2.4 Sheet Metal Inspection, Damage Evaluation & Flush/Lap Repairs
Key Takeaways
- Aircraft structures are categorized into Primary (flight-critical/load-bearing), Secondary (aerodynamic load-carrying), and Tertiary (non-structural) components.
- Damage is classified into four standard levels per AC 43.13-1B and SRM: Negligible Damage, Repairable by Patching, Repairable by Insertion, and Damage Requiring Replacement.
- Fatigue cracks are arrested by stop-drilling the microscopic crack tip with a #30 or #40 drill bit after positive identification using dye penetrant or 10X optical magnification.
- Flush patches require a cutout with radiused corners (minimum 0.50 in), an internal doubler made of the same alloy and one gauge thicker than the skin, and a flush filler plug of identical gauge and alloy.
- Machine countersinking is restricted to sheets where thickness exceeds rivet head depth (typically T >= 0.040 in); thinner sheets (< 0.040 in) must be coin dimpled or radius dimpled to prevent knife-edge bearing failure.
Last updated: August 2026
Sheet Metal Inspection, Damage Evaluation & Flush/Lap Repairs
FAA Airframe Exam Focus: Airframe mechanics must evaluate structural damage against the aircraft Structural Repair Manual (SRM) and FAA AC 43.13-1B, execute precise crack arrest (stop-drilling), calculate doubler and fastener layouts for flush and lap repairs, and select between machine countersinking and coin dimpling.
1. Aircraft Structural Classifications & Stress Distribution
Aircraft structural components are categorized according to their criticality to flight safety and the primary mechanical loads they sustain:
PRIMARY AIRCRAFT STRUCTURAL STRESSES:
[ TENSION ] [ COMPRESSION ] [ SHEAR ] [ TORSION ] [ BENDING ]
◄───[ ]───► ───►[ ]◄─── ▲ ┌───┐ ┌───┐ ↻ ▼ ┌───────┐ ▼
│ │ │ │ │ ↺ │ │
Pulling Apart Crushing Force ▼ └───┘ └───┘ ──┴───────┴──
(Lower Wing Skin) (Upper Wing Skin) (Rivet Joints) (Wing Torque Box) (Wing Spars)
| Classification | Definition & Criticality | Typical Airframe Components | Approved Repair Authority |
|---|---|---|---|
| Primary Structure | Flight-critical load-bearing members whose failure would result in catastrophic structural collapse or loss of aircraft control. | Wing spars, spar caps, fuselage longerons, carry-through bulkheads, pressurized cabin skins, control surface spars. | Manufacturer SRM or FAA-approved engineering data (DER / Form 337). |
| Secondary Structure | Highly stressed members that carry aerodynamic or internal loads, but whose failure would not cause immediate loss of flight safety. | Wing ribs, stringers, leading edge skin panels, floor beams, control surface skins. | Manufacturer SRM, or AC 43.13-1B (if SRM unavailable). |
| Tertiary Structure | Lightly loaded or non-structural elements that define aerodynamic fairings or internal cabin layout. | Wheel fairings, non-pressurized access doors, cabin partitions, wing tips. | Standard sheet metal repair practices per AC 43.13-1B. |
2. Damage Assessment & Classification Standards
Per FAA AC 43.13-1B (Chapter 4) and manufacturer Structural Repair Manuals (SRM), structural damage is categorized into four distinct levels:
- Negligible Damage: Minor scratches, small surface abrasions, tiny shallow dents, or shallow corrosion within published limits that do not compromise structural strength or aerodynamic smoothness.
- Action: Blend out surface scratches with fine abrasive pads (maintaining a minimum 20:1 smooth blend taper ratio), clean, treat with chemical conversion coating (Alodine), and prime.
- Damage Repairable by Patching: Moderate skin punctures, small localized tears, or cracks in low-speed or secondary structure that can be reinforced by adding an external lap patch or internal doubler.
- Damage Repairable by Insertion (Flush Patching): Severe skin damage on high-speed aerodynamic surfaces or pressurized fuselages where the damaged section is cut out and replaced with an internal doubler and flush filler plug.
- Damage Requiring Replacement: Severe structural deformation, widespread distortion of primary load-carrying spars, buckled pressure bulkheads, or damage exceeding maximum SRM repair limits.
3. Crack Arrest Procedures (Stop-Drilling)
Fatigue cracks propagate due to extreme tensile stress concentrations at the sharp, microscopic root of the crack tip. Stop-drilling eliminates this stress notch, distributing loads around a smooth radiused hole.
STOP-DRILLING PROCEDURE:
1. Pinpoint Microscopic Crack Tip (Dye Penetrant / 10X Glass)
═══════════════════════════════════════► (Crack Tip)
2. Drill Stop Hole with #30 or #40 Bit
═══════════════════════════════════════( O )
▲
└── Crack Tip Fully Contained in Hole
Critical Stop-Drilling Protocol
- Inspect and Locate Tip: Clean all paint and grease. Use liquid fluorescent/dye penetrant or a 10X magnifying inspection glass to find the exact microscopic endpoint of the crack tip.
- Drill Size Selection: Drill through the exact crack tip using a #30 ($0.1285"$) or #40 ($0.0980"$) drill bit (or $1/8"$ bit per SRM specifications).
- Post-Drill Inspection: Re-inspect the inside bore of the drilled hole with a magnifier or penetrant to confirm that the crack does not extend beyond the boundary of the hole. If crack remnants remain, re-drill with a larger bit.
- Structural Doubler Requirement: Stop-drilling alone is only a temporary emergency measure or an initial step prior to installing an approved structural doubler patch.
4. Lap Patch vs. Flush Patch Structural Design
LAP PATCH (Surface) vs. FLUSH PATCH (Aerodynamic/Pressurized):
[ LAP PATCH ] [ FLUSH PATCH ]
┌───────────────────────────┐ ◄── Lap Patch ┌──────────────┐ ◄── Flush Filler Plug (Same Gauge)
│ (•) (•) (•) (•) │ (1 Gauge Thicker) ════╧══ ══╧════ ◄── Original Skin
════╧═════╧═════╧═════╧═════╡ │ (•) (•) (•) │
════════════════════════════╛ ◄── Original Skin └────────────────────┘ ◄── Internal Doubler
(1 Gauge Thicker)
Lap Patch (Surface Patch) Design Rules
- Application: Unpressurized fuselages, secondary structure, and lower-speed surfaces where parasite aerodynamic drag is negligible.
- Patch Thickness: Fabricated from the same alloy and temper as the original skin, and typically one standard gauge thicker (or equal thickness) to restore $100%$ original tensile strength.
- Edge Chamfering: Outer perimeter chamfered at $45^\circ$ to minimize air drag and peeling forces.
Flush Patch (Aerodynamic Insertion) Design Rules
- Application: High-speed aerodynamic surfaces, pressurized fuselage cabins, and control surface leading edges.
- Three-Component Assembly:
- Cutout: Damaged area is cut out cleanly into a circular, oval, or rectangular shape with rounded corners (minimum corner radius $R = 0.50"$) to prevent stress risers.
- Internal Doubler: Installed behind the cutout. Must be fabricated from the same alloy/temper and one standard gauge thicker than the original skin (e.g., $0.040"$ skin requires $0.050"$ doubler).
- Flush Filler Plug: Cut to match the exact shape of the cutout with a uniform $1/32"$ to $1/16"$ clearance gap around the perimeter. Must be the exact same gauge and alloy as the original skin.
- Fastener Layout: Fasteners are arranged in multiple staggered rows (minimum 2 rows per side) to transfer full shear load around the cutout.
5. Dimpling vs. Machine Countersinking
Flush riveting (using $100^\circ$ countersunk MS20426 rivets) requires creating a conical recess in the skin:
MACHINE COUNTERSINKING vs. COIN DIMPLING:
[ MACHINE COUNTERSINKING (Thick Sheets >= 0.040") ]
▼ 100° Cutter
╲ ╱
═══╲ ╱═══ ◄── Material Cut Away (Knife-edge avoided only if T >= 1.5 * Head Depth)
═════ ═══
[ COIN DIMPLING (Thin Sheets < 0.040") ]
▼ Male Die
╲ ╱
═══╲ ╱═══ ◄── Metal Formed & Coined (No Material Removed; 100% Strength Retained)
█ ◄── Female Coining Die
| Parameter | Machine Countersinking | Coin / Radius Dimpling |
|---|---|---|
| Mechanism | Rotating cutting tool removes metal to form $100^\circ$ recess. | Male and female die set plastically deforms metal into $100^\circ$ cone without removing material. |
| Skin Thickness Rule | Permitted only if sheet thickness $T \ge 1.5 \times \text{Rivet Head Depth}$ (typically $T \ge 0.040"$ for $3/32"$ rivets; $T \ge 0.050"$ for $1/8"$ rivets). | Mandatory for thin sheets ($T < 0.040"$) where machine countersinking would create a knife-edge. |
| Structural Risk | On thin sheet, creates a razor-sharp knife-edge in the hole, causing sheet cracking, hole elongation, and rivet pull-through. | If dies misaligned, can cause radial cracking or sheet warping. |
| Hot Dimpling | Not applicable. | High-strength/brittle alloys (7075-T6, titanium, magnesium) are heated with resistance dies ($300^\circ\text{F} ext{--}600^\circ\text{F}$) during dimpling to prevent cracking. |
6. Corrosion Removal & Chemical Conversion Coating
- Mechanical Cleaning: Strip paint and remove all corrosion products mechanically using non-metallic Scotch-Brite abrasive pads or aluminum oxide abrasive paper. Never use carbon steel brushes or steel wool, which embed microscopic iron particles and cause rapid galvanic corrosion.
- Chemical Conversion (Alodining): Apply MIL-DTL-5541 (Alodine 1200 / Bonderite) chromate conversion coating to bare aluminum. Alodine chemically reacts with aluminum to form a microscopic, corrosion-resistant passive oxide film that provides an exceptional bonding base for primers.
- Priming: Apply two-part polyamide epoxy zinc-chromate primer (MIL-PRF-23377) within the specified cure window before final rivet assembly and painting.
Test Your Knowledge
When designing a structural flush patch for a damaged 0.040-inch 2024-T3 aluminum wing skin, what are the correct material thickness specifications for the internal doubler and the flush filler plug?
A
B
C
D
Test Your Knowledge
What is the primary engineering reason that machine countersinking is prohibited on thin aluminum skin sheets (typically less than 0.040 inches thick)?
A
B
C
D
Test Your Knowledge
What is the immediate primary purpose of stop-drilling the microscopic tip of a fatigue crack discovered in an aircraft fuselage skin?
A
B
C
D
Test Your Knowledge
Why is the use of carbon steel wire brushes or steel wool strictly prohibited when removing corrosion from aluminum alloy aircraft skins?
A
B
C
D