11.1 Aircraft Cable Construction, Sizing & Swaged Terminal Fittings

11.1 Aircraft Cable Construction, Sizing & Swaged Terminal Fittings

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

In mechanical and hydromechanical flight control systems, flexible steel cables serve as the primary link transmitting pilot control column, wheel, and rudder pedal inputs across long fuselage spans to aerodynamic control surfaces (ailerons, elevators, and rudders). Because flight control cables operate under continuous tension, dynamic airframe flexing, cyclic reverse-bending over pulleys, and extreme environmental exposure, understanding their construction, metallurgical properties, dimensional sizing, and terminal swaging standards is essential for the EASA Part-66 licensed maintenance engineer.

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Aircraft Cable Architecture: Wires, Strands & Cores

Aircraft control cables are precision mechanical assemblies fabricated by helically winding individual high-tensile metal wires into structural strands, which are subsequently laid helically around a central core strand. The internal geometry dictates the cable's flexibility, tensile breaking strength, and resistance to bending fatigue.

+-------------------------------------------------------------------------+
|                   AIRCRAFT CONTROL CABLE NOMENCLATURE                   |
|                                                                         |
|          [Wire]  --> Single high-tensile steel filament                 |
|             |                                                           |
|             v                                                           |
|         [Strand] --> Multiple wires helically twisted together          |
|             |                                                           |
|             v                                                           |
|          [Cable] --> Multiple strands helically laid around a core      |
|                                                                         |
|   Example: 7 x 19 Cable = 7 Strands total, each containing 19 wires     |
|                           (1 Center Core Strand + 6 Outer Strands)      |
|                           Total Wires = 7 x 19 = 133 individual wires   |
+-------------------------------------------------------------------------+

1. Non-Flexible Cable ($1 \times 7$ and $1 \times 19$)

• Construction: A single strand containing 7 or 19 individual high-tensile wires twisted together.
• Mechanical Characteristics: Very high tensile modulus and high breaking strength, but extremely stiff with negligible bending endurance.
• Applications: Strictly restricted to straight structural runs where the cable never passes over a pulley or undergoes directional bending. Common applications include external wing bracing wires, empennage tie rods, fuselage internal diagonal bracing, and fixed antenna stays. They are never permitted for primary flight control systems.

2. Medium-Flexible Cable ($7 \times 7$ Standard Aircraft Cable)

• Construction: Fabricated from 6 outer strands wrapped around a single core strand, with each individual strand containing 7 wires (total of 49 individual wires).
• Mechanical Characteristics: Moderate flexibility with relatively large individual wire diameters. The thicker outer wires provide excellent resistance to surface abrasion and mechanical scuffing, but exhibit low resistance to high-frequency reverse-bending fatigue.
• Applications: Approved for straight cable runs, engine mechanical controls, mixture/propeller governor linkages, trim tab operating systems, and secondary release mechanisms where pulley diameters are large or where pulleys are omitted entirely.

3. Extra-Flexible Cable ($7 \times 19$ Aircraft Cable)

• Construction: Composed of 6 outer strands wound around a single independent core strand, with each strand containing 19 individual wires (total of 133 individual wires).
• Mechanical Characteristics: High flexibility, superior fatigue life, and outstanding resistance to cyclic reverse bending over small-diameter pulleys. Because individual wire diameters are fine, the cable bends around sharp sheaves with minimal internal stress; however, the fine outer wires are more susceptible to surface abrasion wear than those of $7 \times 7$ cable.
• Applications: Mandatory for primary flight control systems (ailerons, elevators, and rudders) across commercial transport, business, and general aviation aircraft where cables route through tortuous airframe paths over directional pulleys and quadrants.

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Cable Materials: Galvanized Carbon Steel vs. CRES Stainless Steel

Aircraft control cables are manufactured from two primary aerospace metal alloys conforming to specification MIL-DTL-83420 (formerly MIL-W-83420):

1. Galvanized Carbon Steel (Composition A)

• Metallurgy: High-carbon structural steel cold-drawn to extreme tensile strength (typically 1,700 to 2,100 MPa), with each individual wire coated with a uniform protective layer of pure zinc via hot-dip or electrolytic galvanizing.
• Corrosion Mechanism: Zinc provides sacrificial galvanic protection. If moisture penetrates the cable, the zinc oxidizes preferentially before the underlying steel can corrode.
• Fatigue Performance: Galvanized carbon steel exhibits approximately 10% to 15% higher cyclic bending fatigue life than stainless steel when operating over pulleys under identical tensile loads.
• Maintenance Consideration: Requires routine lubrication with approved mineral oil or corrosion-inhibiting compounds (such as MIL-PRF-16173) to prevent the breakdown of the zinc coating.

2. Corrosion-Resistant Stainless Steel (CRES, Composition B)

• Metallurgy: Austenitic stainless steel conforming to AISI 302, 304, or 305 (18% Chromium, 8% Nickel). Passivated to produce a protective chromium-oxide barrier film.
• Corrosion Performance: Exceptional resistance to atmospheric corrosion, salt-spray marine environments, engine exhaust gases, and battery acid acid-fume exposure.
• Magnetic Properties: Non-magnetic in the annealed state and possesses very low magnetic permeability even after cold-drawing. For this reason, CRES cables are mandatory in proximity (within 12 to 18 inches / 300 to 450 mm) to magnetic compasses, flux valve heading detectors, and sensitive avionic magnetometers to prevent magnetic field disturbance.
• Operational Limitations: CRES cable possesses roughly 10% lower breaking strength and lower fatigue endurance under tight reverse-bending cycles than galvanized steel. Under continuous friction, stainless steel wires can work-harden and gall rapidly if unlubricated.

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Cable Diameter Sizing Conventions & Minimum Standards

A critical technical distinction tested in EASA Part-66 examinations is the dimensional sizing unit for aircraft cables compared to rigid tubing:

• Aircraft Control Cables are sized strictly in thirty-seconds of an inch ($1/32"$).
  • A 1/16" cable is $2/32"$ (Dash 2)
  • A 3/32" cable is $3/32"$ (Dash 3)
  • A 1/8" cable is $4/32"$ (Dash 4)
  • A 5/32" cable is $5/32"$ (Dash 5)
  • A 3/16" cable is $6/32"$ (Dash 6)
  • A 1/4" cable is $8/32"$ (Dash 8)
• (Note the contrast: Rigid fluid tubing is sized in sixteenths of an inch [$1/16"$], whereas control cables are sized in thirty-seconds of an inch [$1/32"$].)

The 1/8" Primary Flight Control Regulatory Floor

Under EASA CS-23.689, CS-25.689, and equivalent FAA FAR 23/25 certification specifications, no cable of less than 1/8 inch (3.2 mm / 4/32") diameter may be used in primary flight control systems (aileron, elevator, or rudder). Cables of 3/32" or 1/16" are restricted strictly to secondary functions, including engine trim tabs, cockpit door emergency jettison lines, mechanical fuel shut-off valves, and throttle interlocks.

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Swaged Terminal Fittings: Types & Geometries

Control cables transmit tension forces into airframe bellcranks, sectors, and quadrants through precision forged steel end fittings attached via mechanical swaging. Standard aeronautical swaged fittings conform to MS (Military Standard) and AN (Air Force-Navy) series specifications:

+-------------------------------------------------------------------------+
|                      SWAGED TERMINAL FITTING TYPES                      |
|                                                                         |
|   1. Eye End (MS20668 / AN668):                                         |
|      Flat forged eye with a reamed hole for a clevis bolt or pin.       |
|      [===Shank===](  O  )                                               |
|                                                                         |
|   2. Fork End / Clevis (MS20667 / AN667):                               |
|      U-shaped bifurcated prongs designed to straddle a bellcrank arm.   |
|      [===Shank===]==[==O                                                |
|                     ==[==O                                              |
|                                                                         |
|   3. Threaded Stud (MS20669 / MS20670 / AN669):                         |
|      Machined with RH or LH threads for insertion into turnbuckle.      |
|      [===Shank===]||||||||||||> (Threads)                               |
|                                                                         |
|   4. Ball End Fittings:                                                 |
|      - Plain Ball (MS20664): Used for emergency releases or stops.      |
|      - Single-Shank Ball (MS20664): Seats into quadrant cable sockets.  |
|      - Double-Shank Ball: Provides symmetrical support in cable drums.  |
+-------------------------------------------------------------------------+

1. Threaded Cable Terminals (MS20669 / MS20670): Feature external threads (available in right-hand or left-hand threads) designed to screw directly into turnbuckle barrels. The terminal body includes a hollow shank for cable insertion.
2. Fork End (Clevis) Terminals (MS20667): Feature two parallel machined ears with precision reamed holes, designed to engage bellcrank arms using standard AN23 through AN30 clevis bolts secured with castellated shear nuts and cotter pins.
3. Eye End Terminals (MS20668): Feature a single circular eye drilled to accept a clevis pin or bolt, providing a pivoting mechanical connection.
4. Ball End Terminals (MS20664): Available in plain ball, single-shank ball, and double-shank ball configurations. Ball ends seat into matching hemispherical pockets in flight control sectors, control column pulleys, or throttle quadrants. Single-shank ball fittings permit swaging onto the cable end while preventing excessive cable bending right at the ball junction.

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Swaging Mechanics & Fabrication Procedure

Swaging is a mechanical cold-forging process in which a seamless, hollow terminal shank is compressed radially inward by high-pressure hardened steel dies. The compressive force exceeds the yield strength of the terminal metal, causing it to plastically flow into the helical grooves and valleys between the outer cable strands, forming a permanent, mechanical interlock.

Swaging Machines

• Rotary Swaging Machine: A stationary workshop machine consisting of a rotating spindle that forces reciprocating hammer dies against the terminal shank thousands of times per minute. Utilized in manufacturing depots for production cable runs.
• Portable Roll Swaging Tool (e.g., Kearney Tool / Hand Roll Swager): A portable maintenance tool utilizing two rotating grooved roller dies driven by a manual ratchet or hydraulic pump. The technician positions the cable and terminal between the dies and advances the pressure screw through successive passes until the dies close completely. Widely utilized in airline line and base maintenance hangars.

+-------------------------------------------------------------------------+
|                    SWAGING DEFORMATION DYNAMICS                         |
|                                                                         |
|   BEFORE SWAGING:                                                       |
|   Terminal Shank OD = Large (uncompressed)                              |
|   Terminal Shank Length = L_initial                                     |
|   Cable inserted fully into hollow bore.                                |
|                                                                         |
|   DURING SWAGING (Radial Compression):                                  |
|   Rotating dies compress metal radially inward.                         |
|   Metal cold-flows into helical cable valleys.                          |
|                                                                         |
|   AFTER SWAGING:                                                        |
|   Terminal Shank OD = Contracted (measured by Go/No-Go Gauge)           |
|   Terminal Shank Length = L_initial + (8% to 10% Longitudinal Growth)   |
|                                                                         |
|   CRITICAL: If longitudinal growth does not occur, the metal has not    |
|   plastically flowed, indicating insufficient swaging pressure!         |
+-------------------------------------------------------------------------+

Step-by-Step Swaging Fabrication Protocol

1. Square Cable Cutting: Cut the cable to length using an approved mechanical cable cutter (shear-type) or dedicated cable cutoff wheel. Never use an oxyacetylene torch or blunt bolt cutters, which unravel strands, anneal the wire tips, or create flat, distorted ends.
2. Cable Insertion Depth Marking: Measure the depth of the terminal shank's hollow bore. Wrap a narrow band of masking tape or apply a fine paint stripe onto the cable at an exact distance equal to the shank bore depth. Insert the cable into the terminal until the cable bottoms firmly against the internal shoulder. The tape or paint mark must align perfectly with the mouth of the terminal shank.
3. Die Selection & Lubrication: Select the exact roller dies matching the cable diameter and terminal dash size. Lubricate the dies and terminal shank with a light coating of SAE 20 or 30 engine oil to prevent metal galling during compression.
4. Roll Swaging Operation: Place the terminal in the swaging machine. Apply successive passes, rotating the terminal slightly between passes if using a non-continuous roll swager to ensure uniform concentricity and eliminate die flash (finning).
5. Longitudinal Elongation Verification: As the metal contracts radially, it must expand axially. A properly swaged terminal shank will elongate longitudinally by approximately 8% to 10% of its original length. Measuring shank length before and after swaging provides positive confirmation that adequate cold-flow deformation has occurred.

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Post-Swaging Quality Assurance & Inspection Protocol

Every swaged terminal fabricated in an aviation workshop must undergo rigorous multi-stage quality assurance before release:

1. Calibrated Go/No-Go Gauge Verification

The finished outside diameter of the swaged shank must be measured along its entire length using a precision Go/No-Go swaging gauge (or calibrated vernier micrometer):

• The "Go" Opening: Must slide freely over the entire length of the swaged shank. If the shank cannot enter the "Go" slot, the terminal is under-swaged. Under-swaged terminals do not possess sufficient metal engagement in the cable valleys and will pull off under flight loads. The terminal must be re-swaged through another pass.
• The "No-Go" Opening: Must NOT fit over any portion of the swaged shank. If the shank slips into the "No-Go" opening, the terminal is over-swaged. Over-swaging is a non-correctable defect: the excessive reduction severely crushes and shears the internal steel cable strands, while work-hardening and embrittling the terminal shank metal. Over-swaged assemblies must be immediately rejected and scrapped.

2. Micro-Crack and Die Mark Inspection

Under 7x to 10x optical magnification, inspect the terminal shank for:

• Longitudinal surface splits or circumferential cracking.
• Excessive die extrusion ridges (die flash exceeding 0.005" / 0.13 mm).
• Severe scoring, galling, or peeling of protective cadmium plating.

3. Straightness (Concentricity) Verification

The swaged terminal shank must remain concentric and parallel with the cable centerline. The maximum permissible angular misalignment between the terminal shank and the cable axis is 0.5° (one-half degree). Any visible bend in the terminal shank introduces severe bending fatigue stresses in service, leading to rapid shank fracture at the cable mouth.

4. Slippage Witness Paint Mark (Torque Seal)

Immediately following dimensional verification, apply a neat stripe of contrasting witness paint (such as red or orange torque seal lacquer) across the juncture where the cable enters the swaged shank. If the cable slips inside the terminal during subsequent proof loading or in flight, the paint bond fractures and separates from the terminal face, providing unmistakable visual evidence of terminal failure.

5. Process-Specific Proof Load Testing

Where the approved cable-fabrication process requires proof loading, the stated load, duration, witness method, and inspection must be followed:

• Illustrative proof-load value: Some legacy procedures use 60% of rated breaking strength; the actual assembly data control.
• Holding Duration: The tensile load must be applied on a calibrated proof-test bed and held for a minimum of 3 to 5 seconds (or the period specified in the AMM).
• Post-Test Examination: Following load release, reinspect the slippage paint mark. Any movement of the cable out of the terminal shank, distortion of the fitting eye/fork, or yielding of the cable mandates immediate scrapping of the assembly.

+-------------------------------------------------------------------------+
|               REPRESENTATIVE CABLE BREAKING & PROOF LOADS               |
|                                                                         |
|   Cable Dia.   Construction   Material    Min Breaking     60% Proof    |
|                                              Strength         Load      |
|   -------------------------------------------------------------------   |
|   1/16" (2/32")   7 x 7      Galv Steel    480 lbs (2.1 kN)   288 lbs   |
|   3/32" (3/32")   7 x 7      Galv Steel    920 lbs (4.1 kN)   552 lbs   |
|   1/8"  (4/32")   7 x 19     Galv Steel  2,000 lbs (8.9 kN) 1,200 lbs   |
|   1/8"  (4/32")   7 x 19     CRES 302/304 1,760 lbs (7.8 kN) 1,056 lbs   |
|   5/32" (5/32")   7 x 19     Galv Steel  2,800 lbs (12.4 kN) 1,680 lbs  |
|   3/16" (6/32")   7 x 19     Galv Steel  4,200 lbs (18.7 kN) 2,520 lbs  |
|   1/4"  (8/32")   7 x 19     Galv Steel  7,000 lbs (31.1 kN) 4,200 lbs  |
+-------------------------------------------------------------------------+

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Workshop Scenario

A licensed B1 maintenance engineer is tasked with fabricating a replacement aileron balance cable for a twin-turboprop aircraft. According to the Aircraft Maintenance Manual (AMM) and Illustrated Parts Catalog (IPC), the cable specification is 1/8-inch, $7 \times 19$ MIL-DTL-83420 Composition A (Galvanized Steel), terminated with an MS20667-4 Fork End at one end and an MS20669-4RH Threaded Stud at the other.

1. Material Cut: The engineer cuts the cable using a mechanical guillotine shear. The cut face is perfectly square without unlaid or frayed wires.
2. Depth Marking: The engineer measures the bore depth of the MS20667-4 fork fitting (1.50 inches), places a sliver of yellow vinyl tape on the cable 1.50 inches from the tip, and inserts the cable until it bottoms. The tape edge aligns flush with the shank face.
3. Roll Swaging: Using a calibrated Kearney portable roll swager with lubricated 1/8" dies, the engineer completes three progressive passes, rotating the fitting 90° between passes to prevent die finning.
4. Dimensional Check: The engineer uses an MS20667 go/no-go gauge. The "Go" opening slides smoothly along the entire shank. The "No-Go" opening halts firmly against the shank exterior and will not pass. Shank length has grown from 1.50 inches to 1.63 inches (an 8.7% longitudinal elongation), confirming proper plastic metal displacement.
5. Slippage Mark & Proof Test: The engineer applies red torque seal across the cable-to-terminal junction. The cable is mounted on a tensile test bench. Consulting the technical data sheet, a 1/8" galvanized $7 \times 19$ cable has an ultimate breaking strength of 2,000 lbs. The engineer applies a tensile load of 1,200 lbs (60%) and holds it for 5 seconds. Upon release, the red witness paint stripe is intact with zero displacement, no cracks are observed under 10x magnification, and the cable is certified for installation.

Common Exam Traps

• Trap 1: Confusing Cable Sizing with Tube Sizing. Aircraft tubing is sized in sixteenths of an inch ($1/16"$), whereas aircraft control cables are sized in thirty-seconds of an inch ($1/32"$). A "Dash 4" tube is 4/16" (1/4"), but a "Dash 4" cable is 4/32" (1/8").
• Trap 2: Stainless Steel vs. Galvanized Fatigue Myth. Candidates frequently assume that stainless steel is mechanically superior in all respects. On EASA examinations, remember that galvanized carbon steel has superior fatigue endurance and higher ultimate breaking strength than CRES stainless steel.
• Trap 3: Over-Swaged Acceptance Trap. Technicians must never accept an over-swaged terminal. An over-swaged terminal shank has suffered excessive work-hardening and micro-cracking, and the internal cable strands have been crushed and partially sheared, severely reducing fatigue life.
• Trap 4: 100% Proof Load Trap. Apply only the proof load specified for the cable assembly; do not invent a percentage. Applying 100% of rated breaking strength would exceed the yield point of the wire material, inducing permanent plastic elongation and catastrophic structural degradation.