4.4 Aircraft Hardware, Fasteners & Safetying Methods

4.4 Aircraft Hardware, Fasteners & Safetying Methods

Aircraft hardware encompasses the vast family of threaded fasteners, rivets, nuts, washers, and locking devices that assemble airframes and powerplants. Because aircraft operate under severe cyclic vibration, aerodynamic reversals, and wide temperature extremes, aircraft hardware is manufactured to exacting government and military specifications (AN — Air Force-Navy, MS — Military Standard, and NAS — National Aerospace Standard). Technicians must never substitute commercial automotive or hardware-store fasteners for genuine aerospace hardware.

1. Aircraft Bolt Identification & Part Numbering (AN/MS/NAS)

Standard aircraft hex-head structural bolts are designated by the AN3 through AN20 specification series.

AN Aircraft Bolt Part Number Breakdown:
  [ AN4 ]       [ C ]       [ H ]       [ 12 ]       [ A ]
    │             │           │           │            │
    ▼             ▼           ▼           ▼            ▼
Diameter:     Material:   Drilled     Length:      Undrilled Shank:
4/16" (1/4")  CRES        Head for    1-2/8"       (For use with
              Stainless   Safety Wire (1-1/4")     Self-Locking Nut)

Diameter and Length Sizing Rules

• Bolt Diameter: Expressed in $1/16\text{-inch}$ increments corresponding to the AN number:
  • AN3: $3/16^{\prime\prime}$ diameter ($0.1875^{\prime\prime}$)
  • AN4: $4/16^{\prime\prime} = 1/4^{\prime\prime}$ diameter ($0.2500^{\prime\prime}$)
  • AN5: $5/16^{\prime\prime}$ diameter ($0.3125^{\prime\prime}$)
  • AN6: $6/16^{\prime\prime} = 3/8^{\prime\prime}$ diameter ($0.3750^{\prime\prime}$)
  • AN8: $8/16^{\prime\prime} = 1/2^{\prime\prime}$ diameter ($0.5000^{\prime\prime}$)
  • AN16: $16/16^{\prime\prime} = 1.0^{\prime\prime}$ diameter ($1.0000^{\prime\prime}$)
• Bolt Length: Expressed in $1/8\text{-inch}$ increments by the dash number:
  • Dash number 1 through 7 represents eighths of an inch (e.g., AN4-7 = $7/8^{\prime\prime}$ long).
  • Dash number 10 and above represents whole inches and eighths: the first digit is inches, the second digit is eighths (e.g., AN4-12 = $1\text{ inch and } 2/8^{\prime\prime} = 1\text{-}1/4^{\prime\prime}$; AN6-24 = $2\text{ inches and } 4/8^{\prime\prime} = 2\text{-}1/2^{\prime\prime}$).

Material & Modification Letter Codes

• Standard Cadmium-Plated Alloy Steel (SAE 8740 / 4130): No letter in material position (e.g., AN4-7).
• Corrosion-Resistant Steel (CRES): Letter C in material position (e.g., AN4C7).
• Aluminum Alloy (2024-T4): Letters DD in material position (e.g., AN4DD7).
• Drilled Head (for Safety Wire): Letter H preceding the dash number (e.g., AN4H7).
• Undrilled Shank (for Self-Locking Nut): Letter A as suffix (e.g., AN4-7A). If the A is absent (e.g., AN4-7), the shank has a drilled cotter pin hole.

Bolt Head Markings

2. Special Purpose Aircraft Bolts

1. Clevis Bolts (AN21 through AN36): Feature a round slotted or cross-recessed (Phillips/Torq-Set) head, a precision unthreaded shank, and a very short threaded end terminating with a cotter pin hole. Designed strictly for SHEAR LOADS ONLY (e.g., flight control cable pulleys, bellcranks, hinge pins). Never use clevis bolts in tension applications.
2. Eyebolts (AN42 through AN49): Feature a forged steel loop head designed to secure control cable turnbuckles, mooring tie-down rings, or hoisting slings under direct tensile loads.
3. Internal Wrenching Bolts (NAS144 through NAS176 / MS20004 through MS20024): High-strength alloy steel ($160 - 180\text{ ksi}$ tensile strength) featuring an internal hex socket head. They require MS20002 high-strength chamfered washers under the head to clear the large head-to-shank fillet radius.
4. Close-Tolerance Bolts (AN173 through AN186 / NAS6200 Series): Precision ground shanks ($+0.0000^{\prime\prime} / -0.0005^{\prime\prime}$) fitted into tight reamed holes to prevent joint movement and bolt fretting under severe reversing shear loads (such as helicopter rotor hubs and wing spar attach fittings).

3. Aircraft Nuts & Locking Devices

Aircraft nuts must be secured by positive locking mechanisms to prevent vibration-induced loosening.

Aircraft Nut Classification Architecture:
  [ Aircraft Nuts ]
         ├── Non-Self-Locking (Requires Cotter Pin / Safety Wire)
         │     ├── AN310 Castle Nut (Tension & Shear; Full Height)
         │     ├── AN320 Shear Castle Nut (Half-Height; Clevis Shear Joints)
         │     ├── AN315 Plain Hex Nut (Requires Lock Washer / Check Nut)
         │     └── AN316 Check Nut / Jam Nut (Locks Plain Nuts & Rod Ends)
         └── Self-Locking (Prevailing Torque)
               ├── Nylon / Fiber Insert (AN365 / MS20365: Limited to < 250°F)
               └── All-Metal Deformed Thread (AN363 / MS21042: High Temp up to 550°F - 800°F)

Non-Self-Locking Nuts

• AN310 Castle Nut: Full-height slotted nut designed for high tensile and shear loads on drilled-shank bolts. Sized to receive an MS24665 cotter pin or safety wire through its castellations.
• AN320 Shear Castle Nut: Has approximately half the height/thickness of an AN310 nut and is rated for shear loads only when paired with short-thread AN clevis bolts.
• AN315 Plain Hex Nut: Full-strength hex nut without locking slots. Must be locked using an AN935 lock washer or jammed with an AN316 check nut.
• AN316 Check Nut (Jam Nut): Thin plain nut used to jam against rod-end bearings and plain nuts to lock them in place.

Self-Locking Nuts & Mandatory Safety Rules

• Nylon-Insert Elastic Stop Nuts (AN365 / MS20365): Contain an unthreaded resilient nylon or fiber collar. When screwed onto a bolt, the bolt threads force their way through the nylon, generating prevailing friction torque. Operating temperature limit: $250^\circ\text{F}$ ($121^\circ\text{C}$).
• All-Metal Self-Locking Nuts (AN363 / MS21042): Utilize an out-of-round deformed upper thread or slotted crimped crown. Rated for high-temperature service from $550^\circ\text{F}$ up to $800^\circ\text{F}$ (exhaust flanges, wheel brakes, engine nacelles).

MANDATORY FAA SAFETY RULES FOR SELF-LOCKING NUTS:

1. ROTATION PROHIBITION: Self-locking nuts must NEVER be used on bolts or joints subject to relative rotation (such as control surface hinges, pulley brackets, or landing gear linkages) where rotational movement could unscrew the nut.
2. THREAD ENGAGEMENT: When installed, at least $1\text{ to }2\text{ full threads}$ of the bolt shank must extend completely beyond the nut locking collar / crown.
3. PREVAILING TORQUE CHECK: If a self-locking nut can be turned onto the bolt threads past the locking element with bare fingers without a wrench, its locking feature has fatigued and the nut must be discarded and replaced.

4. Aircraft Washers

• AN960 Flat Washers: Standard cadmium-plated steel flat washers used under bolt heads and nuts to provide a smooth bearing surface and prevent structural galling. Available in standard thickness ($1/16^{\prime\prime}$) and thin 'light' thickness (AN960-xD / 'L' washers, $1/32^{\prime\prime}$) used to adjust bolt grip length. Rule: Never stack more than 3 washers under a nut.
• AN935 Split-Lock Washers: Spring-tempered steel split ring. Not permitted on primary structural airframe joints or structures exposed to critical vibration.
• AN970 Wide-Area Flat Washers: Large outside diameter ($3\times$ standard washer diameter) designed to distribute clamping pressure over soft structural materials (wood, composite sandwich panels, and soft sheet).

5. Solid Shank Rivet Specifications & Sizing Standards

Solid shank aluminum rivets are the primary fasteners used in all-metal aircraft construction. Sizing follows a universal standard:

• Diameter: Measured in $1/32\text{-inch}$ increments.
• Length: Measured in $1/16\text{-inch}$ increments (overall length for flush countersunk rivets; under-head shank length for universal head rivets).

Solid Rivet Part Number Decoding (MS20470AD4-6):
  [ MS20470 ]          [ AD ]              [ 4 ]             [ 6 ]
       │                 │                   │                 │
       ▼                 ▼                   ▼                 ▼
  Universal Head    2117-T4 Alloy       Diameter:         Length:
  (Replaces AN470)  (Dimpled Head)      4/32" (1/8")      6/16" (3/8")

Rivet Head Styles

• MS20470 / AN470 Universal Head: Low rounded head replacing round (AN430), flat (AN442), and brazier (AN455) heads on interior structures and non-aerodynamic external skins.
• MS20426 / AN426 $100^\circ$ Countersunk Flush Head: Flat head seated flush inside machine-countersunk or dimpled sheet metal skins for high-speed aerodynamic laminar surfaces.

Rivet Alloy Classifications & Head Markings

Rivet Sizing & Shop Head Dimensions (FAA-H-8083-30B)

Proper Bucked Rivet Shop Head Proportions:
           [ Manufactured Head (MS20470 Universal) ]
           ───────────────────┬───────────────────
                              │
       Grip Length (T)        │  Shank Diameter (D)
       (Total Sheet Pack)     │
           ───────────────────┴───────────────────
             ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░
             ▒▒▒ [ Formed Shop (Bucked) Head ] ▒▒▒  ▲ Height = 0.5 × D (1/2 D)
             ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░  ▼
             |<────── Width = 1.5 × D ──────>|

• Rivet Shank Diameter Rule: The rivet shank diameter ($D$) should equal approximately $3\text{ times}$ the thickness of the thickest sheet ($3t$) in the joint stack.
• Total Rivet Length Formula: Total length ($L$) required before driving equals total grip thickness ($T$) plus $1.5\text{ times}$ the rivet shank diameter ($1.5D$): $L = T + 1.5D$
• Proper Formed Shop Head (Bucked Head) Dimensions:
  • Shop Head Height: $0.5 \times D$ ($1/2$ shank diameter)
  • Shop Head Width (Diameter): $1.5 \times D$ ($1\text{-}1/2$ times shank diameter)

6. Blind Fasteners (CherryMAX, Huck)

Blind fasteners are engineered for structural joints where access is physically limited to only one side of the structure.

• CherryMAX (NAS1400 / NAS1398): Mechanically locked spindle blind rivets consisting of a hollow rivet sleeve, driving anvil, pulling mandrel stem, and a mechanical locking collar. During pulling, the blind head expands, the shear ring shifts, the locking collar is swaged into the head recess, and the pulling stem fractures flush with the manufactured head at a predetermined tensile load.
• Inspection Protocol: The locking collar must be fully seated flush within the head recess, and the fractured mandrel stem must be locked tightly in place. If the stem moves or the collar is loose, the rivet is unairworthy and must be drilled out and replaced.

7. Safetying Methods & Procedures

Safetying is the positive locking process that prevents aircraft fasteners from backing off under dynamic flight vibrations.

Safety Wire Double-Twist Mechanics (Positive Safetying):
        ( Bolt 1 )                             ( Bolt 2 )
      ┌────────────┐                         ┌────────────┐
      │     H      │                         │     H      │
      └─────┬──────┘                         └─────┬──────┘
            │   Wire Pulls Tightening (CW)         │
            └──*═══════════════════════════════════┘
               6 to 8 Twists Per Inch (TPI)
               Terminates in Pigtail: 3 to 6 twists (1/4" to 1/2" long) bent inward

Safety Wire Standards (FAA AC 43.13-1B)

• Wire Material & Sizes: Stainless steel (CRES), Monel, or brass wire. Standard diameter: $0.032\text{-inch}$ for all general airframe structural bolts. $0.020\text{-inch}$ wire is used for small electrical connectors and closely spaced instrument screws ($<1/8^{\prime\prime}$ hole).
• Double-Twist Method: Two strands of wire are intertwined using safety wire twister pliers at $6\text{ to }8\text{ twists per inch (TPI)}$.
• Direction of Pull (Positive Safetying): The safety wire must be routed so that any loosening tendency of the bolt pulls the wire tighter in the clockwise (tightening) direction.
• Grouping Limits: Maximum of 3 bolts in a single safety wire series (or a maximum pattern length of $24\text{ inches}$).
• Pigtail Termination: The safety wire ends must be twisted together for $3\text{ to }6\text{ twists}$ ($1/4^{\prime\prime}\text{ to }1/2^{\prime\prime}$ long) and bent backward/inward toward the bolt head to prevent snagging and puncture injuries.

Cotter Pins (MS24665)

• Used with AN310/AN320 castle nuts and clevis pins. Sized to fit the drilled hole snugly without slop.
• Preferred (Wrap-Around) Method: One prong is bent straight up and over the end of the bolt shank (cut off flush so it does not extend beyond the bolt outer diameter). The lower prong is bent downward flat against the side flat of the castle nut (cut off above the washer).
• Optional (Head-Over / Side-Bent) Method: Prongs bent laterally around the sides of the nut flats for low-clearance installations.