Learning Objective: Recognize the causes of damage to metallic structures and the procedures for their repair.

One of the most important jobs you will encounter is the repair of damaged skin and material. All repairs must be of the highest quality and must conform to certain requirements and specifications.

You must be familiar with the principle of streamlining, the behavior of various metals in high-velocity air currents, and the torsioned stress encountered during high-speed flying and maneuvering.

When any part of the airframe has been damaged, the first step is to clean all grease, dirt, and paint in the vicinity of the damage so the extent of the damage may be determined. The adjacent structure must be inspected to determine what secondary damage may have resulted from the transmission of the load or loads that caused the initial damage. You should thoroughly inspect the adjacent structures for dents, scratches, abrasions, punctures, cracks, loose seams, and distortions. Check all bolted fittings that may have been damaged or loosened by the load that caused the damage to the structure.

Damages to the airframe are many and may vary from those that are classified as negligible to those that are so extensive that an entire member of the airframe must be replaced. The slightest damage could affect the flight characteristics of the aircraft. The most common causes of damage to the airframe are collision, stress, heat, corrosion, foreign objects, fatigue, and combat damage.

COLLISION. This type of damage is often the result of carelessness by maintenance personnel. It varies from minor damage, such as dented or broken areas of skin, to extensive damage, such as torn or crushed structural members and misalignment of the aircraft. You should exercise extreme care in all ground-handling operations.

CORROSION. Damage to airframe components and the structure caused by corrosion will develop into permanent damage or failure if not properly treated. The corrosion control section of the maintenance instructions manual describes the maximum damage limits. These limits should be checked carefully, and if they are exceeded, the component or structure must be repaired or replaced.

FATIGUE. This type of damage is more notice-able as the operating time of the aircraft accumulates. The damage will begin as small cracks, caused by vibration and other loads imposed on skin fittings and load-bearing members, where the fittings are attached.

FOREIGN OBJECT. This damage is caused by hand tools, bolts, rivets, and nuts left adrift during ground operations of the aircraft. Because of jet aircraft design, large volumes of air are required for its efficient operation. During ground operations, the inlet ducts induce a strong suction that picks up objects that are left adrift. Therefore, it is of utmost importance that the area around the aircraft be clean and free of foreign material before ground operations begin.

COMBAT. Damage from enemy gunfire is usually quite extensive and often not repairable. When a projectile strikes sheet metal, it heats the metal in the vicinity of the damage. The metal becomes brittle around the damaged area as a result of the heat, and minute cracks are created by the impact of the projectile. These cracks open up under vibration. If the projectile passes through the component or structure, it will leave a larger hole on the opposite side from where it entered. The repair procedures for combat damage should be followed with extreme care only after a rigid inspection of the damage has been completed in accordance with the General Manual for Aircraft Battle Damage Repair, NAVAIR 01-1A-39.

HEAT. Certain areas of high-performance air-craft are exposed to high temperatures. These areas usually include the engine bleed lines, fuselage sections around the engine, the aft fuselage and horizontal stabilizer, and the wing sections around the boundary layer control system. Some aircraft structural repair manuals include diagrams that illustrate the heat danger areas.

STRESS. This type of damage is usually identi-fied by loosened, sheared, or popped rivets; wrinkled skin or webs; and cracked or deformed structural members. This damage is usually caused by violent maneuvers or hard landings. When the pilot reports these discrepancies on the yellow sheet, a thorough inspection of the entire aircraft must be performed.