Aircraft Control Cable Inspection: A Step-by-Step Corrosion and Wear Checklist (AC 43.13-1B)

Under AC 43.13-1B paragraph 7-149, a control cable with one broken wire in a critical fatigue area must be replaced. A cable with corrosion on its interior strands has failed and must be replaced. Surface rust alone can be cleaned off, but only after you relieve tension and open the cable by reverse twisting to confirm the inside is clean. On the turnbuckle, no more than three threads may show outside the barrel.

Control cables are easy to overlook: they run behind panels, through fairleads and around pulleys, and the damage that matters is often hidden inside the lay. AC 43.13-1B, Chapter 7, Sections 8 to 10, gives a precise method for inspecting them. This checklist follows it paragraph by paragraph. The Kopik AC 43.13 base lets you query the full text, including Tables 7-3 to 7-9.

Step 1: Find the critical fatigue areas and hunt for broken wires

Paragraph 7-149a: at each annual or 100-hour inspection, all control cables must be inspected for broken wire strands, and any cable assembly with one broken wire strand located in a critical fatigue area must be replaced. The AC defines a critical fatigue area as:

• the working length where the cable runs over, under or around a pulley, sleeve, or through a fairlead;
• any section where the cable is flexed, rubbed or worked in any manner;
• any point within 1 foot of a swaged-on fitting (eye, fork, ball, ball and shank, threaded stud, turnbuckle, compression sleeve and so on), because swaging pressure can pinch the small wires.

The method (7-149d): pass a cloth over the area so it snags on broken wires, then make a very careful visual inspection, because a broken wire does not always stick out: it can lie in the strand and show only as a hairline crack. If you suspect one, the AC recommends a magnifying glass of 7 power or greater. Figure 7-16 shows broken wires missed by wiping and revealed only when the cable was removed and bent.

Step 2: The corrosion check and the reverse-twist test

Paragraph 7-149i names the areas especially conducive to cable corrosion: battery compartments, lavatories, wheel wells, wherever corrosive fumes, vapors and liquids accumulate. It also gives a useful clue: carefully examine any cable for corrosion when it has a broken wire in a section not in contact with a pulley, fairlead or other wear-producing component. The procedure for a corroded surface:

1. Relieve cable tension.
2. Carefully force the cable open by reverse twisting and visually inspect the interior.
3. If there is corrosion on the interior strands, that "constitutes failure, and the cable must be replaced."
4. If not, remove loose external rust with a clean, dry, coarse-weave rag or fiber brush.
5. Do not use metallic wool or solvents on installed cables: metallic wool embeds dissimilar metal particles, and solvents strip the internal lubricant so strands abrade and corrode.
6. After cleaning, sparingly apply MIL-C-16173, grade 4 corrosion-preventive compound, not so thick that it interferes with fairleads, pulleys or grooved bellcranks.

So the answer to "my cable has surface rust, can I just clean it and keep flying?" is only if the reverse-twist inspection shows no internal corrosion, and only after the cleaning and protection steps above.

Step 3: Wear, kinks and other replacement criteria

• External wear: replace flexible and nonflexible cables when the individual wires in each strand appear to blend together, meaning outer wires worn 40 to 50 percent (7-149g, Figure 7-17).
• Internal wear: it progresses at the same time as external wear, especially over pulleys and quadrants, and can be faster than surface wear. It is only visible when strands are separated (7-149h).
• Kinks: even if straightened, the strands are disturbed and the cable should be replaced (7-149e).
• Popped core or loose strands: replace regardless of wear or broken wires (7-149e).
• Nylon-jacketed cable: replace when the jacket shows cracks or necking down in diameter (7-149f).

Don't reject a cable for a manufacturer's wire splice, though. Paragraph 7-151 explains that visible individual wire-end splices and slight flattening from die-sizing are normal and "not a cause for cable rejection" (Figure 7-22).

Step 4: Routing, pulleys and fairleads

• Check routing, fraying, twisting and wear at fairleads, pulleys, anti-abrasion strips and guards; look for interference with structure, wiring, plumbing and other controls (7-149j).
• With gust locks installed, try to move the controls to detect slack; with locks removed, feel for friction or hard movement, which indicates excessive cable tension. Stiff controls after work on the surfaces may mean parallel cables twisted around each other or connected in reverse.
• Check swaged terminal reference marks for cable slippage (7-149k).
• Inspect pulleys for roughness, sharp edges and embedded foreign material; rotate pulleys that only turn through a small arc to give the cable a new bearing surface (7-149l).
• At fairleads, deflection angles must be no greater than 3 degrees maximum (7-149n).

Reference data: minimum breaking strength by cable size

When a cable has to be replaced, paragraph 7-147 asks for an exact duplicate in the same size and quality as the original. The minimum breaking strengths in Tables 7-3 and 7-4 are, per paragraph 7-145, "acceptable for repair and modification of civil aircraft."

Related rules from paragraphs 7-146 to 7-148: proof-test terminals and splices at 60 percent of the breaking strength for 3 minutes with a guard over the cable; cut cable mechanically, as a torch is not permitted; keep splices at least 2 inches from any fairlead or pulley; and never solder cable ends before swaging, since solder greatly increases the tendency to pull out of the terminal.

Step 5: Turnbuckles and safetying

• Screw both terminals into the barrel an equal amount, so that not more than three threads on the terminal are exposed (7-166, Figure 7-23, item A).
• On initial installation, terminals should not be screwed inside the barrel more than four threads (Figure 7-23, item B).
• Where the barrel has a witness hole, use it (visually or with a piece of safety wire as a probe) to confirm thread engagement (7-167).
• Replace turnbuckles showing thread distortion or bending (7-165 note).
• Safety every turnbuckle with safety wire (double-wrap preferred, single-wrap acceptable) or an approved locking device per TSO-C21, and do not reuse safety wire (7-179). Wraps go at least four turns around the shank.

Try the base with "How many threads can be exposed on a turnbuckle terminal once it's properly adjusted?" or "My plane's control cable has surface rust, can I just clean it and keep flying?" The source is chapter 7 of AC 43.13-1B.