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FAA Aviation Mechanic — Airframe (AMA)

Practice Questions Study Guide Flashcards

2.4 Sheet Metal Inspection, Damage Evaluation & Flush/Lap Repairs

Q: 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?

Doubler = 0.050 inch (one gauge thicker); Filler Plug = 0.040 inch (same gauge). In standard flush patch design per AC 43.13-1B and manufacturer SRMs, the internal backing doubler must be fabricated from the same alloy and one standard gauge thicker (0.050 inch) to restore structural shear and tensile capability, while the flush filler plug must match the exact original skin thickness (0.040 inch) to maintain aerodynamic contour.

Q: What is the primary engineering reason that machine countersinking is prohibited on thin aluminum skin sheets (typically less than 0.040 inches thick)?

The cutter removes excessive hole wall thickness, producing a sharp knife-edge that causes sheet tearing and rivet pull-through. Machine countersinking thin sheets removes so much parent metal that the straight cylindrical portion of the hole wall is eliminated, creating a razor-sharp "knife-edge." Under structural loads, this knife-edge causes bearing failure, hole elongation, and rivet pull-through. Thin sheets must be dimpled instead.

Q: What is the immediate primary purpose of stop-drilling the microscopic tip of a fatigue crack discovered in an aircraft fuselage skin?

To eliminate the sharp stress concentration notch at the crack tip and redistribute tensile stresses around a smooth radius. Stop-drilling (typically using a #30 or #40 drill bit) removes the sharp, high-stress notch at the microscopic root of the crack tip. Replacing the sharp notch with a round hole redistributes tensile stresses over a circular perimeter, arresting further crack propagation.

Q: Why is the use of carbon steel wire brushes or steel wool strictly prohibited when removing corrosion from aluminum alloy aircraft skins?

Steel particles embed in the aluminum surface, initiating rapid galvanic dissimilar metal corrosion. Carbon steel brushes or steel wool deposit microscopic steel/iron particles into the softer aluminum matrix. In the presence of atmospheric moisture, the dissimilar metal contact between iron and aluminum sets up severe galvanic corrosion cells, rapidly pitting and degrading the structure.

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:
1. 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.
2. 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).
3. 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.

\text{Rivet Count per Side } (N) = \frac{\text{Length of Damage } \times \text{Skin Thickness } \times \text{Skin Tensile Strength}}{\text{Single Shear Strength of Selected Rivet}}

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 ( $300^\circ\text{F}\text{--}600^\circ\text{F}$ ) 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?

Doubler = 0.032 inch (one gauge thinner); Filler Plug = 0.040 inch

Doubler = 0.040 inch (same gauge); Filler Plug = 0.050 inch (one gauge thicker)

Doubler = 0.050 inch (one gauge thicker); Filler Plug = 0.050 inch (one gauge thicker)

Doubler = 0.050 inch (one gauge thicker); Filler Plug = 0.040 inch (same gauge)

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)?

Machine countersink cutters will overheat and anneal the surrounding aluminum alloy

Countersinking creates severe work-hardening that prevents subsequent rivet upsetting

The cutter removes excessive hole wall thickness, producing a sharp knife-edge that causes sheet tearing and rivet pull-through

Countersunk holes cannot be treated with chemical conversion coating (Alodine)

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?

To provide an opening for injecting chemical conversion coating into the crack

To eliminate the sharp stress concentration notch at the crack tip and redistribute tensile stresses around a smooth radius

To permit insertion of temporary Cleco fasteners during flight operations

To create a reference alignment hole for installing an external lap patch

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?

Steel particles embed in the aluminum surface, initiating rapid galvanic dissimilar metal corrosion

Steel tools generate excessive friction heat that transforms the -T6 temper into an annealed -O temper

Steel abrasives react chemically with Alodine, generating flammable hydrogen gas

Steel bristles create static electrical charges that destroy solid-state avionics

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