable to use only one type of attachment, but in the above example, the dimensions of the legs of the bulb angle indicated rivets for the upper leg and bolts for the bulb leg.

(5) Splicing of intermediate frames. (i) The same principles that are used for stringer splicing may be applied to intermediate frames, when the following point is also considered:

(ii) Conventional frames of channel or Z section are relatively deep and thin compared to stringers, and usually fail by twisting or by buckling of the free flange. The splice joint should be reinforced against this type of failure by using a splice plate heavier than the frame and by splicing the free flange of the frame with a flange of the splice plate, as illustrated in figure 4-24. Since a frame is likely to be subjected to bending loads, the length of splice plate "L" should be more than twice the width,

"W2," and the rivets spread out to cover the plate.

(ix) Repairing cracked members. Acceptable methods of repairing various types of cracks occurring in service in structural elements from various causes are shown in figures 4-25 to 4-28. The following general procedure should be followed in repairing such defects:

(a) Small holes three thirty-seconds inch (or 8 inch) should be drilled at the extreme ends of the cracks to mitigate the possibility of their spreading further.

(b) Reinforcements as shown in these figures should be added to carry the stresses across the damaged portion and to stiffen the joints.

The condition causing such cracks to develop at a particular point is stress concentration at that point in conjunction with repetition of stress (such as produced by vibration of the structure). The stress concentration may be due to the design or to defects such as nicks, scratches, tool marks, and initial stresses or cracks from forming or heat-treating operations. It should be noted that an increase in sheet thickness alone is usually beneficial but does not necessarily remedy the conditions leading to cracking.

(1) Fittings—(1) Steel fittings—(1) Inspections for defects. (a) Fittings should be free from scratches, vise and nibbler marks, and sharp corners. A careful examination of the fitting with a medium power (at least 10 power) magnifying glass will be considered an acceptable inspection.

(b) When repairing aircraft after an accident or in the course of a major overhaul, all highly stressed main fittings should be inspected in accordance with the provisions of § 18.30-8, and, if necessary, corrosion prevention measures taken as recommended in § 18.30-7.

(ii) Torn, kinked, or cracked fittings. Torn, kinked, or cracked fittings should be replaced and not repaired.

(iii) Elongated or worn bolt holes. Elongated holes in fittings which were designed without bushings should not be reamed oversize but such fittings should be replaced unless the method of repair is approved by a representative of the Federal Aviation Agency. Holes should not be filled with welding rod. Acceptable methods of repairing elongated or worn bolt holes in landing gear, stabilizer, interplane or cabane strut ends

only, not originally equipped with pin plates, are shown in figure 4-29. (See also figure 4-8 on longeron repair at a fitting.)

(2) Aluminum and aluminum alloy fittings. Damaged fittings should be replaced with new parts having the same material specifications or the method of repair should be specifically approved by a representative of the Federal Aviation Agency.

(g) Castings. Damaged castings should be replaced and not repaired unless the method of repair is specifically approved by a representative of the Federal Aviation Agency.

§ 18.30-5

Control cables and terminals (FAA policies which apply to § 18.30).

(a) Control cables and wires. Control cables and wires should be replaced if injured, distorted, worn, or corroded even though the strands are not broken. However, cable sections may be spliced using the procedures of subparagraph (1) of this paragraph. Cable tension should be checked after installation to insure proper rigging.

(1) Splicing. (i) Control cables may be spliced when they become worn, distorted, corroded, or otherwise injured. The cable, thimbles, shackles, turnbuckles, bolts, and other parts should be of the same size, material, and quality as the original parts or of such size that the repaired cable will be of strength equivalent to the original. However, AN-666 through AN-669 standard swaged cable terminals develop the full cable strength and may be substituted for the original terminals wherever practical. If facilities and supplies are limited, repair may sometimes be accomplished, using thimbles, bushings, and turnbuckles in place of original terminals. When this is done, flexible cables 7 x 7 and 7 x 19, having a diameter of three thirtyseconds inch or over, may be woven spliced by means of the 5-tuck method. Flexible cable less than three thirtyseconds inch in diameter and nonflexible carbon steel 19 wire cable (MIL-C-6940) may be wrap-soldered. Directions for fabricating these splices and limitations as to their use are contained in the following paragraphs.

(ii) All splices should be installed so that no portion of the splice comes closer than 2 inches to any fairlead or pulley