5.2 Aircraft Bolts, Screws & Studs

5.2 Aircraft Bolts, Screws & Studs

Aircraft structural hardware operates under extreme environmental and mechanical conditions. Unlike commercial industrial hardware, fasteners used in aviation are manufactured to rigorous aerospace specifications that dictate alloy composition, grain flow, heat treatment, thread rolling, protective plating, and quality inspection. Fasteners must maintain structural integrity under high cyclic tension, severe shear loads, dynamic vibration, and corrosive operational environments.

Under EASA Part-66 Module 06, certifying technicians must be proficient in identifying, selecting, and installing aircraft bolts, structural screws, and studs in strict accordance with approved aeronautical standards.

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Aircraft Fastener Standardization Systems

Three primary standardization systems dominate civil and military aviation:

1. AN (Air Force-Navy): The historic baseline standard developed during World War II for joint military operations. AN specifications established standard dimensions, thread fits, and strength ratings for basic airframe fasteners.
2. MS (Military Standard): Succeeded and modernized many AN standards, introducing tighter manufacturing controls, advanced alloys, and standardized corrosion protection.
3. NAS (National Aerospace Standard): Maintained by the Aerospace Industries Association (AIA). NAS standards represent advanced, high-performance fasteners, including ultra-high tensile 12-point bolts, close-tolerance shear bolts, and specialized composite fasteners.

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AN Bolt Numbering System

Standard hex-head aircraft bolts (AN3 through AN20) are the workhorses of airframe structural assembly. The AN part number encodes the bolt's diameter, material, length, and drilling options.

                    AN Bolt Part Number Decoding

       AN    4    C    H    1    4    A
        │    │    │    │    └────┬────┘  │
        │    │    │    │         │       └─ Suffix 'A' = Un-drilled Shank (Plain)
        │    │    │    │         │          (Omission of 'A' = Drilled Shank)
        │    │    │    │         └────────── Length Dash Number: 1-4/8" = 1-1/2"
        │    │    │    └──────────────────── Letter 'H' = Drilled Head (Safety Wire)
        │    │    └───────────────────────── Material: 'C' = CRES Stainless Steel
        │    │                               (Blank = Cad-Plated Alloy Steel)
        │    │                               ('DD' = 2024-T4 Aluminium Alloy)
        │    └────────────────────────────── Diameter in 1/16" increments (4/16" = 1/4")
        └─────────────────────────────────── Air Force-Navy Standard

1. Bolt Diameter Coding

The diameter of an AN bolt is designated by the number immediately following the 'AN' prefix, representing the nominal shank diameter in sixteenths of an inch (1/16"):

• AN3: $3/16\text{ in}$ (0.1875 in)
• AN4: $4/16\text{ in} = 1/4\text{ in}$ (0.2500 in)
• AN5: $5/16\text{ in}$ (0.3125 in)
• AN6: $6/16\text{ in} = 3/8\text{ in}$ (0.3750 in)
• AN8: $8/16\text{ in} = 1/2\text{ in}$ (0.5000 in)
• AN10: $10/16\text{ in} = 5/8\text{ in}$ (0.6250 in)
• AN20: $20/16\text{ in} = 1-1/4\text{ in}$ (1.2500 in)

2. Bolt Length and Dash Numbers

The dash number indicates the overall bolt length from beneath the head to the tip of the shank:

• Lengths under 1 inch: A single dash number represents the length in eighths of an inch (1/8").
  • Example: AN4-7 is $7/8\text{ in}$ long.
  • Dash numbers ending in 8 or 9 do not exist; lengths roll over directly to the next whole inch.
• Lengths of 1 inch and greater: The first digit represents whole inches, while the second digit represents additional eighths of an inch.
  • Example: AN4-10 is $1\text{ in}$ long ($1\text{ in} + 0/8\text{ in}$).
  • Example: AN4-12 is $1-1/4\text{ in}$ long ($1\text{ in} + 2/8\text{ in}$).
  • Example: AN4-25 is $2-5/8\text{ in}$ long ($2\text{ in} + 5/8\text{ in}$).

3. Material Designations

• No Material Letter (Standard): Cadmium-plated SAE 4037 or 8740 nickel-chromium-molybdenum alloy steel heat treated to a minimum tensile strength of 125,000 psi (125 ksi / 860 MPa).
• Letter 'C': Corrosion-Resistant Steel (CRES, AISI 300 series austenitic or 400 series martensitic stainless steel).
• Letters 'DD': 2024-T4 aluminium alloy (used in non-structural or light structural joints where magnetic neutrality or weight reduction is paramount; tensile strength ~62 ksi).

4. Shank and Head Drilling Suffixes

• Omission of Suffix 'A': The shank tip has a drilled hole to accept a split cotter pin (used with castellated nuts like AN310).
• Suffix 'A' (e.g., AN4-12A): The shank is un-drilled (plain). Designed exclusively for use with self-locking nuts (e.g., AN365 / MS20365).
• Letter 'H' (e.g., AN4H12 or AN4H12A): The bolt head is drilled through the corners or flats to accept safety wire.

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The Golden Rule of Grip Length

The grip length is the length of the unthreaded cylindrical shank measured from beneath the bolt head to the beginning of the thread runout.

                        Aircraft Bolt Grip Geometry

            ├─── Head ───┤├──────── Grip Length ────────┤├── Thread ──┤
            ┌────────────┐┌──────────────────────────────┐
            │            ││                              │\\\\\\\\\\\\
            │  AN HEAD   ││     SMOOTH SOLID SHANK       │\\\\\\\\\\\\
            │  MARKING   ││    (CARRIES SHEAR LOAD)      │\\\\\\\\\\\\
            └────────────┘└──────────────────────────────┘
                         ▲                              ▲             ▲
                         │                              │             │
                         └── Under-Head Fillet          └── Shear     └── Shank Tip
                                                            Plane

Airworthiness Rules for Grip Selection

1. Grip Length Must Equal Material Thickness: The grip length must strictly match the total thickness of the material sheets being clamped together.
2. No Threads in the Shear Plane: Threads have a smaller cross-sectional area (up to 25% lower than the shank) and sharp root notches that act as severe stress raisers. If threads are placed in the shear plane of a joint, the bolt can fail rapidly under cyclic shear fatigue.
3. No Shank Should Bottom Out in the Nut: The thread must extend sufficiently into the nut to allow full torque clamping before the nut reaches the unthreaded shank shoulder (thread runout).
4. Washer Compensation Limits: Under FAA AC 43.13-1B and EASA Part-M/145 guidelines:
   • Shimming washers (AN960 standard or AN960L light series) may be added under the nut to prevent bottoming.
   • Maximum Washers Allowed: A maximum of three washers may be used under the nut. Never exceed three washers under any circumstances.
   • If more than three washers are required, the bolt is too long and must be exchanged for a bolt with a shorter dash number.
   • A maximum of one washer may be placed under the bolt head if necessary to prevent the bolt head fillet from riding on the edge of the hole.
5. Thread Protrusion and the 3 Threads Limit: When installed and properly torqued, standard aircraft bolts must expose a minimum of 1 to 2 full threads beyond the nut face (for self-locking nuts, at least 1.5 to 2 threads or the full chamfer plus 1 thread must emerge past the locking collar to ensure full locking engagement). Standard airworthiness rules enforce a strict maximum of 3 threads limit (no more than 3 exposed threads protruding beyond the nut face); excessive thread protrusion indicates the bolt grip length is too short, creating the danger of non-grip thread engagement in the shear plane, excess weight, or clearance fouling with adjacent components.

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Bolt Head Identification Markings

Aircraft technicians must identify bolt materials and types by inspecting the standardized raised or stamped symbols on the bolt head:

Exam Warning / Common Trap: A plain, completely unmarked bolt head signifies a commercial hardware-grade bolt (SAE Grade 1 or 2) with low, uncertified tensile strength (typically < 60 ksi). Unmarked commercial bolts are STRICTLY PROHIBITED on type-certificated aircraft. Installing an unmarked commercial bolt in an aircraft structure is an immediate airworthiness violation.

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Close-Tolerance Bolts

Close-tolerance bolts (such as AN173 through AN182 and NAS6200 / NAS6600 series) are specialized aerospace fasteners manufactured for zero-backlash structural joints.

• Precision Ground Shank: The cylindrical shank is ground to an ultra-precise diameter within $\pm 0.0005\text{ inch}$ ($\pm 0.013\text{ mm}$) of nominal size.
• Identification: Marked with a raised equilateral triangle (△) on the head.
• Installation Standard: Must be installed in reamed, high-tolerance holes to create a light drive fit. The bolt must not slide in loosely by hand; it is driven home using gentle taps from a brass drift pin or dead-blow hammer. Never strike a close-tolerance bolt with a steel hammer, which damages cadmium plating and mushrooms the shank.
• Applications: Wing spar attach fittings, horizontal stabilizer pivot hinges, engine pylon links, and flight control mass balance hinges where any mechanical play would induce destructive structural flutter.

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Specialty Aircraft Bolts

1. Clevis Bolts (AN21 through AN36):
   • Feature a round, flat head with a deep screwdriver slot or cruciform recess, and a very short threaded section.
   • Strict Limitation: SHEAR LOADS ONLY. The unthreaded shank carries heavy shear loads (e.g., flight control cable fork ends, bellcrank attachments, and pulley brackets). They must never be used in tension applications.
2. Eyebolts (AN42 through AN49):
   • Feature a forged circular looped head designed to attach flight control cables, turnbuckle fork ends, or mooring/towing equipment. Can be subjected to tension and shear, but require specialized spherical washers if loaded at an angle.
3. 12-Point Internal/External Wrenching Bolts (MS21250, NAS144 through NAS158):
   • High-tensile alloy steel or high-nickel superalloys (Inconel 718, MP35N) heat treated to 160,000 to 220,000 psi tensile strength.
   • 12-point (double-hex) head allows high torque application in confined spaces without socket slippage. Standard on jet engine turbine mounts, main rotor hubs, and airliner landing gear attach pins.

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Aircraft Screws: Machine Screws vs. Structural Screws

A common maintenance error is treating all threaded screws as interchangeable. In aviation, screws are strictly divided into two distinct structural classes:

High-Performance Structural Screw Recesses

Standard flat-blade slotted screws are obsolete on modern airframe skins due to tool slippage that scores aircraft skin. Aerospace engineers developed high-torque driver recesses:

• Phillips: Cruciform recess with tapered walls. Designed to cam out at high torque to prevent over-torquing, but prone to head stripping if worn drivers are used.
• Torq-Set: Offset cruciform arms that do not intersect at a common center. Provides high torque transfer without cam-out; standard on flush exterior aerodynamic skins.
• Tri-Wing: Three radial drive slots arranged symmetrically around a central triangular hub. Extremely resistant to cam-out and tamper-resistant; universal on commercial transport jetliners (Boeing, Airbus).
• Hi-Torque: A shallow, curved undercut slot that locks the driver blade into the head under torque. Delivers the highest torque-to-size ratio for flush aerodynamic structural screws.

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Aircraft Studs

An aircraft stud is a cylindrical rod threaded on both ends with an unthreaded plain section in the middle.

                         Aircraft Stepped Stud Anatomy

            ├── Coarse Thread (UNC) ──┤├── Plain Shank ──┤├── Fine Thread (UNF) ──┤
            \\\\\\\\\\\\\\\\\\\\\\\\\\                     ////////////////////////
            \\\\\\\\\\\\\\\\\\\\\\\\\\                     ////////////////////////
            ▲                                            ▲
            │                                            │
            └── Screws Permanently into Casting          └── Receives Aircraft Nut
                (Aluminium / Magnesium Crankcase)            (Secures Component)

Why Coarse Threads in Castings?

Aircraft studs are installed permanently into engine crankcases, cylinder heads, and accessory gearboxes cast from aluminium or magnesium alloys. Because cast light alloys have significantly lower shear strength than steel, the end driven into the casting features coarse (UNC or American National Coarse) threads. Coarse threads provide a deeper, wider tooth profile and larger shear engagement area in the soft metal, preventing the threads from pulling out under operational tension.

The protruding outer end features fine (UNF) threads to receive high-strength steel aircraft nuts (such as AN310 or MS21042), providing fine torque clamping and high tensile strength.

Stud Classifications and Repair Types

1. Straight Studs: Constant diameter across both threaded ends and plain shank.
2. Stepped Studs (Oversize Repair): The casting end is manufactured with an oversize diameter (e.g., 1/16" larger) to anchor into a casting hole where the original threads stripped, while the nut end maintains the original standard diameter.

Stud Installation and Engagement Rules

• Installation Methods: Installed using a specialized mechanical stud driver or the double-nut (jam nut) method (two plain nuts locked against each other on the fine-threaded end to permit wrench driving).
• Minimum Thread Engagement Depth:
  • In steel or bronze: Minimum engagement depth = $1.0 \times \text{Diameter}$.
  • In soft aluminium or magnesium castings: Minimum engagement depth = $1.5 \text{ to } 2.0 \times \text{Diameter}$.

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Aircraft Maintenance Scenarios & Common Exam Traps

Maintenance Scenario: A technician reattaches a composite engine cowl bracket using an AN3-14A bolt. The sheet pack thickness is 0.50 inches, but the bolt selected has a grip length of 0.75 inches. To compensate, the technician stacks five AN960 washers under the nut so that the nut tightens without bottoming on the thread runout. During pre-flight inspection, the lead technician grounds the aircraft. The installation violates the airworthiness limit of a maximum of three washers. The technician must replace the fastener with an AN3-12A bolt matching the exact 0.50-inch grip requirement.

Exam Warning / Common Trap:

• Trap 1: Dash Number Arithmetic: In lengths under 1 inch, the dash number is eighths of an inch (e.g., -6 is 6/8" = 3/4"). In lengths of 1 inch and greater, -16 does NOT mean 16/8" (which would be 2 inches); it means $1\text{ in} + 6/8\text{ in} = 1-3/4\text{ in}$.
• Trap 2: Clevis Bolts in Tension: Never install a clevis bolt where tension loads exist. Clevis bolts have thin heads and short threads designed strictly for shear loads.
• Trap 3: Machine vs Structural Screws: Never substitute an AN500 machine screw for an AN509 structural screw. AN500 is low-tensile (60 ksi) non-structural steel; AN509 is 125 ksi structural alloy steel.