6.2 Blind Fasteners, Structural Fasteners & Inspection

6.2 Blind Fasteners, Structural Fasteners & Inspection

Modern airframes incorporate complex closed-cell assemblies—such as tubular control surfaces, dry bay stringers, sealed fuel tanks, wing leading edge slats, and floor crossbeams—where access to the reverse side of the structure is physically impossible. In these locations, conventional solid rivets cannot be bucked, necessitating the use of blind fasteners.

Furthermore, high-stress primary structural joints subject to extreme cyclic fatigue and high shear loads require specialized fasteners that bridge the gap between standard rivets and aircraft bolts, including Hi-Shear pins, Hi-Lok torque-controlled systems, and swaged Lockbolts.

Under EASA Part-66 Module 06, maintenance certifying staff must master the mechanics, grip range tolerances, installation tools, and in-service inspection criteria of these advanced fastening systems.

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Blind Fasteners: Classification and Mechanics

A blind fastener is designed to be inserted and fully set from one side of the workpiece only (the accessible or "manufactured" side). All blind rivets consist of two basic components: a tubular outer sleeve (hollow rivet body) and a central mandrel or pulling stem.

          BEFORE PULLING:                                 AFTER PULLING (MECHANICAL LOCK):

       Pulling Stem / Mandrel                          Locked Stem (Flush Break)
       |  |                                                |
       |  |                                                v
    .--------.  <-- Driving Anvil                      .--------.  <-- Swaged Locking Ring
   (__________) <-- Manufactured Head                 (__________)
      | || |                                             | || |    
  ====|=||=|==== Top Sheet                           ====|=||=|==== Top Sheet
  ====|=||=|==== Bottom Sheet                        ====|=||=|==== Bottom Sheet
      | || |                                            (======)   <-- Expanded Blind Head
      ( oo )   <-- Expanding Bulb                       (______)
       '--'                                                ^ Blind Side (Inaccessible)

1. Friction-Lock Blind Rivets

First-generation blind fasteners (e.g., original Cherry friction-lock, self-plugging, or hollow pull-through rivets) rely strictly on friction between the expanded outer sleeve and the remaining stem segment to retain the stem after the pull stem breaks.

Operational Limitations

• Stem Loss Under Vibration: Engine vibration, aerodynamic buffeting, and cyclic wing flexure readily overcome the frictional fit, causing the stem to shake loose and fall out.
• Catastrophic Strength Loss: When the stem falls out, the remaining thin-walled hollow sleeve loses up to 70% of its initial shear strength and has zero tensile capability.
• Environmental Hazard: An empty rivet sleeve acts as an open channel for moisture and atmospheric contaminants, causing rapid inter-sheet corrosion.
• Regulatory Restriction: EASA CS-25 and FAA AC 43.13-1B prohibit friction-lock blind rivets in primary structural applications. Their use is strictly confined to secondary, non-critical assemblies, interior cabin fixtures, and low-pressure air distribution ducting.

2. Mechanical-Lock Blind Rivets

To overcome the severe hazards of friction-lock designs, aerospace engineers developed mechanical-lock blind rivets. In these designs, a malleable metal locking ring or collar is positively forced into an annular locking groove on the pulling stem during the driving sequence, mechanically locking the stem inside the sleeve before the stem snaps off.

The CherryMAX System (NAS9301–NAS9312)

CherryMAX is the worldwide maintenance standard for structural blind riveting. A complete CherryMAX fastener consists of four precision pre-assembled components:

1. Outer Sleeve: Aluminium alloy (5056), alloy steel, or Inconel.
2. Pulling Stem: High-strength alloy steel with a breakneck groove, annular locking groove, and expanding blind head.
3. Driving Anvil: A small, precision-engineered metal ring seated directly atop the manufactured head.
4. Locking Collar: A soft malleable ring situated between the anvil and the stem.

CherryMAX Installation Sequence

1. Insertion: The rivet is inserted into a clean, deburred hole of exact tolerance.
2. Tension Application: The pulling gun jaws grip the stem serrations. As the gun pulls the stem, the blind head draws against the blind sheet surface, expanding the sleeve into a large, uniform, fully formed blind bulb that clamps the sheets tightly together.
3. Anvil Driving: When the sheet stackup is fully clamped, resistance halts the bulb expansion. Continued pulling force drives the integral driving anvil downward into the manufactured head.
4. Locking Swage: The descending anvil drives and swages the locking collar into the annular groove of the stem, permanently locking the stem to the sleeve.
5. Stem Fracture: Once the collar is locked, tension reaches the calibrated breaking point at the breakneck groove. The stem snaps off cleanly, flush with the manufactured head.

The Major Advantage of CherryMAX: The integral driving anvil eliminates the need to change pulling heads for different diameters and head styles. One pulling head (such as the Cherry G746A or G704B gun with an H701B-456 nosepiece) can install 1/8", 5/32", and 3/16" fasteners.

Blind Fastener Grip Length and Inspection Gauges

Blind rivets have a very narrow grip range (typically in $1/16\text{ inch}$ increments, such as Grip 2 for $0.063"–0.125"$, Grip 3 for $0.126"–0.187"$).

• If the grip is too short, the expanding bulb will form inside the hole barrel rather than against the backside sheet, bulging the hole, crushing the metal, and failing to clamp the joint.
• If the grip is too long, the blind head forms too far away from the sheet; the locking collar cannot swage properly, the stem breaks high or below the surface, and the sheets remain loose.
• Grip Gauge: Technicians must insert a specialized hook-type grip gauge through the drilled hole, hook the rear surface of the blind sheet, pull the gauge flat, and read the exact required grip dash number directly from the scale before selecting the fastener.

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High-Strength Structural Pin-and-Collar Fasteners

In primary airframe joints carrying extreme shear, tensile, or cyclic vibrational stresses (e.g., wing spar chords, landing gear trunnions, engine pylon bulkheads), solid rivets lack adequate shear strength, and standard threaded bolts are too heavy or susceptible to vibrational loosening. Specialized pin-and-collar fasteners solve these challenges.

     Hi-Shear Fastener                             Hi-Lok Fastener
   (Swaged Collar Pin)                      (Torque-Controlled Collar)

      Manufactured Head                            Hex Recess (Allen Key)
         .-------.                                      .-------.     
         |_______|                                      |_______|     
         | |   | |                                      | |   | |     
     ====|=|===|=|==== Sheets                       ====|=|===|=|==== Sheets
     ====|=|===|=|====                              ====|=|===|=|==== 
         | |   | |                                      | |   | |     
         | |   | |                                     /=========\  <-- Threaded Collar
        (=========) <-- Swaged Collar                 (___________) 
        (_________)                                         :       <-- Shear-Off Nut
             v                                              v          (Snaps off at torque)
        Annular Groove                               Calibrated Neck

1. Hi-Shear Rivets

A Hi-Shear rivet is essentially an unthreaded, high-strength alloy steel (AISI 4130/4340 or 17-4PH stainless, heat-treated to 125–160 ksi shear strength) or titanium pin held in place by a swaged 2024 or 6061 aluminium alloy collar.

• Characteristics: Possesses the identical shear strength of an AN/MS aircraft bolt of the same diameter, but weighs 40% less than a bolt-and-nut combination and installs in a fraction of the time.
• Installation: The pin is inserted through a close-tolerance hole (light interference or push fit). The aluminium collar is placed over the grooved end. A special cupped Hi-Shear rivet set in a pneumatic rivet gun (or squeezer) is driven against a bucking bar. The set drives the collar over an annular groove, swaging the malleable aluminium into the groove while a cutting edge on the set automatically shears and ejects the excess collar material.
• Inspection: The collar must be swaged smoothly into the groove with the trimmed collar ring cleanly detached; no excess material may prevent full engagement.

2. Hi-Lok and Hi-Tique Fasteners

Hi-Lok fasteners are the premier structural fasteners in modern commercial airliners (Boeing, Airbus) and military aircraft. They provide guaranteed, uniform preloading and torque control without requiring a torque wrench.

Hi-Lok Construction

• Threaded Pin: Manufactured from high-strength alloy steel (4340), titanium (Ti-6Al-4V), or Inconel 718. The threaded tip contains an internal hex socket (Allen recess).
• Threaded Collar: Features an internal thread, a smooth counterbore that accommodates sheet thickness variations, and an external driving hex nut separated from the collar body by a narrow, calibrated shear-off groove (breakoff neck).

Installation Mechanics

1. The pin is placed through the close-tolerance hole.
2. A specialized pneumatic or hand-ratchet installation tool engages both the pin and the collar simultaneously: an internal Allen hex wrench fits into the pin's socket to prevent the pin from rotating, while an outer socket rotates the collar.
3. As the collar tightens against the sheet stackup, clamp-up tension increases rapidly.
4. When the exact engineered torque preload is achieved, the torsional shear stress across the calibrated neck exceeds its ultimate strength. The hex driving nut automatically shears off, leaving a smooth, lightweight collar permanently locked at the exact required tension.

Hi-Lok Inspection

Inspection is exceptionally simple and 100% visual: if the hex drive portion is cleanly sheared off, the fastener has attained its correct torque preload. Technicians inspect the pin protrusion through the collar (typically 1 to 2 threads showing, but collar counterbore must not bottom out on pin unthreaded shank).

Hi-Tique Variant

The Hi-Tique fastener is an advanced derivative of the Hi-Lok. It incorporates a subtle, bead-rolled band along its shank. When driven into an interference-fit hole, the bead cold-works and plastically expands the hole wall, setting up beneficial residual compressive stresses that delay fatigue crack initiation and increase joint fatigue life by several hundred percent.

3. Lockbolts (Huckbolts / Cherrybuck)

Lockbolts are permanent, non-threaded structural fasteners combining the clamping strength of a heavy bolt with the rapid installation of a rivet.

• Design: A high-strength pin featuring locking grooves, a breakneck groove, and a pulling pintail, accompanied by a malleable metal collar.
• Operation: A hydraulic or pneumatic pull gun grips the pintail serrations. The tool pulls with thousands of pounds of force, drawing the sheets into intimate contact. A hardened anvil within the nose assembly descends over the collar, cold-swaging the collar metal into the pin's annular locking grooves. Once swaging is complete, continued tensile pull breaks the pintail off at the breakneck groove.
• Types: Available in Tension Lockbolts (wide head, heavy collar for high tensile and shear loads) and Shear Lockbolts (compact head, short collar for pure shear applications).

4. Jo-Bolts and Rivnuts

• Jo-Bolt: A high-strength structural blind fastener consisting of three parts: a heat-treated alloy steel bolt, a threaded nut, and an expandable stainless steel sleeve. Driven with a high-torque pneumatic adapter that holds the bolt while torquing the nut, expanding the sleeve against the blind sheet. It matches the shear and tensile capacity of a standard AN bolt and is certified for primary blind airframe joints.
• Rivnut (BF Goodrich): An internally threaded hollow tubular blind rivet invented to attach de-icer boots to aircraft wing leading edges. A threaded mandrel in a mechanical or pneumatic puller threads into the internal threads, then retracts to collapse (bulb) the unthreaded thin-walled upper section against the blind sheet. Once set, the tool unscrews, leaving an internal machine screw thread available for securing inspection covers, fairings, radomes, and de-icing boots. Available with a key under the head to prevent spinning during screw installation.

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Fastener Inspection & Defect Analysis

Airframe structural integrity requires continuous surveillance of fasteners during scheduled base maintenance (C-Checks, D-Checks) and pre-flight inspections.

1. Flushness and Aerodynamic Tolerances

On high-speed commercial transports, countersunk fasteners on external skins must conform to strict aerodynamic surface waviness and flushness limits:

• Allowable Protrusion ("Proud"): Typically no more than +0.005 inch (+0.13 mm) above the adjacent skin line.
• Allowable Recession: Typically no more than -0.002 to -0.004 inch (-0.05 to -0.10 mm) below the skin line.
• Flushness is verified using a calibrated dial indicator flushness gauge seated across the joint.

2. "Smoking Rivets" (Fretting Corrosion)

The appearance of "smoking rivets" is one of the most common and critical structural defect findings on operational transport aircraft.

         WIND / AIRFLOW DIRECTION ===>

               Black/Grey Oxide Plume
            .-----------------------------.
           /  ___                          \
          |  /   \     ( ( ( ( ( ( ( ( ( (  |
          | | (•) |   ( ( ( ( ( ( ( ( ( ( ( |
          |  \___/     ( ( ( ( ( ( ( ( ( (  |
           \_______________________________/
             Rivet Head with Fretting Halo

Mechanism of Formation

1. Loss of Clamp-Up Preload: Due to cyclic airframe flexing, severe vibration, or undersized hole filling, the fastener loses its clamping force.
2. Relative Micro-Motion: Under aerodynamic flight loads and pressurization cycles, the rivet shank rubs microscopic distances against the aluminium hole wall (fretting).
3. Abrasion and Rapid Oxidation: The natural protective aluminium oxide ($Al_2O_3$) film is continuously scraped off by the rubbing action, exposing bare aluminium atoms. These bare metal particles oxidize instantaneously into microscopic aluminium oxide debris.
4. Aerodynamic Streaking: The abrasive oxide particles combine with atmospheric moisture, de-icing fluid, or engine exhaust soot to form a distinct fine black or dark-grey powder. Aerodynamic airflow blows this black residue backward from beneath the rivet head, forming characteristic dark streaks ("smoke plumes") trailing across the aircraft skin.

Corrective Maintenance Action

A "smoking rivet" indicates a failed, loose fastener and an enlarged or wallowed hole. The technician must:

1. Drill out the affected rivet following standard removal protocols.
2. Perform Non-Destructive Testing (NDT)—typically high-frequency Eddy Current inspection—to verify that cyclic fatigue cracks have not initiated from the hole bore.
3. Ream the hole round to the next standard oversize (typically $1/64\text{ inch}$ oversize).
4. Install a matching oversize rivet or close-tolerance fastener with wet epoxy primer or polysulfide sealant to restore joint clamping and corrosion resistance.

3. Knife-Edge Countersinking Defects

When machine countersinking is performed on thin sheet metal where sheet thickness $t$ is less than the head depth $h$, the cutter removes all cylindrical bearing surface in the hole, creating a razor-thin knife edge at the bottom of the countersink. Under cyclic flight stress, this knife edge creates severe stress concentrations, causing catastrophic fatigue cracks to radiate outward between fastener holes. If skin thickness is less than standard countersink depth, dimpling is mandatory.