Repairability of Sheet Metal Structures (Part Five)

in Aircraft Metal Structural Repair

Stringer Repair

The fuselage stringers extend from the nose of the aircraft to the tail, and the wing stringers extend from the fuselage to the wing tip. Surface control stringers usually extend the length of the control surface. The skin of the fuselage, wing, or control surface is riveted to stringers.


Stringers may be damaged by vibration, corrosion, or collision. Because stringers are made in many different shapes, repair procedures differ. The repair may require the use of preformed or extruded repair material, or it may require material formed by the airframe technician. Some repairs may need both kinds of repair material. When repairing a stringer, first determine the extent of the damage and remove the rivets from the surrounding area. [Figure 4-184] Then, remove the damaged area by using a hacksaw, keyhole saw, drill, or file. In most cases, a stringer repair requires the use of insert and splice angle. When locating the splice angle on the stringer during repair, be sure to consult the applicable structural repair manual for the repair piece’s position. Some stringers are repaired by placing the splice angle on the inside, whereas others are repaired by placing it on the outside.

Figure 4-184. Stringer repair.

Figure 4-184. Stringer repair.

Extrusions and preformed materials are commonly used to repair angles and insertions or fillers. If repair angles and fillers must be formed from flat sheet stock, use the brake. It may be necessary to use bend allowance and sight lines when making the layout and bends for these formed parts. For repairs to curved stringers, make the repair parts so that they fit the original contour.

Figure 4-185. Stringer repair by patching.

Figure 4-185. Stringer repair by patching.

Figure 4-185 shows a stringer repair by patching. This repair is permissible when the damage does not exceed two-thirds of the width of one leg and is not more than 12-inch long. Damage exceeding these limits can be repaired by one of the following methods.

Figure 4-186. Stringer repair by insertion when damage exceeds two-thirds of one leg in width.

Figure 4-186. Stringer repair by insertion when damage exceeds two-thirds of one leg in width.

Figure 4-186 illustrates repair by insertion where damage exceeds two-thirds of the width of one leg and after a portion of the stringer is removed. Figure 4-187 shows repair by insertion when the damage affects only one stringer and exceeds 12-inch in length. Figure 4-188 illustrates repair by an insertion when damage affects more than one stringer.

Figure 4-187. Stringer repair by insertion when damage affects only one stringer.

Figure 4-187. Stringer repair by insertion when damage affects only one stringer.

Figure 4-188. Stringer repair by insertion when damage affects more than one stringer.

Figure 4-188. Stringer repair by insertion when damage affects more than one stringer.

Former or Bulkhead Repair

Bulkheads are the oval-shaped members of the fuselage that give form to and maintain the shape of the structure. Bulkheads or formers are often called forming rings, body frames, circumferential rings, belt frames, and other similar names. They are designed to carry concentrated stressed loads.

There are various types of bulkheads. The most common type is a curved channel formed from sheet stock with stiffeners added. Others have a web made from sheet stock with extruded angles riveted in place as stiffeners and flanges. Most of these members are made from aluminum alloy. Corrosion-resistant steel formers are used in areas that are exposed to high temperatures.

Bulkhead damages are classified in the same manner as other damages. Specifications for each type of damage are established by the manufacturer and specific information is given in the maintenance manual or SRM for the aircraft. Bulkheads are identified with station numbers that are very helpful in locating repair information. Figure 4-189 is an example of a typical repair for a former, frame section, or bulkhead repair.

Figure 4-189. Bulkhead repair.

Figure 4-189. Bulkhead repair.

  1. Stop drill the crack ends with a No. 40 size drill.
  2. Fabricate a doubler of the same material but one size thicker than the part being repaired. The doubler should be of a size large enough to accommodate 1⁄8- inch rivet holes spaced one inch apart, with a minimum edge distance of 0.30-inch and 0.50-inch spacing between staggered rows. [Figure 4-190]
  3. Attach the doubler to the part with clamps and drill holes.
  4. Install rivets.

Figure 4-190. Wing spar repair.

Figure 4-190. Wing spar repair. [click image to enlarge]

Most repairs to bulkheads are made from flat sheet stock if spare parts are not available. When fabricating the repair from flat sheet, remember the substitute material must provide cross-sectional tensile, compressive, shear, and bearing strength equal to the original material. Never substitute material that is thinner or has a cross-sectional area less than the original material. Curved repair parts made from flat sheet must be in the “0” condition before forming, and then must be heat treated before installation.

Longeron Repair

Generally, longerons are comparatively heavy members that serve approximately the same function as stringers. Consequently, longeron repair is similar to stringer repair. Because the longeron is a heavy member and more strength is needed than with a stringer, heavy rivets are used in the repair. Sometimes bolts are used to install a longeron repair, due to the need for greater accuracy, they are not as suitable as rivets. Also, bolts require more time for installation.

If the longeron consists of a formed section and an extruded angle section, consider each section separately. A longeron repair is similar to a stringer repair, but keep the rivet pitch between 4 and 6 rivet diameters. If bolts are used, drill the bolt holes for a light drive fit.

Spar Repair

The spar is the main supporting member of the wing. Other components may also have supporting members called spars that serve the same function as the spar does in the wing. Think of spars as the hub, or base, of the section in which they are located, even though they are not in the center. The spar is usually the first member located during the construction of the section, and the other components are fastened directly or indirectly to it. Because of the load the spar carries, it is very important that particular care be taken when repairing this member to ensure the original strength of the structure is not impaired. The spar is constructed so that two general classes of repairs, web repairs and cap strip repairs, are usually necessary.

Figure 4-191. Wing spar repair.

Figure 4-191. Wing spar repair. [click image to enlarge]

Figures 4-190 and 4-191 are examples of typical spar repairs. The damage to the spar web can be repaired with a round or rectangular doubler. Damage smaller than 1-inch is typically repaired with a round doubler and larger damage is repaired with a rectangular doubler.

  1. Remove the damage and radius all corners to 0.5-inch.
  2. Fabricate doubler; use same material and thickness. The doubler size depends on edge distance (minimum of 2D) and rivet spacing (4-6D).
  3. Drill through the doubler and the original skin and secure doubler with Clecos.
  4. Install rivets.