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.
Stop holes—drill 3/32" diameter holes in each sharp corner orOriginal damage crack or break and clean up edges Skin Reinforcement material—ALCLAD 2024-T3 same gauge or one gauge heavier Rivets—material thickness of 0.032 inch or less. Use 1/8" rivets—material thickness greater than 0.032", use 5/32" rivets. Space rivets aproximately 1" apart in staggered rows 1/2" apart. Maintain minimum edge distance of 1" when skin thickness is 0.032" or less and 1/8" when skin thickness is more than 0.032". Minimum edge distance using 1/8" rivets is 1/4" and using 5/32" rivets is 5/16". 1/2 T Rivet holeEdge distance 5° Neutral axis45° T Damage Doubler Damaged area cut to a smooth rectangle with corner radil Filler Doubler riveted in place Filler riveted in place bend of the crack before applying the patch. These holes relieve the stress at these points and prevent the crack from spreading. The patch must be large enough to install the required number of rivets. It may be cut circular, square, or rectangular. If it is cut square or rectangular, the corners are rounded to a radius no smaller than 1⁄4-inch. The edges must be chamfered to an angle of 45° for 1⁄2 the thickness of the material, and bent down 5° over the edge distance to seal the edges. This reduces the chance that the repair is affected by the airflow over it. These dimensions are shown in Figure 4-175.
Flush Patch A flush patch is a filler patch that is flush to the skin when applied it is supported by and riveted to a reinforcement plate which is, in turn, riveted to the inside of the skin. is inserted through the opening and rotated until it slides in place under the skin. The filler must be of the same gauge and material as the original skin. The doubler should be of material one gauge heavier than the skin. 4-96 Open & Closed Skin Area Repair The factors that determine the methods to be used in skin repair are accessibility to the damaged area and the instructions found in the aircraft maintenance manual. The skin on most areas of an aircraft is inaccessible for making the repair from the inside and is known as closed skin. Skin that is accessible from both sides is called open skin.
Usually, repairs to open skin can be made in the conventional manner using standard rivets, but in repairing closed skin, some type of special fastener must be used. The exact type to be used depends on the type of repair being made and the recommendations of the aircraft manufacturer. Design of a Patch for a Non-pressurized Area Damage to the aircraft skin in a non-pressurized area can be repaired by a flush patch if a smooth skin surface is required or by an external patch in noncritical areas. [Figure 4-177] The first step is to remove the damage. Cut the damage to a round, oval, or rectangular shape. Round all corners of a rectangular patch to a minimum radius of 0.5-inch. The minimum edge distance used is 2 times the diameter and the rivet spacing is typically between 4-6 times the diameter. The size of the doubler depends on the edge distance and rivet spacing. The doubler material is of the same material as the damaged skin, but of one thickness greater than the damaged skin. The size of the doubler depends on the edge distance and rivet spacing. The insert is made of the same material and thickness as the damaged skin. The size and type of rivets should be the same as rivets used for similar joints on the aircraft. The SRM indicates what size and type of rivets to use.
Typical Repairs for Aircraft Structures This section describes typical repairs of the major structural parts of an airplane. When repairing a damaged component or part, consult the applicable section of the manufacturer’s SRM for the aircraft. Normally, a similar repair is illustrated, and the types of material, rivets, and rivet spacing and the methods and procedures to be used are listed. Any additional knowledge needed to make a repair is also detailed. If the necessary information is not found in the SRM, attempt to find a similar repair or assembly installed by the manufacturer of the aircraft.
Floats To maintain the float in an airworthy condition, periodic and frequent inspections should be made because of the rapidity of corrosion on metal parts, particularly when the aircraft is operated in salt water. Inspection of floats and hulls involves examination for damage due to corrosion, collision with other objects, hard landings, and other conditions that may lead to failure. Insertion Doubler Skin Insertion Patch Skin SkinSkin Doubler 1/4 inch deep dent Insertion patch method Cover patch method P E Note: Blind rivets should not be used on floats or amphibian hulls below the water line.
Sheet-metal floats should be repaired using approved practices; however, the seams between sections of sheet metal should be waterproofed with suitable fabric and sealing compound. A float that has undergone hull repairs should be tested by filling it with water and allowing it to stand for at least 24 hours to see if any leaks develop. [Figure 4-178] Corrugated Skin Repair Some of the flight controls of smaller general aviation aircraft have beads in their skin panels. The beads give some stiffness to the thin skin panels. The beads for the repair patch can be formed with a rotary former or press brake. [Figure 4-179] Replacement of a Panel Damage to metal aircraft skin that exceeds repairable limits 4-97 Extrusion angle stiffener Repair to step station Replace skin Splice in new portion Replace skin A Replace skeg Detail A Splice Station 5 Splice Shims Repair to stepRepairs to keelson 4-98 Skin Use MS20470AD4 or MS20600 self-plugging rivets or equivalent 0.25" edge distance Patch 0.016" AlcladTM 2024-T4 0.75˝ rivetspacing 0.25" radius Cut out damaged area requires replacement of the entire panel. [Figure 4-180] A panel must also be replaced when there are too many previous repairs in a given section or area.
In aircraft construction, a panel is any single sheet of metal covering. A panel section is the part of a panel between adjacent stringers and bulk heads. Where a section of skin is damaged to such an extent that it is impossible to install a standard skin repair, a special type of repair is necessary. The particular type of repair required depends on whether the damage is repairable outside the member, inside the member, or to the edges of the panel. Outside the Member For damage that, after being trimmed, has 81⁄2 rivet diameters or more of material, extend the patch to include the manufacturer’s row of rivets and add an extra row inside the members.
Inside the Member For damage that, after being trimmed, has less than 8 1⁄2 manufacturer’s rivet diameters of material inside the members, use a patch that extends over the members and an extra row of rivets along the outside of the members. Edges of the Panel For damage that extends to the edge of a panel, use 4-99