Repairability of Sheet Metal Structures (Part Two)

in Aircraft Metal Structural Repair

Damage Removal

To prepare a damaged area for repair:

  1. Remove all distorted skin and structure in damaged area.
  2. Remove damaged material so that the edges of the completed repair match existing structure and aircraft lines.
  3. Round all square corners.
  4. Smooth out any abrasions and/or dents.
  5. Remove and incorporate into the new repair any previous repairs joining the area of the new repair.

Repair Material Selection

The repair material must duplicate the strength of the original structure. If an alloy weaker than the original material has to be used, a heavier gauge must be used to give equivalent cross-sectional strength. A lighter gauge material should not be used even when using a stronger alloy.

Repair Parts Layout

All new sections fabricated for repairing or replacing damaged parts in a given aircraft should be carefully laid out to the dimensions listed in the applicable aircraft manual before fitting the parts into the structure.

Rivet Selection

Normally, the rivet size and material should be the same as the original rivets in the part being repaired. If a rivet hole has been enlarged or deformed, the next larger size rivet must be used after reworking the hole. When this is done, the proper edge distance for the larger rivet must be maintained. Where access to the inside of the structure is impossible and blind rivets must be used in making the repair, always consult the applicable aircraft maintenance manual for the recommended type, size, spacing, and number of rivets needed to replace either the original installed rivets or those that are required for the type of repair being performed.

Rivet Spacing and Edge Distance

The rivet pattern for a repair must conform to instructions in the applicable aircraft manual. The existing rivet pattern is used whenever possible.

Corrosion Treatment

Prior to assembly of repair or replacement parts, make certain that all existing corrosion has been removed in the area and that the parts are properly insulated one from the other.

Approval of Repair

Once the need for an aircraft repair has been established, Title 14 of the Code of Federal Regulations (14 CFR) defines the approval process. 14 CFR part 43, section 43.13(a) states that each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer’s maintenance manual or instructions for continued airworthiness prepared by its manufacturer, or other methods, techniques, or practices acceptable to the Administrator. AC 43.13-1 contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions. This data generally pertains to minor repairs. The repairs identified in this AC may only be used as a basis for FAA approval for major repairs. The repair data may also be used as approved data, and the AC chapter, page, and paragraph listed in block 8 of FAA Form 337 when:

  • The user has determined that it is appropriate to the product being repaired;
  • It is directly applicable to the repair being made; and
  • It is not contrary to manufacturer’s data.

Engineering support from the aircraft manufacturer is required for repair techniques and methods that are not described in the aircraft maintenance manual or SRM.

FAA Form 337, Major Repair and Alteration, must be completed for repairs to the following parts of an airframe and repairs of the following types involving the strengthening, reinforcing, splicing, and manufacturing of primary structural members or their replacement, when replacement is by fabrication, such as riveting or welding. [Figure 4-174]

Figure 4-174. FAA Form 337.

Figure 4-174. FAA Form 337. [click image to enlarge]

  • Box beams
  • Monocoque or semimonocoque wings or control surfaces
  • Wing stringers or chord members
  • Spars
  • Spar flanges
  • Members of truss-type beams
  • Thin sheet webs of beams
  • Keel and chine members of boat hulls or floats
  • Corrugated sheet compression members that act as flange material of wings or tail surfaces
  • Wing main ribs and compression members
  • Wing or tail surface brace struts, fuselage longerons
  • Members of the side truss, horizontal truss, or bulkheads
  • Main seat support braces and brackets
  • Landing gear brace struts
  • Repairs involving the substitution of material
  • Repair of damaged areas in metal or plywood stressed covering exceeding six inches in any direction
  • Repair of portions of skin sheets by making additional seams
  • Splicing of thin sheets
  • Repair of three or more adjacent wing or control surface ribs or the leading edge of wings and control surfaces between such adjacent ribs

For major repairs made in accordance with a manual or specifications acceptable to the Administrator, a certificated repair station may use the customer’s work order upon which the repair is recorded in place of the FAA Form 337.