Service and Repair Manual

3. Extent of Damage.

     The extent of damage must be carefully ascertained. Determine the path taken by every missile which enters the wings, fuselage, or control surfaces and examine the attachment aft every item which has been struck. Look for damage caused by the travel of shock loads along large members. If necessary form semi structural doors In the skin, (section VI, paragraph 3.§,(2)) to facilitate inspection. Check particularly for the following: Dents and cracks in extrusions and fittings; elongated rivet and bolt holes, cracks and sharp cornered dents or wrinkles, and corrosion in sheet materials; and rivets which are stretched or sheared leaving their heads intact.

4. Support of Structure During repair.

     When repair operations are being performed which necessitate the removal of structural material, care must be taken not to overload other parts of the structure. A fixture of some type must be used to brace or hold that portion of the structure being worked upon. It is essential that the damaged structure be suitably and firmly supported against distortion.

5. Heat Treatment.

     The material used in the construction of the airplane is almost entirely aluminum alloy. High strength rolled sheet is used for the stressed skin, and formed sheet is used for the spars, webs, bulkheads and -ribs, and extrusions for the stringers and cap strips. All re placement or reinforcement material used must have the same structural characteristics as the damaged part for which it is used. Repair stock for the assembly covered n each section is listed in a table at the end of the respective section. A list of the commonly used heat-treat specifications, along with a brief description of each is given at the end of this section.

     The heat-treated and cold worked alloys, 17SRT and 24SRT, can stand very little bending without cracking. The strong alloys of 17S and 24S can be formed in their annealed temper and heat-treated to develop the required strength before assembly. In cases where the annealed metal is not available, it is possible to heat the metal and quench it according to regular heat treating practice and then form it before the age hardening sets in. The forming must be completed in 1 hour on "17S" and 20 minutes on "24S" after quenching or the material will become too hard to work. Heat treating the strong alloys of 17S and 24S consists of heating to a temperature ranging from 504.14° ą 5.5°C (940° ą l0°F) for "17S" material and 493.33° ą 5.5°C (920°C ą 10°F) for "24S." Hold at the required temperature until the piece is uniformly heated throughout and quench it immediately in cold water or oil. Thus, all strong aluminum-alloy bar, sheet and tube used for the airplane parts must be in the fully heat-treated condition upon final fabrication.

It is only in this condition that the material develops its maximum physical properties and resistance to corrosion. Aluminum forgings, castings, and fittings must be replaced, as reheat treat weakens the tensile strength of the part.

     All structural steel fittings are also heat-treated. These fittings must be replaced because of their respective structural and tensile strength value.

6. Welding.

  a. Welded Aluminum Tanks and Fuel Bag Shells. Repair welded aluminum tanks and fuel bag shells according to regular welding procedure. There are several ways and means of accomplishing good last ing welds if the following precautions are adhered to. However, when effecting repairs, care must be taken to eliminate a combination of slag and gas as they produce defects that have a very definite bearing on the strength and pliancy of the weld, and depending on their size and location can seriously endanger any welded joint. The elimination of slag and gas in the weld metal is dependent largely upon proper welding procedure and good welding operator. Nevertheless, some slag will be formed and this must be removed with emery paper.

  b. Welding Direction. - When performing welds on the fuel tanks and fuel bag shells, the tip size and gas pressure for oxyhydrogen welding should be according to the thickness of the metal. For metal 0.050 inch - 0.064 inch, the diameter .of the orifice should be 0.065 inch. The oxygen pressure should be 2 and the hydrogen pressure should be 1 pound per square inch. For metal 1/8 inch - 3/16 inch, the diameter of the orifice should be 0.095 inch, the oxygen pressure 3 and the hydrogen pressure 2 pounds per square inch.

  c. Weldable Parts. - Only parts which were welded during manufacture may be repaired or replaced by welding. When repairing the engine mount, reference should be made to the Caution note contained in section II, paragraph l.b. All welded repairs must be similar to those made on the original part; for in stance tack welding should be replaced by tack welding.

7. Bowed Tubes.

  Engine mount tubes bowed in excess of 1/800 of their length must be replaced.

8. Solid Rivets.

  Removal of formed rivets: When removing old rivets the following precautions should be observed. Use No. 30 (.128-inch)drill for 1/ 8-inch rivets, 5/32 inch drill for a 5/32-inch rivet and 3/16-inch drill for a 3/16-inch rivet. On round-head rivets, a flat should be filed on the head before drilling. Drill to a depth equal to the depth of the rivet head or slightly beyond the head and drive the rivet out with a punch. If the rivet will not come out and the drill hole is centered in the rivet, the drill may be run partly into