7.4 Rivet Joint Inspection, Failure Modes & Drill-Out Procedures

7.4 Rivet Joint Inspection, Failure Modes & Drill-Out Procedures

Approved-Data Control

Values and examples explain principles. Current approved maintenance data, product instructions, organisation procedures, and applicable law control actual limits, materials, intervals, methods, and acceptance.

Quality control in aircraft riveting extends well beyond the driving process. Every newly fabricated joint must undergo rigorous dimensional and non-destructive inspection prior to certification, and operational aircraft must be systematically inspected for in-service degradation such as fatigue cracking, looseness, and fretting corrosion. When a rivet is found defective, it must be removed without inflicting collateral damage on the parent airframe sheet metal. For certifying engineers under EASA Part-66 Module 7, mastering inspection standards, joint failure mechanics, in-service diagnostic indicators (such as 'smoking rivets'), and precision drill-out procedures is vital for airworthiness compliance.

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Visual & Tactile Inspection Standards

Riveted joint inspection incorporates visual assessment under 10x magnification, precision dimensional gaging, and tactile checking for movement.

1. Go / No-Go Rivet Gauges

Technicians verify formed shop head dimensions using precision sheet-metal Go/No-Go gauges:

• Height Slot: Checks that shop head height is not less than $0.5D$. If the head slips into an undersized slot, it is under-height (over-driven).
• Width Opening: Checks that shop head width is not less than $1.5D$. If the head fails to bridge the minimum diameter gauge, it is under-width (under-driven).

2. Identifying Common Driving Defects

• Tipped (Canted) Shop Head: Formed at an angle because the bucking bar face was not held square to the rivet shank axis. Acceptable only if the minimum height and width criteria are met across the entire perimeter without cutting into the sheet.
• Clenched (Bent) Head: The shank bent over sideways instead of upsetting axially. Caused by excessive rivet length ($>1.5D$ protruding), bucking bar bounce, or mismatched bar mass. Mandates rejection.
• Under-Driven Head: Width $<1.5D$ or height $>0.5D$. Shank did not expand sufficiently to fill the hole; joint lacks designed shear capacity and clamping pre-load. Must be re-driven or replaced.
• Over-Driven Head: Width $>1.5D$ or height $<0.5D$. Severe impact over-stresses and work-hardens the rivet metal, risking head cracking and parent sheet stretching. Mandates replacement.
• Cracked Shop Heads: Driving cold or age-hardened rivets produces shear cracks. Minor superficial surface crazing within allowable SRM limits may be acceptable, but any through-crack extending into the shank, or cracks spanning more than 50% of the head diameter, require immediate drill-out and replacement.
• Sheet Distortion & Puckering: Skin warpage caused by over-driving, lack of Cleco clamping, or riveting in an irregular sequence. Riveting must always proceed from the centre of a repair outward toward the free edges.
• Inter-Sheet Gaps (Faying Surface Separation): Sheets must be in complete, intimate contact. As an industry-standard inspection rule, if a 0.002-inch (0.05 mm) feeler gauge can be inserted between sheets adjacent to a rivet, the fastener has failed to clamp the joint; the rivet must be drilled out and replaced.

Common Solid Rivet Driving Defects:

      Ideal Shop Head            Tipped Shop Head          Clenched / Bent Head
    (1.5D Wide x 0.5D High)    (Bucking Bar Tilted)       (Excess Length / Slips)

         .------------.               .--------'                 .----.
        (              )             (         /                /      \
    ====+--------------+====     ====+--------+======       ===+--------+========
             |    |                   |      /                  \        \
             |    |                   |     /                    '--------'

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Structural Joint Failure Modes: Shear, Bearing & Tear-Out

A riveted joint subjected to extreme aerodynamic, pressurization, or gust loads can fail through three primary structural mechanisms:

+-------------------------------------------------------------------------+
|                     RIVETED JOINT FAILURE MODES                         |
|                                                                         |
|   1. RIVET SHEAR FAILURE       2. SHEET BEARING FAILURE   3. SHEET TEAR-OUT     |
|                                                                         |
|   P <=== [ Sheet 1 ]           P <=== [ Sheet 1 ]         P <=== [ Sheet 1 ]    |
|             --- Cut Line              ( O ) Hole Ovalizes        ( / / ) Torn   |
|          [ Sheet 2 ] ===> P        [ Sheet 2 ] ===> P        [ Sheet 2 ] ===> P |
|   (Shank Slices Across)        (Hole Stretches & Crushes) (Rivet Rips to Edge)  |
+-------------------------------------------------------------------------+

1. Rivet Shank Shear Failure

• Mechanism: The rivet shank slices completely across its transverse cross-section along the slip plane between the overlapping sheets.
• Root Cause: Shear stress applied to the joint exceeds the total shear strength of the fasteners ($P > A_{\text{rivet}} \times \tau_{\text{allowable}}$). Occurs when rivets are undersized relative to sheet thickness ($D < 2T$) or too few rivets are installed.

2. Sheet Hole Bearing Failure

• Mechanism: The cylindrical rivet hole in the parent sheet stretches, elongates, or crushes into an oval shape, while the rivet shank remains intact.
• Root Cause: Bearing stress on the sheet exceeds the parent metal's compressive yield strength ($P > D \times T \times \sigma_{\text{bearing}}$). Occurs when rivets are excessively large relative to thin sheet metal ($D > 4T$) or when the sheet alloy is too soft.

3. Sheet Edge Tear-Out (Cleavage Failure)

• Mechanism: The rivet shank tears completely through the margin of the sheet metal toward the free edge, splitting the parent sheet open.
• Root Cause: Inadequate Edge Distance ($ED < 2D$ for universal, $< 2.5D$ for countersunk). The cross-sectional shear area between the hole and the sheet edge is insufficient to resist the transmitted load.

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In-Service Defect Diagnosis: The "Smoking Rivet" Phenomenon

During routine transit checks and scheduled base maintenance inspections, certifying engineers inspect painted and unpainted airframe skins for subtle signs of structural distress.

Physics of Fretting Corrosion ("Smoking Rivets")

• Visual Appearance: Characterized by black, soot-like powder or black greasy streaks radiating outward in aerodynamic trails from beneath rivet heads. Commonly termed "smoking rivets".
• Underlying Mechanism: When a riveted joint experiences cyclic flight loads, any slight initial looseness allows relative micro-motion (fretting) between the rivet shank, the rivet head, and the sheet metal hole. This micro-rubbing continually strips away the microscopic, protective aluminium oxide ($Al_2O_3$) surface film. The freshly exposed, virgin aluminium immediately oxidizes upon atmospheric exposure and is ground into a fine black powdery soot. Moisture and boundary layer slipstream airflow wash this black fretting soot out from under the rivet head, depositing conspicuous black trails across the skin.
• Maintenance Remediation: "Smoking rivets" are definitive proof of a loose, working fastener that has lost its structural clamp-up. They must never be washed off or painted over. The certifying engineer must:
  1. Perform tactile checks and dye penetrant inspection to verify whether surrounding skin has developed fatigue cracks radiating from the hole.
  2. Drill out the affected rivets.
  3. Inspect the hole for elongation or fretting wear.
  4. Ream the hole to the next 1/64-inch oversize diameter and install appropriate oversize fasteners.

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Standardized Rivet Removal Protocol (The Drill-Out Procedure)

Improper rivet removal is one of the leading causes of airframe damage during maintenance. Gouging the parent sheet, drilling off-centre, or enlarging the hole requires costly oversize fasteners or structural doublers. Technicians must execute the standardized five-step drill-out protocol:

Standardized Rivet Removal Protocol (The 5-Step Drill-Out):

    Step 1: Centre Punch       Step 2: Drill Head Only       Step 3 & 4: Snap Head & Drift Shank

          Prick Punch                 Drill (Size = Shank)          Pin Punch
               |                                |                          |
               v                                v                          v
         .-----+-----.                    .-----+-----.                 .----+----. (Snap off!)
        (      *      )                  (     \ /     )               (     |     )
    ====+-------------+====          ====+======v======+====       ====+-----+-----+====
             |   |                            |   |                     |    |   ^
             |   |                            |   |                     |    |   | Backed up by
             '---'                            '---'                     '---'    | bucking bar!
     (Exact Center Indent)           (Drill ONLY to base of head!)    (Gently drift shank out)

Step 1: Precision Centre Punching

Using a fine prick punch and a light ball-peen hammer, make a small, precise indentation dead centre in the manufactured head. Never strike hard, which would distort thin sheet metal or expand the rivet shank further into the hole.

Step 2: Drilling Through the Head Only

Select a twist drill bit matching the same diameter or one size smaller than the rivet shank (e.g., for a 1/8" [0.125"] rivet, select a #31 [0.120"] or #30 [0.1285"] drill bit). Drill squarely into the centre punch mark.

• THE GOLDEN DRILLING RULE: Drill ONLY to the depth of the manufactured head (the juncture where the head meets the parent skin). NEVER drill through the rivet shank or into the airframe sheet! Drilling into the shank causes the drill bit to wander off-centre, severely elongating, ovalizing, or gouging the airframe hole.

Step 3: Snapping Off the Head

Insert a pin punch of matching diameter into the drilled hole. Gently pry sideways. The weakened manufactured head will snap cleanly off the shank at the sheet boundary.

Step 4: Drifting Out the Shank

Position a pin punch against the exposed shank end. Crucial Step: The surrounding structure on the opposite side must be backed up with a heavy bucking bar featuring a clearance hole. Gently tap the pin punch with a hammer to drift the shank out of the hole. Backing up the sheet prevents bending, dishing, or delaminating the surrounding parent metal.

Step 5: Post-Removal Hole Bore Inspection

Inspect the empty hole using a 10x magnifying glass and an internal bore micrometer. The hole must remain perfectly round, deburred, and free of scratches or elongation. If the hole was enlarged during removal, it must be reamed to the next standard oversize diameter (1/64" oversize) and fitted with an approved oversize rivet (e.g., NAS1097 flush or oversize NAS9300 series).

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

During an annual zonal inspection on an Airbus A330 wing-to-body fairing support angle, a technician identifies a row of eight rivets exhibiting dark fretting soot halos ('smoking rivets'). A 0.002-inch feeler gauge easily enters the faying surface between the support angle and the fuselage skin.

The technician marks all eight fasteners, lightly centre-punches the factory heads, and drills strictly through each head using a #31 drill bit (one size smaller than the 1/8" shank). Inserting an 1/8" pin punch, the technician snaps the heads off and drifts the shanks into a backed-up bucking bar. Inspection reveals that two of the holes are elongated by 0.005" due to fretting wear. The technician reams the elongated holes to 9/64" (1/64" oversize) and installs approved oversize rivets, restoring structural compliance with the Airbus SRM.

Common Exam Traps

• Trap 1: Drilling all the way through the rivet during removal. A classic exam trap asserts that a technician should drill completely through the rivet shank. This is strictly prohibited; the drill bit must only penetrate the depth of the head, and the shank must be drifted out with a pin punch.
• Trap 2: Chiseling rivet heads off with a cold chisel. Using a cold chisel or pneumatic chisel to shear off rivet heads stretches and gouges the airframe sheet metal and is strictly forbidden on aircraft skin.
• Trap 3: Dismissing smoking rivets as cosmetic exhaust soot. Fretting soot is black aluminium oxide generated by relative micro-motion of loose rivets; cleaning the soot without replacing the fasteners leaves a compromised structure.