6.1 Solid Aircraft Rivets & Installation Practice

6.1 Solid Aircraft Rivets & Installation Practice

Solid shank rivets remain the primary permanent mechanical fastening method for metallic aircraft structures. Airframes such as modern commercial transports and general aviation aircraft incorporate tens of thousands of solid rivets to assemble stressed skins, spar webs, bulkheads, stringers, and rib flanges. Riveted joints provide superior fatigue endurance, high strength-to-weight ratios, and fail-safe redundancy under severe aerodynamic and cyclic flight loads.

Under EASA Part-66 Module 06 (Materials and Hardware), certifying technicians must possess comprehensive knowledge of rivet alloy chemistry, identification markings, thermal handling regimens, geometric layout calculations, driving dynamics, and non-destructive inspection and removal protocols.

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Solid Rivet Alloys and Head Identification Markings

Solid aircraft rivets are manufactured from specific aluminium alloys, nickel-copper alloys, or titanium, each engineered to fulfill distinct structural and environmental demands. Because an installed rivet cannot be distinguished by its shank alloy, standard aerospace specifications mandate distinct raised or recessed markings forged into the center of the manufactured head during heading.

The "Field Rivet": 2117-T4 (AD)

The 2117-T4 (AD) rivet is commonly designated the "field rivet" because maintenance technicians can drive it directly from stock bins without prior heat treatment, refrigeration, or special conditioning. The addition of copper (~2.6%) provides moderate shear strength (205 MPa), while retaining sufficient ductility to allow severe upset without cracking. It exhibits excellent atmospheric corrosion resistance and forms the backbone of airframe repair.

"Icebox Rivets": 2017-T4 (D) and 2024-T4 (DD)

High-strength aluminium-copper alloys like 2017 and 2024 possess superior ultimate shear strength but work-harden rapidly. If driven in their fully aged stable condition, the excessive cold working causes severe radial cracking around the perimeter of the formed shop head and risks cracking the sheet material.

To permit successful driving:

1. Solution Heat Treatment: Rivets are heated in a salt bath or air furnace to their solution temperature (500°C for 2017; 495°C for 2024) until all alloying constituents dissolve into solid solution.
2. Quench: The rivets are rapidly quenched in cold water, producing a supersaturated, highly ductile solid solution.
3. Natural Age-Hardening Kinetics: At ambient room temperature (20°C / 68°F), atomic precipitation begins immediately:
   • 2017-T4 (D) rivets remain sufficiently ductile to drive for approximately 1 hour after quenching.
   • 2024-T4 (DD) rivets harden much faster; they must be driven within 10 to 20 minutes after removal from the quench bath or storage freezer.
4. Refrigeration Retardation: Storing the quenched rivets at sub-zero temperatures drastically slows atomic diffusion and halts age-hardening:
   • Storage at -18°C (0°F) or lower preserves drivability for several days to one week.
   • Storage at -20°C to -40°C (-40°F) preserves drivability for several weeks or months.
   • Once removed from the "icebox", any 2024-DD rivet not driven within 10 to 20 minutes must be returned for re-solution heat treatment. Specifications generally limit the number of allowable re-heat treatment cycles (typically maximum 3 to 5 cycles) to prevent intergranular copper depletion and excessive cladding diffusion.

Exam Warning / Common Trap: Never attempt to drive a 2024-DD rivet that has warmed up to room temperature past its 20-minute limit. The rivet shank will work-harden prematurely, failing to swell uniformly in the hole and producing ring cracks around the bucktail. The technician will damage the surrounding skin through excessive bucking blows.

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Rivet Part Numbering and Head Styles

Aerospace rivets conform to standardized military and national aerospace standards, primarily MS (Military Standard), AN (Air Force-Navy), and NAS (National Aerospace Standard).

Principal Head Geometries

• Universal Head (MS20470 / AN470): Replaced historic round head (AN430), flat head (AN442), and brazier head (AN455) designs. It features a wide, low-profile dome that distributes clamping stress across the outer sheet while minimizing aerodynamic parasitic drag on non-flush external or internal structures.
• 100° Countersunk Flush Head (MS20426 / AN426): Designed for external aerodynamic surfaces where laminar airflow is paramount. The head angle is standardized at exactly 100° in American aerospace practice (unlike commercial hardware which often uses 82° or 90°). British and European aircraft historically utilized 120° or 90° countersinks, but 100° remains the worldwide civil jet standard.

   Universal Head (MS20470)              100° Countersunk Head (MS20426)
         ____---____                                 ___________  
      .-'           '-.                             \         /  
     (_________________)  <-- Head Surface           \  100° /   <-- Included Angle
           |     |                                    \_____/    
           |     |                                     |   |     
           |     |    <-- Shank                        |   |     <-- Shank
           |_____|                                     |___|     

Sizing Code Anatomy

Consider the standard rivet callout: MS20470AD4-6

$\mathbf{MS20470} \quad \mathbf{AD} \quad \mathbf{4} \quad - \quad \mathbf{6}$

1. MS20470 (Specification & Head Style): Military Standard universal protruding head rivet (MS20426 indicates 100° countersunk flush head).
2. AD (Material / Alloy Code): 2117-T4 aluminium alloy (identifiable by its single center dimple).
3. 4 (Shank Diameter in 32nds of an inch): $\text{Diameter} = \frac{4}{32}\text{ inch} = \frac{1}{8}\text{ inch} \approx 3.175\text{ mm}$
4. 6 (Length in 16ths of an inch): $\text{Length} = \frac{6}{16}\text{ inch} = \frac{3}{8}\text{ inch} \approx 9.525\text{ mm}$

Measurement Critical Difference:

• For Universal / Protruding Head Rivets (MS20470), length is measured strictly from the flat underside of the head to the flat end of the shank.
• For Countersunk Head Rivets (MS20426), length is measured overall, including the entire depth of the countersunk head from top surface to shank end.

Notice that standard rivet hole clearance drill sizes are slightly larger (typically 0.002" to 0.004" clearance) than the nominal rivet shank to permit easy insertion without binding, allowing the shank to swell radially during driving to achieve 100% hole filling.

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Structural Rivet Layout Geometry

To ensure structural integrity and prevent fatigue failures, rivet patterns must strictly adhere to aerodynamic and structural spacing criteria governed by FAA AC 43.13-1B and EASA Part-M/Part-145 repair specifications.

Sizing the Rivet Diameter ($D$)

As a general engineering rule, the rivet diameter $D$ must be selected relative to the thickness of the sheets being joined:

$D \approx 3 \times t_{\text{thickest}}$

• If the rivet diameter is too small ($D < t$), the shear strength is inadequate, requiring an excessive number of fasteners that weaken the skin like a postage stamp perforation.
• If the rivet diameter is too large ($D > 3t$), the high driving force required to upset the thick shank will dish, stretch, and severely deform the thin sheet metal surrounding the hole.

1. Edge Distance ($ED$)

Edge distance is the linear distance measured from the geometric center of the rivet hole to the nearest free edge of the metal sheet.

• Minimum Edge Distance ($ED_{\min}$):
  • For Universal Head (MS20470): $2.0 \times D$ (twice the rivet shank diameter).
  • For Countersunk Head (MS20426): $2.5 \times D$ (to compensate for the material removed by countersinking, which creates a knife-edge thin perimeter if positioned too close to the edge).
• Nominal Edge Distance: $2.5 \times D$.
• Maximum Edge Distance ($ED_{\max}$): $4.0 \times D$.

Consequence of Violating Limits:

• $ED < 2D$: Extreme risk of edge tear-out or sheet cleavage under operational shear loading.
• $ED > 4D$: The unsupported sheet edge beyond the rivet line tends to curl up, vibrate, permit environmental moisture ingress, and generate inter-sheet crevice corrosion and aerodynamic drag.

2. Rivet Pitch ($P$)

Rivet pitch is the center-to-center distance between adjacent rivets in the same row.

• Minimum Pitch ($P_{\min}$): $3.0 \times D$ (absolute minimum). If rivets are spaced closer than $3D$, the stress concentration fields around adjacent holes overlap, creating high localized stress lines that tear under cyclic load.
• Nominal Structural Pitch: $4.0 \times D$ to $6.0 \times D$.
• Maximum Pitch ($P_{\max}$): $8.0 \times D$ (or up to $10D-12D$ in non-structural fairings). Spacing exceeding $8D$ in structural skins permits the sheets to buckle or "oil-can" between rivets under compressive flight loads.

3. Transverse Pitch / Gauge ($G$)

Transverse pitch (or row spacing) is the perpendicular distance between the centerlines of adjacent parallel rivet rows.

• Nominal Transverse Pitch: Approximately $0.75 \times \text{Pitch}$ (typically $3.0 \times D$ nominal).
• In staggered rivet patterns, the diagonal distance between rivets in adjacent rows must never be less than the minimum pitch ($3D$).

|<-- ED -->| <------- Pitch (P) -------> |
+----------+-----------------------------+----------+
|          |                             |          |
|   (O)    |              (O)            |   (O)    |  <-- Row 1
|    |     |               |             |          |
|----+-----+---------------+-------------+----------|
|    |                     |                        |
|    |<-- Transverse ----->|                        |
|    |    Pitch (G)        |                        |
|    |                     |                        |
|    |            (O)      |             (O)        |  <-- Row 2 (Staggered)
|    |             |       |              |         |
+----+-------------+-------+--------------+---------+
     ^ Edge

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Shop Head (Bucktail) Dimensions & Shank Length Calculation

During driving, compressive impact or steady squeeze forces cause the unsupported shank extending beyond the work to swell plastically, forming the shop head (also termed the bucktail or upset head).

Required Shank Extension Allowance

To form a geometrically compliant shop head, the length of the shank protruding beyond the assembled material stackup before driving must equal exactly $1.5$ times the shank diameter ($1.5D$):

$\text{Shank Extension Allowance} = 1.5 \times D$

$\text{Total Required Rivet Length } L = \text{Grip Length } (G) + 1.5 \times D$

Where Grip Length ($G$) is the combined total thickness of all sheets of metal being joined ($t_1 + t_2 + \dots + t_n$).

Formed Shop Head Inspection Dimensions

Once properly bucked, the resulting shop head must conform to standard geometric inspection limits:

• Shop Head Width (Diameter): Nominal $1.5 \times D$ (acceptable inspection range: $1.3D$ to $1.5D$).
• Shop Head Height (Thickness): Nominal $0.5 \times D$ (acceptable inspection range: $0.4D$ to $0.5D$).

$\text{Width} = 1.5 \times D \qquad \text{Height} = 0.5 \times D$

Memory Aid Rule of Thumb: "One and a half diameter out before bucking gives one and a half diameter wide by half a diameter high after bucking."

BEFORE DRIVING:                           AFTER DRIVING (FORMED SHOP HEAD):

    Manufactured Head                         Manufactured Head
       .-------.                                 .-------.     
       |_______|                                 |_______|     
       | |   | |                                 | |   | |     
  =====|=|===|=|===== Top Sheet             =====|=|===|=|===== Top Sheet
  =====|=|===|=|===== Bottom Sheet          =====|=|===|=|===== Bottom Sheet
       | |   | |                                 | |   | |     
       | |   | |                                 .-------.  <-- Height = 0.5D
       '-------' <-- Protrusion = 1.5D          (_________) <-- Width = 1.5D

Calculation Scenario

An aircraft technician is joining two sheets of 2024-T3 alclad aluminium alloy. Sheet 1 has a thickness of $0.040\text{ inch}$ ($1.016\text{ mm}$) and Sheet 2 has a thickness of $0.060\text{ inch}$ ($1.524\text{ mm}$). The blueprint specifies an MS20470AD rivet with a diameter of $1/8\text{ inch}$ ($-4$ diameter):

1. Calculate Total Grip Length ($G$): $G = 0.040" + 0.060" = 0.100"$
2. Calculate Shank Extension Allowance ($1.5D$): $1.5 \times D = 1.5 \times 0.125" = 0.1875" = \frac{3}{16}"$
3. Calculate Total Theoretical Rivet Length ($L$): $L = G + 1.5D = 0.100" + 0.1875" = 0.2875"$
4. Convert to Sixteenths of an Inch for Rivet Callout: $\frac{0.2875"}{1/16"} = 0.2875 \times 16 = 4.6 \text{ sixteenths}$ Rivet lengths are manufactured in discrete $1/16\text{ inch}$ increments. A technician should choose a dash -5 ($5/16" = 0.3125"$) length rivet. Using a dash -4 ($4/16" = 0.250"$) would leave only $0.150"$ extension (less than the required $0.1875"$), producing an undersized, weak bucktail. The final part callout is: MS20470AD4-5.

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Rivet Driving Tools & Techniques

Solid rivets are upset using either impact riveting (pneumatic rivet gun combined with a bucking bar) or compression riveting (rivet squeezer).

Pneumatic Rivet Guns

• Slow-Hitter Gun: Delivers 900 to 2500 heavy, deliberate blows per minute. Preferred for structural aircraft riveting because each solid blow displaces substantial metal, upsetting the shank completely before work-hardening can take place.
• Fast-Hitter Gun (Buzz Gun): Delivers 2500 to 5000 light, rapid blows per minute. Used primarily on small, soft rivets (such as 1100-A or small 2117-AD in non-structural skins). Rapid blows on hard alloys work-harden the metal prematurely.
• One-Shot Gun: Drives the rivet in a single heavy blow. Used mainly in high-volume production jigs.
• Corner Riveters: Feature short offset barrels designed to reach confined structural corners.

Rivet Sets and Bucking Bars

• Rivet Sets: Manufactured from hardened alloy steel. The cupped recess in the rivet set face must precisely match the radius and curvature of the manufactured rivet head. An undersized cup marks the crown of the head; an oversized cup marks the sheet around the head perimeter. For flush rivets, a wide, flat flush set fitted with a spring-loaded rubber non-slip ring is employed to prevent the set from walking across the skin.
• Bucking Bars: Dense forged steel or tungsten alloy blocks held against the shank end to absorb the impact energy and force the metal to upset laterally. Tungsten bucking bars are more than twice as dense as steel, delivering equivalent upsetting inertia in half the physical volume, making them indispensable in tight wing cells and empennage bays. The bucking bar face must be held dead flat and perpendicular to the shank.

Squeeze Riveters

Pneumatic or hydraulic rivet squeezers apply a smooth, continuous, controlled compressive force that expands the shank uniformly to fill the hole and creates a flawless shop head in a single stroke. Squeezers eliminate human error, hand vibration, sheet dimpling, and noise. They are limited by the depth of the squeezer yoke (throat depth) and are primarily used on subassemblies, ribs, and spar flanges.

Flush Riveting: Machine Countersinking vs. Dimpling

To install flush countersunk rivets (MS20426):

• Machine Countersinking: A rotating cutter removes a conical section of metal from the top sheet. Rule: Machine countersinking is permitted ONLY if the thickness of the top sheet is greater than the depth of the rivet head. If used on thin sheets, the cutter cuts through the entire sheet thickness, creating an enlarged hole with a razor-thin knife-edge that tears easily under cyclic fatigue.
• Dimpling: Used when the skin is thinner than the rivet head depth. The sheet is pressed between matched male and female dies to form a conical depression without removing any metal.
  • Coin Dimpling: Uses an integral coining ram to compress the metal during forming, yielding sharp, perfectly nested dimples.
  • Hot Dimpling: Utilizes electrically heated dies; mandatory for high-strength 7075-T6 and 2024-T3 alloys to prevent radial cracking around the dimple bend radius.

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Defective Rivets and Precise Removal Procedures

Quality assurance requires inspecting every driven rivet before an aircraft is returned to service.

Common Rivet Defects

1. Under-Driven Shop Head: Height $> 0.5D$, Width $< 1.3D$. The shank failed to swell sufficiently to fill the hole; joint possesses substandard shear and tensile strength.
2. Over-Driven Shop Head (Pancake Head): Height $< 0.4D$, Width $> 1.7D$. Excessive hammering work-hardens the bucktail, creating perimeter cracks and over-expanding the hole, causing sheet distortion.
3. Tipped / Cocked Shop Head: Caused by holding the bucking bar at an angle to the shank axis.
4. Clinched / Bent Shank: The rivet shank bent over sideways instead of swelling. Caused by excessive shank extension ($> 1.5D$), improper drill clearance, or the bucking bar bouncing off center.
5. Sheet Separation ("Clinching" / Gap): Gaps between joined sheets caused by inadequate pre-clamping (insufficient Cleco temporary fasteners) or metal drilling chips trapped between the faying surfaces.
6. Mutilated Skin ("Smiles"): Crescent-shaped gouges in the sheet metal caused by the rivet set slipping off the head or being tilted during driving.

FAA and EASA Standard Rivet Removal Procedure

Improper rivet removal can ruin an expensive spar cap or skin panel by enlarging or elongating the hole. Maintenance personnel must follow this standardized five-step procedure:

STEP 1: Center Punch       STEP 2: Drill Head Only    STEP 3: Snap Off Head      STEP 4: Back-Up & Drive Shank
     (Automatic)               (Twist Drill)              (Pin Punch)                (Drive Pin Punch)

        | |                        | |                                                    | |
        v v                        v v                        \ /                         v v
       .---.                      .---.                      .---.                      .---.
      (  •  )                    (  v  )                    (     )                    (     )
     ===========                ===========                ===========                ===========  Skin
     ===========                ===========                ===========                ===========  Skin
         | |                        | |                        | |                        | |
         |_|                        |_|                        |_|                        |_|  
                                                                                       [Backing Bar]

1. Center Punch: Accurately mark the exact center of the manufactured head using an automatic center punch or prick punch. If the head is countersunk, take extreme care to place the indentation in the true center.
2. Drill the Head: Select a twist drill bit of the same nominal diameter or one size smaller than the rivet shank (e.g., use a #30 or #31 drill for a 1/8" rivet). Drill carefully through the manufactured head only to the depth of the head (the junction between the head and the shank). Stop immediately before the drill bit touches the aircraft skin surface!
3. Shear the Head: Insert a matching pin punch (or drift punch) into the drilled pilot hole. Gently pry or twist the punch sideways to snap and shear the drilled head off the shank cleanly along the head-shank shear line.
4. Drive the Shank: Support the backside of the surrounding structure with a hollow backing block or bucking bar to prevent sheet dishing. Place a drive pin punch against the exposed shank center and lightly tap with a small mallet to knock the shank out of the hole.
5. Inspect the Hole: Clean and inspect the empty hole. The barrel must remain round and free of scores or gouges. If the hole was inadvertently elongated, it must be reamed to the next standard oversize rivet (e.g., 1/64" larger).