Par 4-58 Page 4-47
(3) To avoid concentration of load on the end rivet and consequent tendency toward progressive rivet failure, the splice is tapered at the ends by tapering the backing angle and by making it shorter than the splice bar. (See Figure 4-18.)
(4) 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, stagger the splices if possible.
k. Size of Splicing Members. When the same material is used for the splicing members as for the original member, the cross-section area (i.e., the shaded areas in Figure 4-17), of the splice material will 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, the bar “B” in Figure 4-18 is assumed to splice the upper leg of the stringer, and the angle “A” is assumed to splice the bulbed leg of the stringer. Since the splice bar “B” is not as wide as the adjacent leg, its thickness must be increased such that the area of bar “B” is at least equal to the area of the upper leg of the stringer.
l. The Diameter of Rivets in Stringers. The diameter of rivets in stringers might preferably be between two and three times the thickness “t” of the leg, but must not be more than 1/4 the width “W” of the leg. Thus, 1/8-inch rivets are chosen in the example in Figure 4-18. If the splices were in the lower surface of a wing, the end rivets would be made the same size as the skin-attaching rivets, or 3/32 inch.
m. The Number of Rivets. 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 in Tables 4-9 through 4-11.
Par 4-58 Page 4-48 TABLE 4-11. NUMBER OF RIVETS REQUIRED FOR SPLICES (SINGLE-LAP JOINT) IN 5052 (ALL HARDNESSES) SHEET Thickness “t” in Inches No. of 2117-T4 (AD) Protruding Head Rivets Required per Inch of Width “W” No. of Bolts Rivet Size 3/32 1/8 5/32 3/16 1/4 AN-3 .016 6.3 4.7 .020 6.3 4.7 3.8 .025 6.3 4.7 3.8 .032 6.3 4.7 3.8 3.2 .036 7.1 4.7 3.8 3.2 2.4 .040 7.9 4.7 3.8 3.2 2.4 .051 10.1 5.6 3.8 3.2 2.4 .064 12.7 7.0 4.6 3.2 2.4 .081 8.9 5.8 4.0 2.4 3.2 .091 10.0 6.5 4.5 2.5 3.2 .102 11.2 7.3 5.1 2.8 3.2 .128 9.2 6.4 3.5 3.2 NOTES:
a. For stringers in the upper surface of a wing, or in a fuselage, 80 percent of the number of rivets shown in the table may be used.
b. For intermediate frames, 60 percent of the number shown may be used.
c. For single lap sheet joints, 75 percent of the number shown may be used.
ENGINEERING NOTES:
a. The load per inch of width of material was calculated by assuming a strip 1 inch wide in tension.
b. Number of rivets required was c alculated for 2117-T4 (AD) rivets, based on a rivet allowable shear stress equal to 70 percent of the sheet allowable tensile stress, and a sheet allowable bearing stress equal to 160 percent of the sheet allowable tensile stress, using nominal hole diameters for rivets.
c. Combinations of sheet thickness and rivet size above the underlined numbers are critical in (i.e., will fail by) bearing on the sheet, those below are critical in shearing of the rivets.
(1) In determining the number of rivets required in the example in Figure 4-18, for attaching the splice bar “B” to the upper leg, the thickness “t” of the element of area being spliced is 1/16 inch (use 0.064), the rivet size is 1/8 inch, and Table 4-9 shows that 9.9 rivets are required per inch of width. Since the width “W” is 1/2 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) x 0.5 (inch width) = 4.95 (use 5 rivets).
(2) For the bulbed leg of the stringer “t” = 1/16 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 1 x 3.3 = 3.3 (use 4 bolts). When both rivets and bolts are used in the same splice, the bolt holes must be accurately
Par 4-58 Page 4-49 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.
n. Splicing of Intermediate Frames. The same principles used for stringer splicing may be applied to intermediate frames when the following point is considered. Conventional frames of channel or Z sections are relatively deep and thin compared to stringers, and usually fail by twisting or by buckling of the free flange. Reinforce the splice joint 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. (See Figure 4-20.) Since a frame is likely to be subjected to bending loads, make the length of splice plate “L” more than twice the width “W2,” and the rivets spread out to cover the plate.
4-59. REPAIRING CRACKED MEMBERS. Acceptable methods of repairing various types of cracks in structural elements are shown in Figures 4-21 through 4-24. The following general procedures apply in repairing such defects.
FIGURE 4-20. EXAMPLE OF INTERMEDIATE FRAME STRINGER SPLICE (MATERIAL: 2017-T3 AL ALLOY)
THE NUMBER OF RIVETS REQUIRED IN EACH LEG ON EACH SIDE OF THE CUT IS DETERMINED BY THE WIDTH "W," THE THICKNESS OF THE FRAME "" AND THE RIVET DIAMETER "d" USING TABLE 4- 10 IN A MANNER SIMILAR TO THAT FOR STRINGERS IN FIGURE 4-
20.
NOTE b. IN TABLE 4-10 INDICATES THAT ONLY 60 PERCENT OF
THE NUMBER OF RIVETS SO CALCULATED NEED BE USED IN SPLICES IN INTERMEDIATE.
EXAMPLE: (FOR 2017-T3 aluminum alloy frame)
FLANGE LEG † = .040" d = 1/8" 2117-T4 (AD) W, & W 3 = 6 inch NO. OF RIVETS PER INCH OF WIDTH FROM TABLE 4-10 = 6.2 No. of rivets required = W x 6.2 = . 6 x 6.2 = 3.72 or 4 rivets.
60 percent of 4 rivets = 2.4 rivets.
USE 3 RIVETS ON EACH SIDE OF THE CUT IN EACH FLANGE LEG.
WEB OF ZEE (OR CHANNEL) † = .040" d = 1/8" 2117-T4 (AD) rivet W = 2.0 inches NO. OF RIVETS PER INCH OF WIDTH FROM TABLE 4-10 = 6.2 No. of rivets required = W x 6.2 =
2.0 x 6.2 = 12.4 or 13 rivets.
60 percent of 13 rivets = 7.8 rivets.
USE 8 RIVETS ON EACH SIDE OF CUT IN THE WEB OF ZEE (OR CHANNEL).
"L" SHOULD BE MORE THAN TWICE W2 Thickness of splice plate to be greater than that of the frame to be spliced.
Par 4-58 Page 4-50
FIGURE 4-21. TYPICAL METHODS OF REPAIRING CRACKED LEADING AND
TRAILING EDGES AND RIB INTERSECTIONS
CHANNELS
TRAILING EDGE
FORWARD
HORIZONTAL
RIBS SKIN
O VERTICAL SPAR CRACKS CRACKS ANCHOR
NUTS FOR RIB
ATTACH-
MENT OF
FIN TO SKIN
STABILIZER
AND TIP CRACK TO FIN. TRAILING EDGE
REAR SHEAR
FRONT BEAM SPAR CRACKS
LEADING
EDGE AILERON NOTE: ALL REINFORCING PLATES TO BE OF REAR * SAME ALLOY & APPROX. 1.5 THICKNESS OF ORIGINAL Par 4-59 Page 4-51
FIGURE 4-22. TYPICAL METHODS OF REPLACING CRACKED MEMBERS
AT FITTINGS
END RIE REAR SPAR
CONTROL
STIFFENING CHANNEL TO SURFACE
FIT UNDER END OF RIB, FITTING
REINFORCEMENT AND UNDER ANCHOR NUT. MATERIAL: • POLE RELIEF DIA.
SAME ALLOY AS ORIGINAL, APPROX
1.5 X THICKNESS OF ORIGINAL OR GREATER
RIVETS JOINING SPAR WEB WITH FRONT
AND REAR CHANNEL TYPE REINFORCEMEN
NOTCHED MEMBER
CRACKS AT
FITTING ANCHOR NUTS
TRAILING EDGE
PORTION OF RIE DRILL RELIEF HOLE REAR SHEAR
REINFORCING PLATE- BEAM
WITH FILLETED NOTCH
MATERIAL OF REINFORCING PLATES SAME ALLOY AS ORIGINAL AND 1.5 (OR MORE) × THICKNESS OF ORIGINAL *HINGE Par 4-59 Page 4-52
FIGURE 4-23. TYPICAL METHODS OF REPAIRING CRACKED FRAME AND
STIFFENER COMBINATION
CRACKS IN SKIN AND
TRANSVERSE STIFFENER
• AT DRAGG RIB JUNCTION _(REPAIR BY OUTSIDE GUSSETS)
DRAG RIB
TRANSVERSE STIFFENER
LONGITUDINAL STIFF =
- CRACK DUE TO FATIGUE
REINFORCE-
DRAG RIB MENT TO BE REPAIR OF CRACKED SAME MATERIAL STIFFENER.
AND AT LEAST
SAME AREA AS
TRANSVERSE STIFFENER.
FAILURES
A COMES Par 4-59 Page 4-53
FIGURE 4-24. TYPICAL REPAIRS TO RUDDER AND TO FUSELAGE AT TAIL POST
FAILURES FRONT SPAR HORN RIB BOTTOM
FUSELAGE (TOP)
FAILURES
FUSELAGE (SIDE) NOTE: USE SAME MATERIAL, NEXT HEAVIER GAUGE FOR REINFORCEMENT.
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a. Drill small holes 3/32 inch (or 1/8 inch) at the extreme ends of the cracks to minimize the possibility of their spreading further.
b. Add reinforcement to carry the stresses across the damaged portion and to stiffen the joints. (See Figures 4-14 through 4-17.) The condition causing 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.
4-60. STEEL AND ALUMINUM FITTINGS.
a. Steel Fittings. Inspect for the following defects.
(1) Fittings are to be free from scratches, vise and nibbler marks, and sharp bends or edges. A careful examination of the fitting with a medium power (at least 10 power) magnifying glass is acceptable as an inspection.
(2) When repairing aircraft after an accident or in the course of a major overhaul, inspect all highly stressed main fittings, as set forth in the manufacturer’s instruction manual.
(3) Replace torn, kinked, or cracked fittings.
(4) Elongated or worn bolt holes in fittings, which were designed without bushings, are not to be reamed oversize. Replace such fittings, unless the method of repair is approved by the FAA. Do not fill holes with welding rod. Acceptable methods of repairing elongated or worn bolt holes in landing gear, stabilizer, interplane, or cabane-strut ends are shown in Figure 4-25.
FIGURE 4-25. TYPICAL METHODS OF REPAIRING ELONGATED OR WORN
BOLT HOLES
BUSHING
ELONGATED OR WORN.
TAPER WASHER OF 4130 ADDED. THICKNESS = NOT LESS THAN 1.5t OR GREATER THAN 2t - t
ELONGATED
HOLE IN
ORIGINAL REMOVE STRUT. ORIGINAL WASHER NOTES:
R=M-†ö 1. METHODS A OR B MAY 3. MEMBERS ORIGINALLY
BE USED IN REPAIRING HEAT TREATED MUST TYPE C STRUT ENDS. BE REHEAT TREATED AFTER WELDING.
2. METHOD B SHOULD BE
USED IN REPAIRING TYPE D & E STRUT ENDS.
b. Aluminum and Aluminum Alloy Fittings.
(1) Replace damaged fittings with new parts that have the same material specifications.
(2) Repairs may be made in accordance with data furnished by the aircraft manufacturer, or data substantiating the method of repair may be submitted to the FAA for approval.
4-61. CASTINGS. Damaged castings are to be replaced and not repaired unless the method of repair is specifically approved by the aircraft manufacturer or substantiating data for the repair has been reviewed by the FAA for approval.
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Par 4-62 Page 4-57
4-62. SELECTIVE PLATING IN AIRCRAFT MAINTENANCE. Selective plating is a method of depositing metal from an electrolyte to the selected area. The electrolyte is held in an absorbent material attached to an inert anode. Plating contact is made by brushing or swabbing the part (cathode) with the electrolyte-bearing anode.
a. Selective Plating Uses. This process can be utilized for any of the following reasons.
(1) To prevent or minimize disassembly, or reassembly.
(2) Resizing worn components (plate to size).
(3) Filling in damaged or corroded areas.
(4) To plate small areas of extremely large parts.
(5) To plate electrical contacts.
(6) To plate parts too large for existing baths.
(7) To supplement conventional plating.
(8) To plate components which become contaminated if immersed in a plating bath.
(9) To cadmium-plate ultrahigh strength steels without hydrogen embrittlement.
(10) Onsite plating.
(11) Reverse current applications (e.g., stain removal, deburring, etching, and dynamic balancing).
b. Specifications. Selective plating (electrodepositions), when properly applied, will meet the following specifications and standards.
(1) AMSQQC320, Chromium Plating.
(2) AMSQQN290, Nickel Plating.
(3) AMSQQP416, Cadmium Plating.
(4) ASTM-B700, Silver Plating.
(5) ASTM-B633, Zinc Plating.
(6) ASTM-B545 and ASTM-B339, Tin Plating.
(7) AMS2418, Copper Plating.
(8) MIL-G-45204, Gold Plating.
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c. General Requirements.
(1) Areas to be repaired by this process should be limited to small areas of large parts, particularly electrical or electronic parts.
(2) All solutions should be kept clean and free from contamination. Care should be taken to ensure that the solutions are not contaminated by used anodes or other plating solutions.
Brush-plating solutions are not designed to remove large amounts of scale, oil, or grease.
Mechanical or chemical methods should be used to remove large amounts of scale or oxide. Use solvents to remove grease or oil.
(3) Brush-plating solutions are 5 to 50 times as concentrated as tank solutions. The current densities used range from 500 to 4,000 amperes (amp)/feet2. The voltages (V) listed on the solution bottles have been precalculated to give proper current densities. Too high a current density burns the plating, while too low a current density produces stressed deposits and low efficiencies. Agitation is provided by anode/cathode motion. Too fast a motion results in low efficiencies and stressed deposits, and too slow a motion causes burning. A dry tool results in burnt plate, coarse grain structure, and unsound deposits. The tool cannot be too wet. Solution temperatures of 110 °F to 120 °F are reached during operation.
(4) Materials such as stainless steel, aluminum, chromium, and nickel (which have a passive surface) will require an activating operation to remove the passive surface. During the activating process, do not use solutions that have been previously used with reverse current (because of solution contamination).
d. Equipment. The power source should operate on either 110 V or 220 V alternating current (AC), 60 Hertz (Hz), single-phase input. It should have a capability to produce direct current (DC) having smooth characteristics with controlled ripple and be able to output a current of at least 25 amp at 0 V to 25 V. Minimum instrumentation of the power source should include a voltmeter, ammeter, and amp-hour meter.
(1) The ammeter should provide a full-scale reading equal to the maximum capacity of the power source, and with an accuracy of ±5 percent of the current being measured.
(2) The voltmeter should have sufficient capacity to provide a full-scale reading equal to the maximum capacity of the power source and an accuracy of ±1.0 V.
(3) An amp-hour meter should be readable to 0.001 amp-hour and have an accuracy of ±0.01 amp-hour.
(4) The stylus should be designed for rapid cooling and to hold anodes of various sizes and configurations. For safety, the anode holder should be insulated.
(5) The containers for holding and catching runoff solutions should be designed to the proper configuration and be inert to the specific solution.
(6) The mechanical cleaning equipment and materials should be designed and selected to prevent contamination of the parts to be cleaned.