Text-only reference. Published from the recorded official FAA Airframe Chapter 4 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
Damage area Splice angles Damaged area cut out smooth Insertion Assembled repair only one stringer. 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. Figures 4-189 and 4-190 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. Damage area Reinforcements Stringer insertion Damaged area cut back so joints will be staggered Rib repaired Damaged skin cut back to smooth contour with corner radii Assembled repair Skin Rib Section A-A more than one stringer.
Rib & Web Repair Web repairs can be classified into two types: 1. Those made to web sections considered critical, such as those in the wing ribs. 2. Those considered less critical, such as those in elevators, rudders, flaps, and the like. 4-104 Crack Stop drill #40 drill hole Crack Doubler Bulkhead Radius to rest in bulkhead Bulkhead 1.5" minimum OR Radius to rest in bulkhead Web sections must be repaired in such a way that the original strength of the member is restored. In the construction of a member using a web, the web member is usually a light gauge aluminum alloy sheet forming the principal depth of the member. The web is bounded by heavy aluminum alloy extrusions known as cap strips. These extrusions carry the loads caused by bending and also provide a foundation for attaching the skin. The web may be stiffened by stamped beads, formed angles, or extruded sections riveted at regular intervals along the web.
The stamped beads are a part of the web itself and are stamped in when the web is made. Stiffeners help to withstand the compressive loads exerted upon the critically stressed web members. Often, ribs are formed by stamping the entire piece from sheet stock. That is, the rib lacks a cap strip, but does have a flange around the entire piece, plus lightening holes in the web of the rib. Ribs may be formed with stamped beads for stiffeners, or they may have extruded angles riveted on the web for stiffeners. Most damages involve two or more members, but only one member may be damaged and need repairing. Generally, if the web is damaged, cleaning out the damaged area and installing a patch plate are all that is required.
The patch plate should be of sufficient size to ensure room for at least two rows of rivets around the perimeter of the damage that includes proper edge distance, pitch, and transverse pitch for the rivets. The patch plate should be of material having the same thickness and composition as the original member. If any forming is necessary when making the patch plate, such as fitting the contour of a lightening hole, use material in the “0” condition and then heat treat it after forming. Damage to ribs and webs that requires a repair larger than a simple plate probably needs a patch plate, splice plates, or angles and an insertion. [Figure 4-191] Leading Edge Repair The leading edge is the front section of a wing, stabilizer, or other airfoil. The purpose of the leading edge is to streamline the forward section of the wings or control surfaces to ensure effective airflow. The space within the leading edge is sometimes used to store fuel. This space may also house extra equipment, such as landing lights, plumbing lines, or thermal anti-icing systems.
The construction of the leading edge section varies with the type of aircraft. Generally, it consists of cap strips, nose ribs, stringers, and skin. The cap strips are the main lengthwise extrusions, and they stiffen the leading edges and furnish a base for the nose ribs and skin. They also fasten the leading edge to the front spar. The nose ribs are stamped from aluminum alloy sheet or machined parts. These ribs are U-shaped and may have their web sections stiffened. Regardless of their design, their purpose is to give contour to the leading edge. Stiffeners are used to stiffen the leading edge and supply a base for fastening the nose skin. When fastening the nose skin, use only flush rivets.
Leading edges constructed with thermal anti-icing systems consist of two layers of skin separated by a thin air space. The inner skin, sometimes corrugated for strength, is perforated to conduct the hot air to the nose skin for anti-icing purposes. Damage can be caused by contact with other objects, namely, pebbles, birds, and hail. However, the major cause of damage is carelessness while the aircraft is on the ground. 4-105 A 2 1 3 Spar chord Web Fillet seal (typical) Seal heads (typical)Make a laying surface seal refer to SRM 51-20-05 Web 3 0.050 0.070 Gap (typical) 2 1 2D minumum (typical) Damage cutout 0.50R minimum all corners AFT B Note: Use this repair at the inboard end of the spar when the damage is near the upper or lower chord.
A damaged leading edge usually involves several structural parts. FOD probably involves the nose skin, nose ribs, stringers, and possibly the cap strip. Damage involving all of these members necessitates installing an access door to make the repair possible. First, the damaged area has to be removed and repair procedures established. The repair needs insertions and splice pieces. If the damage is serious enough, it may require repair of the cap strip and stringer, a new nose rib, and a skin panel. When repairing a leading edge, follow the procedures prescribed in the appropriate repair manual for this type of repair. [Figure 4-192] Repairs to leading edges are more difficult to accomplish than repairs to flat and straight structures because the repair parts need to be formed to fit the existing structure.
Trailing Edge Repair A trailing edge is the rear-most part of an airfoil found on the wings, ailerons, rudders, elevators, and stabilizers. It is usually a metal strip that forms the shape of the edge by tying the ends of a rib section together and joining the upper and lower skins. Trailing edges are not structural members, but they are considered to be highly stressed in all cases. Damage to a trailing edge may be limited to one point or extended over the entire length between two or more rib sections. Besides damage resulting from collision and 4-106 Upper flange Case A Case B Spar web Damage Lower flange Patch Patch Same material and thickness If web stiffener is within 1/2" of hole and is not damaged.
Drill out stiffener rivets. After repair is made, rivet stiffener at original location. Add new stiffener if stiffener is damaged. Rib Original damaged web area Reinforcement material—same as original and of same gauge or one gauge heavier. Clean holes smooth Reinforcement plate Pick up rivets along flange— add reinforcing rivets spaced 3/4" as shown, maintaining 21/2 times rivet diameter for proper edge 4-107