Control Cable Corrosion and Wear: What the FAA's AC 43.13-1B Says, Explained for UK Readers

The FAA's AC 43.13-1B (paragraph 7-149) reduces control cable inspection to a few hard rules: one broken wire in a critical fatigue area means replacement; corrosion on the interior strands is failure, which you can only see after relieving tension and opening the cable by reverse twisting; and outer wires worn 40 to 50 percent mean replacement. Turnbuckles must show no more than three threads outside the barrel. These are FAA-acceptable methods, not UK or EASA requirements.

This article is for owners, engineers and students in Britain who meet AC 43.13-1B on American-built aircraft, or who use it as a reference text. It answers five practical questions from the circular's Chapter 7, Sections 8 to 10, with the paragraph numbers so you can check the source. The full circular and related FAA documents are indexed in the Kopik AC 43.13 base.

Question 1: Where on the cable should I look hardest?

In the critical fatigue areas defined by paragraph 7-149b: wherever the cable runs over, under or around a pulley or sleeve, or through a fairlead; any section that is flexed, rubbed or worked; and any point within 1 foot of a swaged-on fitting. Swaging can pinch the small wires, which then fail prematurely (7-149c).

The circular also points to the places where cables corrode: battery compartments, lavatories and wheel wells, where corrosive fumes, vapours and liquids collect (7-149i). A broken wire found away from any pulley or fairlead is a reason to examine that cable carefully for corrosion.

Question 2: How do I find a broken wire that doesn't stick out?

1. Wipe a cloth along the critical areas: it snags on broken wires and cleans the cable for a visual check (7-149d).
2. Then look closely: a broken wire may lie in the strand in its original helix and show only as a hairline crack.
3. If in doubt, use a magnifying glass of 7 power or greater, as the AC recommends.
4. Remember Figure 7-16: some broken wires were invisible to wiping and only appeared when the cable was removed and bent. The AC says cables should be removed periodically for a more detailed inspection.

The verdict is in 7-149a: at each annual or 100-hour inspection (the FAA's inspection terms), all control cables are inspected for broken wire strands, and one broken wire strand in a critical fatigue area means the cable assembly must be replaced.

Question 3: The cable is rusty. Is it scrap?

Not automatically. The deciding factor is what is inside. Paragraph 7-149i sets the sequence:

1. Relieve cable tension.
2. Carefully force the cable open by reverse twisting and inspect the interior.
3. Internal corrosion = failure; the cable must be replaced.
4. No internal corrosion: remove loose external rust with a clean, dry, coarse-weave rag or fibre brush.
5. Never use metallic wool (it embeds dissimilar metal particles) or solvents (they remove the internal lubricant).
6. Protect with a sparing coat of MIL-C-16173, grade 4 compound, kept thin enough not to foul fairleads, pulleys or grooved bellcranks.

For routine protection, paragraph 7-150 adds that tinned steel cable should be coated with rust-preventive oil, while corrosion-resistant steel cable does not need that treatment. Carbon steel cable can be lubricated and protected in one operation with MIL-C-16173 grade 4 or MIL-C-11796 Class I.

Question 4: What other defects mean replacement?

Internal wear deserves a mention: paragraph 7-149h warns that wear between inner wires, over pulleys and quadrants, is not easily detected unless the strands are separated, and can proceed faster than surface wear.

If you replace it: matching the cable

Paragraph 7-147 asks for an exact duplicate of the original, same size and quality. Tables 7-3 and 7-4 give minimum breaking strengths, which paragraph 7-145 calls acceptable for repair and modification of civil aircraft. A few examples, in pounds: 3/32 in 7 x 7: 920 (MIL-W-83420 Comp A), 920 (Comp B), 700 (MIL-C-18375); 5/32 in 7 x 19: 2,800, 2,400 and 2,000; 3/16 in 7 x 19: 4,200, 3,700 and 2,900 (Table 7-3). For nonflexible 1 x 7 cable of 1/16 in, Table 7-4 gives 500 lb.

Fabrication rules worth knowing: cut cable mechanically (no torch, 7-147b); proof-load terminals and splices to 60 percent of breaking strength for 3 minutes (7-146); no part of a splice closer than 2 inches to a fairlead or pulley (7-147a).

Swaged terminals: checking for slippage

Paragraph 7-148 records cases where only 1/4 inch of cable had actually been swaged into the terminal. To guard against this, the AC describes a simple marking routine, which also gives inspectors something to look at years later:

1. Measure the length of the terminal end of the fitting to know how much cable must go into the barrel.
2. Mark that length on the cable with masking tape, which won't slip during swaging.
3. After swaging, check the tape marker to confirm the cable has not slipped.
4. Remove the tape and paint the junction of fitting and cable.
5. At every later inspection, look for a gap in the painted section: a gap means slippage has occurred.

After swaging, the terminal should be free of die marks and splits and not out of round, with no cut or broken strands, and its shank diameter checked with a go/no-go gauge or micrometer against Table 7-5. For a 1/8 in 7 x 19 cable, for instance, Table 7-5 lists a shank diameter after swaging of .219 in and a minimum breaking strength of 2,000 lb. The cable is then proof-loaded to 60 percent of its rated breaking strength.

Question 5: Is the turnbuckle right?

• Both terminals screwed in equally, with not more than three threads exposed (7-166, Figure 7-23 item A).
• On initial installation, not more than four threads inside the barrel (Figure 7-23 item B).
• Use the witness hole where fitted (7-167).
• Bent or thread-distorted turnbuckles are replaced (7-165).
• Safety-wired (double-wrap preferred) or fitted with an approved clip; safety wire is never reused (7-179). Table 7-8 gives wire size and material, e.g. 1/16 in cable, double wrap: 0.020 in stainless steel, Monel or "K" Monel.
• Tension itself comes from the airframe manufacturer (7-153); the AC gives no tension figures.

Last, the routing checks in 7-149j to 7-149n: no interference with structure, wiring or plumbing; pulleys free of sharp edges and embedded debris; fairlead deflection angles no greater than 3 degrees; and stiff controls after work on the surfaces may mean parallel cables twisted together or connected in reverse.

You can ask the base exactly the question that brought you here: "My plane's control cable has surface rust, can I just clean it and keep flying?" The source text is the FAA's AC 43.13-1B.