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.
Par 4-59 Page 4-54
Par 4-59 Page 4-55
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.
Par 4-60 Page 4-56
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.
Par 4-62 Page 4-58
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.
Par 4-62 Page 4-59
e. Materials. The anodes should be of high-purity dense graphite or platinum-iridium alloys. Do not mix solutions from different suppliers. This could result in contamination.
f. Detail Requirements. On large parts, no area greater than approximately 10 percent of the total area of the part should be plated by this selective plating process. Small parts may be partially or completely plated. Special cases exceeding these limitations should be coordinated with the manufacturer of the plating equipment being used and their recommendations should be followed.
g. Anode Selection. As a general guide, the contact area of the anode should be approximately one-third the size of the area to be plated. When selecting the anode, the configuration of the part will dictate the shape of the anode.
h. Required Amp-Hour Calculation. The selected plating solution has a factor which is equal to the amp-hours required to deposit 0.0001-inch on 1 square inch of surface. Determine the thickness of plating desired on a certain area, and multiply the solution factor times the plating thickness times the area in square inches to determine the amp-hours required. This factor may vary because of temperature, current density, etc.
i. Cleaning. Remove corrosion, scale, oxide, and unacceptable plating prior to processing.
Use a suitable solvent or cleaner to remove grease or oil.
j. Plating on Aluminum and Aluminum Base Alloys.
(1) Electroclean the area using DC until water does not break on the surface. This electroclean process should be accomplished at 10 V to 15 V using the appropriate electroclean solution.
(2) Rinse the area in cold, clean tap water.
(3) Activate the area with reverse current, 7 V to 10 V, in conjunction with the proper activating solution until a uniform, gray-to-black surface is obtained.
(4) Rinse thoroughly in cold, clean tap water.
(5) Immediately electroplate to color while the area is still wet, using the appropriate nickel solution.
(6) Rinse thoroughly.
(7) Immediately continue plating with any other solution to desired thickness.
(8) Rinse and dry.
Par 4-62 Page 4-60
k. Plating on Copper and Copper Base Alloys.
(1) Electroclean the area using DC until water does not break on the surface. The electroclean process should be accomplished at 8 V to 12 V using the appropriate electroclean solution.
(2) Rinse the area in cold, clean tap water.
(3) Immediately electroplate the area with any of the plating solutions, except silver.
Silver requires an undercoat.
(4) Rinse and dry.
l. Plating on 300 and 400 Series Stainless Steels, Nickel Base Alloys, Chrome Base Alloys, High Nickel Ferrous Alloys, Cobalt Base Alloys, Nickel Plate, and Chrome Plate.
(1) Electroclean the area using DC until water does not break on the surface. This electroclean process should be accomplished at 12 V to 20 V using the appropriate electrocleaning solution.
(2) Rinse the area in cold, clean tap water.
(3) Activate the surface using DC for 1 to 2 minutes, using the activating solution, and accomplish at 6 V to 20 V.
(4) Do not rinse.
(5) Immediately nickel-flash the surface to a thickness of 0.00005 to 0.0001 inch, using the appropriate nickel solution.
(6) Rinse thoroughly.
(7) Immediately continue plating with any other solution to desired thickness.
(8) Rinse and dry.
m. Plating on Low-Carbon Steels (Heat Treated to 180,000 psi).
(1) Electroclean the area using DC until water does not break on the surface. This electroclean process should be accomplished at 12 V to 20 V, using the appropriate electrocleaning solution.
(2) Rinse the area in cold, clean tap water.
(3) Reverse-current etch at 8 V to 10 V, using the appropriate activating solution, until a uniform gray surface is obtained.
(4) Rinse thoroughly.
Par 4-62 Page 4-61
(5) Immediately electroplate the part using any solutions, except copper or silver. Both of these require undercoats.
(6) Rinse and dry.
n. Plating on Cast Iron and High-Carbon Steels (Steels Heat Treated to 180,000 psi).
(1) Electroclean the area using DC until water does not break on the surface. This electroclean process should be accomplished at 12 V to 20 V, using the appropriate electrocleaning solution.
(2) Rinse the area thoroughly in cold, clean tap water.
(3) Reverse-current etch at 8 V to 10 V, using the appropriate etching solution, until a uniform gray is obtained.
(4) Rinse thoroughly.
(5) Remove surface smut with 15 V to 25 V using the appropriate activating solution.
(6) Rinse thoroughly.
(7) Electroplate immediately, using any of the solutions, except copper or silver (both of these require undercoats).
(8) Rinse and dry.
o. Plating on Ultrahigh Strength Steels (Heat Treated Above 180,000 psi).
(1) Electroclean the area using reverse current until water does not break on the surface.
This electroclean process should be accomplished at 8 V to 12 V using the appropriate electroclean solution.
(2) Rinse the area thoroughly in cold, clean tap water.
(3) Immediately electroplate the part, using either nickel, chromium, gold, or cadmium.
Other metals require an undercoat of one of the above. Plate initially at the highest voltage recommended for the solution so as to develop an initial barrier layer. Then reduce to standard voltage.
(4) Rinse and dry.
(5) Bake the part for 4 hours at 375 °F ±25 °F.
NOTE: Where the solution vendor provides substantiating data that hydrogen embrittlement will not result from plating with a particular solution, then a postbake is not required. This substantiating data can be in the form of aircraft industry manufacturer’s process specifications, military specifications, or other suitable data.
Par 4-62 Page 4-62 NOTE: Acid etching should be avoided, if possible. Where etching is absolutely necessary, it should always be done with reverse current. Use alkaline solutions for initial deposits.
p. Dissimilar Metals and Changing Base. As a general rule, when plating two dissimilar metals, follow the plating procedure for the one with the most steps or activation. If activating steps have to be mixed, use reverse current activation steps prior to DC activation steps.
q. Plating Solution Selection.
(1) Alkaline and neutral solutions are to be used on porous base metals, white metals, high-strength steel, and for improved coating ability. Acid solutions are to be used for rapid buildup and as a laminating structure material in conjunction with alkaline-type solutions.
(2) Chrome brush-plating solutions do not yield as hard a deposit as bath-plating solutions. The hardness is about 600 Brinell as compared to 1,000 Brinell for hard chrome deposited from a tank.
(3) Silver-immersion deposits will form with no current flowing on most base metals from the silver brush-plating solutions. Such deposits have poor adhesion to the base metal.
Consequently, a flash of a more noble metal should be deposited prior to silver plating to develop a good bond.
(4) In general, brush plating gives less hydrogen embrittlement and a lower fatigue strength loss than does equivalent tank deposits. However, all brush-plated, ultrahigh strength steel parts (heat treated above 180,000 psi) should be baked, as mentioned, unless it is specifically known that embrittlement is not a factor.
r. Qualification Tests. All brush-plated surfaces should be tested for adhesion of the electrodeposit. Apply a 1-inch wide strip of 3M Scotch Flatback Masking Tape 250, or an approved equal, with the adhesive side to the freshly plated surface. Apply the tape with heavy hand pressure and remove it with one quick motion perpendicular to the plated surface. Any plating adhering to the tape should be cause for rejection.
s. Personnel Training for Quality Control. Manufacturers of selective-plating equipment provide training in application techniques at their facilities. Personnel performing selective plating must have adequate knowledge of the methods, techniques, and practices involved. These personnel should be certified as qualified operators by the manufacturers of the products used.
4-63.–4-73. [RESERVED.]
Par 4-74 Page 4-63 SECTION 5. WELDING AND BRAZING
4-74. GENERAL. This section covers weld repairs to aircraft and component parts only.
Observe the following procedures when using such equipment as gas tungsten arc welding (GTAW), Gas Metal Arc Welding (GMAW), plasma arc welding, and oxyacetylene gas welding.
When repairs of any of these flight-critical parts are required, it is extremely important to make the weld repairs equal to the original weld. Identifying the kind of metal to be welded, identifying the kind of welding process used in building the part originally, and determining the best way to make welded repairs are of utmost importance.
a. Welding is one of the three commonly used methods of joining metals without the use of fasteners. Welding is done by melting the edges of two pieces of metal to be joined and allowing the molten material to flow together so the two pieces will become one.
b. Brazing is similar to welding in that heat is used to join the material; but rather than melting, the metal is heated only enough to melt a brazing rod having a much lower melting point. When this brazing rod melts, it wets the surfaces to be joined, and when it cools and solidifies, it bonds the pieces together.
c. Soldering is similar to brazing except that brazing materials normally melt at temperatures above 425 ºC (800 °F), while solders melt at temperatures considerably lower.
d. The next step in making airworthy weld repairs is to decide the best process to use, considering the available state-of-the-art welding equipment, and then deciding the correct weld-filler material to use. Before any weld repairs can be made, the metal parts to be welded must be cleaned properly, fitted and jigged properly, and all defective welds must be removed to prepare for an aircraft quality weld repair.
e. Finally, after the weld is completed, the weld must be inspected for defects. All these things are necessary in order to make an airworthy weld repair.
f. Aircraft Welding Qualifications. Four groups of metals a person can be certified and qualified to use are:
(1) Group 1, 4130 Steel.
(2) Group 2, Stainless Steel.
(3) Group 3, Aluminum
(4) Group 4, Titanium.
g. For other group listing of metal the welder may qualify, refer to AMSSTD1595.
h. Most large business or agencies conduct their own certification tests, or they have an outside testing lab validate the certification tests.
Par 4-75 Page 4-64
4-75. EQUIPMENT SELECTION. Use the welding equipment manufacturer’s information to determine if the equipment will satisfy the requirements for the type of welding operation being undertaken. Disregarding such detailed operating instructions may cause substandard welds. For example, when using GTAW equipment, a weld can be contaminated with tungsten if the proper size electrode is not used when welding with DC reverse polarity. Another example, the depletion of the inert gas supply below the critical level causes a reduction in the gas flow and will increase the danger of atmospheric contamination.
a. Electric welding equipment versatility requires careful selection of the type of current and polarity to be used. Since the composition and thickness of metals are deciding factors, the selection may vary with each specific application. Metals having refractory surface oxide films
(i.e., magnesium alloys and aluminum and its alloys), are generally welded with AC, while DC is used for carbon, low alloy, noncorrodible, and heat-resisting steels. General recommendations covering current and polarity are shown in Table 4-12.
TABLE 4-12. CURRENT AND POLARITY SELECTION FOR INERT GAS WELDING
MATERIAL
ALTERNATING
CURRENT (AC) DIRECT
CURRENT (DC) With High-Frequency Stabilization Straight Polarity Magnesium up to 1/8 in. thick 1 N.R.
Magnesium above 3/16 in. thick 1 N.R.
Magnesium castings 1 N.R.
Aluminum up to 3/32 in. thick 1 N.R.
Aluminum over 3/32 in. thick 1 N.R.
Aluminum castings 1 N.R.
Stainless steel 1 Low carbon steel, 0.015 to 0.030 in. 1 Low carbon steel, 0.030 to 0.125 in. N.R. 1 1 Recommended N.R. Not Recommended
b. Oxyacetylene gas equipment is suitable for welding most metals. It is not, however, the best method to use on such materials as stainless steel, magnesium, and aluminum alloys;
because of base metal oxidization, distortion, and loss of ductility.
NOTE: If oxyacetylene is used for welding stainless steel or aluminum, all flux must be removed, as it may cause corrosion.
4-76. ACCURATELY IDENTIFY THE TYPE OF MATERIAL TO BE REPAIRED. If positive identification of the material is not possible, contact the aircraft manufacturer or subject the item to a metallurgical laboratory analysis. Before any welding is attempted, carefully consider the weldability of the alloy, since all alloys are not readily weldable. The following steels are readily weldable: plain carbon (of the 1000 series), nickel steel (of the SAE 2300 series), chrome-nickel alloys (of the SAE 3100 series), chrome-molybdenum steels (of the SAE 4100 series), and low nickel-chrome-molybdenum steel (of the SAE 8600 series).