10.1 Aircraft Hardware, Control Cables, Turnbuckles & Safetying Methods

10.1 Aircraft Hardware, Control Cables, Turnbuckles & Safetying Methods

[!NOTE] Where this lives: AC 43.13-1B, Chapter 7, Aircraft Hardware, Control Cables, and Turnbuckles. Section 1 covers rivets; the later sections cover bolts, nuts, washers, safetying methods, and — at Section 8 — the inspection and repair of control cables and turnbuckles. Section 10 covers safety methods for turnbuckles and contains Table 7-8, one of the few tables the IAR test asks about by its actual numbers.

Hardware is where careless work becomes fatal work. A control system is a chain of small parts, and the exam tests hardware the way an accident investigator would: by asking whether the specific fastener, wire, or cable installed was the correct one.

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AN Hardware Sizing

AN bolt part numbers state shank diameter in sixteenths of an inch. This convention answers a family of exam questions. (Solid rivets use a different scale — the diameter dash number on a rivet such as AN470AD4 is in thirty-seconds, so an AD4 rivet is 4/32, or 1/8, inch. Do not carry the bolt convention across to rivets.)

Given a bolt pattern drilled to 0.3125 inch, multiply by 16 to recover the AN number: 0.3125 × 16 = 5, so the pattern takes AN5 bolts. The dash number following the AN number gives grip length, not diameter.

Grip Length

The grip is the unthreaded portion of the shank. Correct practice puts the grip through the material being joined so that no threads bear in shear inside the hole. An inspector checking a bolted joint looks for:

• No more than one thread inside the hole — threads in bearing are a rejectable condition, because the minor diameter carries the load instead of the shank.
• At least one thread projecting past the nut, and not so many that a shorter bolt was clearly required.
• Washers used to fine-tune grip, but not stacked in numbers that indicate the wrong bolt length was chosen.

Nuts and Their Reuse

• Self-locking (fiber or nylon insert) nuts may not be used where the bolt rotates, and may not be reused if the nut can be run down the threads by hand. AC 43.13-1B also restricts them from certain locations where the fiber could be exposed to high temperature.
• Castellated nuts are used with drilled-shank bolts and secured with a cotter pin. The cotter pin prong bent over the bolt must be seated firmly against the bolt shank and must not exceed the bolt diameter; the prong bent over the nut must lie flat against the nut and must not contact the washer surface.
• Plain nuts in structural or control applications require a separate locking means — safety wire, cotter pin, or lock washer as specified.

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Safety Wiring Fundamentals

Safety wire is installed so that any tendency of the fastener to loosen tightens the wire. Standard practice from AC 43.13-1B Chapter 7:

• Wire is twisted 6 to 8 twists per inch for common sizes, tight enough to remove slack but not so tight as to notch or fatigue the wire.
• No more than three fasteners are wired in a series in a general application; closely spaced items in a geometric pattern may be grouped as shown in the AC.
• Wire is not reused. Once twisted and formed, it is work-hardened.
• The pigtail is bent under so that it cannot snag a hand or a control cable.
• Materials must be compatible with the environment: corrosion-resistant steel is the general choice, and the AC warns that smaller wire sizes tend to crack when twisted, particularly where corrosion is a concern.

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Turnbuckle Safetying: AC 43.13-1B Table 7-8

Turnbuckles are the adjustment point of a control cable system, and a turnbuckle that unwinds in flight is a control system failure. The AC prescribes both the method (safety wire or an approved locking clip) and the wire size and material, and it does so by cable size in Table 7-8, Turnbuckle safetying guide:

The AC notes that galvanized or tinned steel, or soft iron wires, are also acceptable materials.

Read the table on two axes. The answer depends on cable size and whether the wrap is single or double — a single wrap always demands either a larger wire or a stronger material than the corresponding double wrap. The most-asked line is the one at the bottom: a 5/32-inch cable safetied with a single wrap of stainless steel wire requires wire of at least 0.057 inch.

AC 43.13-1B is also explicit that the single-wrap methods, while acceptable, are not the equal of the double-wrap methods. Where a choice exists, double wrap.

Thread Engagement

Independently of safetying, a turnbuckle must be screwed together far enough to carry load. The universal inspection rule is that no more than three threads may be exposed on either side of the barrel. An inspector who can see four threads at one end has found a rigging discrepancy regardless of how well the assembly is safetied. Approved wire-locking clips may be substituted for safety wire only where the turnbuckle assembly was designed to accept them.

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Control Cable Construction, Inspection, and Wear Limits

AC 43.13-1B Chapter 7 Section 8 defines cable components against MIL-W-83420, MIL-C-18375, and MIL-W-87161 — the wire center, the strand center or core, and the independent wire rope core.

Cable designation states construction: a 7 × 19 cable has 7 strands of 19 wires each and is the flexible construction used where the cable passes over pulleys; 7 × 7 is less flexible and used in straight runs; 1 × 19 is non-flexible and used for bracing and streamlined wires, never over pulleys.

Inspection technique: wipe a dry cloth or rag along the length of the cable. Snags mark broken wires. Then inspect closely wherever the cable passes through a fairlead, over a pulley, or through a pressure seal — these are the wear points.

Reject criteria an IA applies:

Cable tension is set with a calibrated tensiometer and corrected for ambient temperature using the manufacturer's chart. Rigging a cable to book tension on a cold morning without temperature correction leaves the system over-tensioned when the airframe warms.

Pulleys and fairleads: a pulley worn with a groove offset from the cable path indicates misalignment; a fairlead should support the cable without deflecting it more than the allowable angle. A cable rubbing a structure anywhere in its travel is a discrepancy, and the check must be made through full travel in both directions, not at neutral.

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What the IA Checks in a Control System

[ ] Cable condition through full travel: broken wires, corrosion, wear at pulleys and fairleads
[ ] No broken wires at any terminal or pressure seal
[ ] Turnbuckle thread engagement: no more than three threads exposed either end
[ ] Turnbuckle safetying: correct wrap, correct wire size and material per Table 7-8
[ ] Cable tension per the maintenance manual, temperature corrected, calibrated tensiometer
[ ] Control surface travel within TCDS limits, measured with a protractor or travel board
[ ] Bolt grip length: no threads bearing in the hole
[ ] Cotter pins and self-locking nuts correct, prongs seated, no reused locking nuts
[ ] Correct hardware: AN/MS/NAS, no commercial substitutes

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High-Yield Exam Traps

• AN number = sixteenths of an inch. A 0.3125-inch hole takes AN5 hardware.
• Table 7-8 answers depend on cable size and wrap type. 5/32 and greater, single wrap, stainless: 0.057 inch minimum.
• Single wrap is acceptable but not equal to double wrap — AC 43.13-1B says so in as many words.
• No more than three threads exposed on a turnbuckle barrel.
• One broken wire at a terminal condemns the cable, even if the same count elsewhere in the run would be within limits.
• Threads in bearing are rejectable — the grip, not the thread, carries shear.