5.1 Metallic Structure Inspection and Damage Patterns

Metallic Structures: Inspection Before Repair

Metal aircraft structure is built to carry specific loads through skins, frames, stringers, bulkheads, spars, ribs, fittings, and fasteners. For the FAA Airframe knowledge test, the important idea is not only naming those parts. The working mechanic must decide whether a defect is cosmetic, repairable under approved data, or a reason to stop and get engineering direction.

A dent in an unloaded fairing is a different maintenance problem from a wrinkle near a spar attach fitting. A smoking rivet can point to movement between sheets. Fretting around a fastener can indicate looseness. Corrosion under a lap joint may hide until the joint is opened. Good troubleshooting begins with the structure's job, the direction of load, and whether the damage interrupts a load path.

Common inspection methods include visual inspection, magnification, straightedges, dye penetrant, eddy current, ultrasonic inspection, and radiography when appropriate and authorized. The test may ask about inspection principles, but the maintenance judgment is broader. A technician verifies access, lighting, cleaning, and reference data before deciding that an area is acceptable.

Use this study pattern when you see a metallic-structure scenario:

• Identify the structure and likely load path.
• Clean enough to inspect without removing protective finish unnecessarily.
• Classify the defect: crack, dent, scratch, corrosion, loose fastener, elongated hole, or heat damage.
• Check the aircraft maintenance manual, structural repair manual, service information, and airworthiness directives.
• Confirm material, thickness, temper, fastener type, edge distance, spacing, and finish requirements.
• Decide whether the repair is within published limits or needs approved engineering data.

A scratch is often judged by depth, direction, location, and whether it crosses a highly stressed area. A crack is normally more serious because it can grow under cyclic load. Stop-drilling may be part of an approved repair, but it is not a universal permission to keep flying. Elongated holes can reduce bearing area and clamp-up. Heat damage in aluminum may change temper and strength even if the surface looks smooth.

Sheet-metal work also creates hazards. Burrs cut hands and start cracks. Drilling chips damage finishes. Poorly supported sheet can be bent while a rivet is driven. Compressed air can drive chips into eyes. Welding gases and cylinders add fire, pressure, and handling risk. The ACS risk elements expect you to think about personal protective equipment, tool control, fire prevention, and the danger of applying a repair material that is not compatible with the aircraft.

Exam questions often hide the key word in the location of the damage. Damage near a row of rivets, a bend radius, a bulkhead, or a control-surface hinge deserves more caution than the same mark in an unstressed cover. When in doubt, choose the answer that inspects first, uses current maintenance data, preserves material strength, and avoids a return-to-service decision based only on appearance.

The best mental model is simple: clean, inspect, measure, compare to approved data, repair only within authority, and document the result. That sequence keeps a technician from turning a small defect into a structural risk.