Hardware

Aircraft Hardware Overview

• Focus on bolts and nuts used in aircraft structures.

• Types of bolts commonly used:

  • AN (Air Force Navy) bolts

  • NAS (National Aircraft Standard) close tolerance bolts.

Identification of Bolts

• Bolts are identified by code markings on their heads, which are crucial for FAA examinations.

• FAA Figure Pages - Familiarize with illustrations presented in FAA figures for identifying bolts:

  • AN Standard Steel Bolt:

    • Marked with a cross or asterisk on the bolt head. Seen in Illustration 1.

  • NAS Close Tolerance Bolt:

    • Indicated with an X inside a triangle on the bolt head. Found in Illustration 2.

  • Corrosion Resistant AN Standard Steel Bolt:

    • Identified with a single raised dash on the bolt head. This is shown in Illustration 3.

Clevis Bolts

• Characteristics of clevis bolts:

  • Round head, either slotted or recessed for screwdrivers.

  • Identification: Look for the round head, which is Illustration 3 in Figure 43.

• Usage:

  • Only for shear load applications.

  • Never used for tension applications.

  • When used with fork end cable terminals:

    • Secured with a shear nut tightened snugly and safety with a cotter pin.

Bolt Grip Length

• Defined as:

  • The unthreaded portion of the bolt extending through the material.

• Importance of grip length:

  • Must equal the thickness of the material it passes through.

  • Grip Length Calculation Example:

    • Given data: Overall length = 1.5 inches, Shank length = 1 3/16 inches, Threaded length = 5/8 inches.

    • Grip Length = Shank Length - Threaded Length

      • Convert:

        • Shank Length: 1 3/16 = 19/16 inches;

        • Threaded Length: 5/8 = 10/16 inches;

      • Grip Length = 19/16 - 10/16 = 9/16 inches.

      • Decimal form: 9/16 = 0.5625 inches.

Threads on Bolts

• Thread Classes: Defines the manufacturing clearance and tolerance:

  • Class 1: Loose fit

  • Class 2: Free fit

  • Class 3: Medium fit (commonly used for aircraft bolts)

  • Class 4: Close fit

Fiber Type Lock Nuts

• Locking feature:

  • Contains unthreaded fiber locking insert in the nut,

  • Should not be used if bolt/nut may rotate, as it won't maintain security.

Standards for Bolt Installation

• Standards for protrusion of bolts, studs, and screws through self-locking nuts are found in:

  • Advisory Circular 43.13-1B - Key reference in aircraft inspection and repair.

• Differences with Advisory Circular 43.13-2A:

  • 43.13-2A focuses on aircraft alterations, not the installation of hardware.

Installation Best Practices

• Bolts typically installed with the head upward or forward for more security in case the nut loosens.

• Torque values:

  • Apply to dry, clean threads (free of grease/oil).

  • If specific torque values are not provided, use Advisory Circular 43.13-1B for recommended values.

• If a cotter pin hole is misaligned, the practice is:

  • Change washers to reattempt alignment without exceeding recommended torque.