11.2 Cable End Fittings, Swaging & Turnbuckles

11.2 Cable End Fittings, Swaging & Turnbuckles

To transmit mechanical motion between cockpit controls and aerodynamic flight surfaces, aircraft control cables must terminate in robust, standardized end fittings that attach to bellcranks, quadrants, control horns, and actuators. Furthermore, because cables require precise tension adjustments during airframe assembly and routine maintenance, adjustable turnbuckles are incorporated into every cable run.

Under EASA Part-66 Module 06 (Materials and Hardware — Sub-module 6.11 Control Cables), certifying technicians are held to strict standards regarding terminal swaging processes, dimensional verification using Go/No-Go gauges, proof loading protocols, turnbuckle thread engagement limits, and positive safety-locking mechanisms.

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Swaged Cable Terminal Fittings (Military Specifications)

The universally approved method for terminating primary aircraft control cables is the swaged terminal fitting. Manufactured from annealed corrosion-resistant steel (AISI 303SE or 304 CRES) or heat-treatable alloy steel, swaged terminals feature a hollow cylindrical shank that slips over the end of the cable. Under extreme radial compressive force in a swaging machine, the shank metal cold-flows into the helical valleys between the outer strands of the cable, forming an unbreakable mechanical interlock that develops 100% of the rated breaking strength of the cable.

   PRINCIPAL SWAGED CABLE TERMINAL CONFIGURATIONS

      MS20668 (Eye Terminal)                    MS20667 (Fork / Clevis Terminal)
         .--------.   .--------.                   .-----.   .--------.
        /    __    \  | Hollow |                  /  / \  \  | Hollow |
       |    /  \    |=| Barrel |                 |  | O |  |=| Barrel |
        \   \__/   /  | Shank  |                  \  \ /  /  | Shank  |
         '--------'   '--------'                   '-----'   '--------'
       Pin/bolt connection to bellcranks         Connects to single lugs / control horns


      MS20664 (Single-Shank Ball)               MS20669 (Threaded Stud Terminal)
           .---.      .--------.                             .------------------.
          /     \     | Hollow |                   =======|| |  Hollow Barrel   |
         |   O   |====| Barrel |                   Threads|| |      Shank       |
          \     /     | Shank  |                   =======|| '------------------'
           '---'      '--------'                 Screws directly into turnbuckle barrels
       Quadrant tracks & throttle levers         (Available in RH and LH threads)

Standard Military Swaged Terminals

1. MS20668 Eye Terminal: Features a circular eye end with a precision reamed hole. Connects via a clevis pin or bolt to double-shear bellcrank fittings or quadrant attachment plates.
2. MS20667 Fork (Clevis) Terminal: Features a U-shaped fork with aligned through-holes. Engages single-shear control horns, bellcrank tabs, or torque tubes, secured with an AN clevis bolt, washer, and cotter pin.
3. MS20664 Single-Shank Ball Terminal: A spherical ball with an integral tubular shank on one side. Fits into matching hemispherical sockets in throttle quadrants, rudder pedals, or release levers. Because of its compact profile, it can pass through small cutouts.
4. MS20663 Double-Shank Ball Terminal: Features tubular shanks extending from both sides of the central ball. Used where a continuous cable run engages a quadrant track or capstan drum, with the ball acting as a positive travel stop or drive anchor.
5. MS20669 Threaded Stud Terminal: Consists of an externally threaded shank on one end and a hollow swaging barrel on the other. Threaded studs screw directly into turnbuckle barrels (available with both Right-Hand and Left-Hand threads) or rod-end linkages.

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The Swaging Process & Quality Assurance Verification

Swaging is a critical cold-forming process governed by FAA AC 43.13-1B and manufacturer structural repair manuals (SRM). It must be performed using approved equipment: either a stationary rotary swager (featuring high-speed spinning dies that hammer the terminal radially) or a portable hydraulic roll/squeeze swager for on-aircraft repairs.

   SWAGING OPERATION AND QUALITY VERIFICATION FLOW

      Pre-Swage Inspection         Cold Plastic Swaging           Post-Swage Verification
         .------------.               .------------.                 .------------.
         | Cable Cut  |               | Rotary /   |                 | Go/No-Go   |
         | Square and | ------------> | Hydraulic  | --------------> | Gauge &    |
         | Bottomed   |               | Dies Flow  |                 | Paint Mark |
         '------------'               '------------'                 '------------'
               |                             |                              |
        Cable inserted                Radial compression             Diameter reduced;
        fully to base                 eliminates voids;              check straightness;
        of hollow shank               100% cable strength            proof load 60%

1. Pre-Swaging Preparation

• Cable Cut: The cable must be cut square and clean using a dedicated mechanical cable shear or high-speed abrasive cutoff wheel. Thermal cutting with an oxy-acetylene torch or electric arc is strictly prohibited, as heat anneals the cold-drawn wires and oxidizes the core.
• Insertion Depth: The cable end must be inserted into the terminal shank until it firmly bottoms against the internal shoulder. If the terminal features an inspection hole, the cable core must be clearly visible through the hole before swaging begins.

2. Post-Swaging Dimensional Check: The Go/No-Go Gauge

During swaging, the tubular shank is compressed radially, reducing its outside diameter while increasing its overall length by approximately 8% to 10% (known as longitudinal elongation). To verify proper compression:

• A precision-machined Go/No-Go gauge (swage gauge) must be used across the swaged shank.
• Inspection Technique: The gauge has a calibrated slot corresponding to the finished terminal diameter. The shank must slide smoothly into the gauge slot along its entire length (verifying sufficient reduction to grip the cable), but must not fit into an undersized slot (verifying the shank was not over-swaged or crushed).
• The gauge must be applied at two positions rotated 90° apart to check for ovality or out-of-round deformation.

   GO / NO-GO SWAGE GAUGE VERIFICATION

         .---------------------------------------------------.   
         |  AIRCRAFT TERMINAL SWAGE GAUGE     (MS20668 / 667) |   
         |                                                   |   
         |    .-----.       .-----.       .-----.    [MADE]  |   
         |    |     |       |     |       |     |    [USA ]  |   
         |    |     |       |     |       |     |            |   
         |    |     |       |     |       |     |            |   
         |    '--+--'       '--+--'       '--+--'            |   
         |       |             |             |               |   
         |     1/8"          5/32"         3/16"             |   
         |     CABLE         CABLE         CABLE             |   
         '---------------------------------------------------'   

      • Swaged shank must fit into the calibrated slot along its FULL length.
      • Rotate gauge 90° to verify shank roundness (no ovality or flash fins).
      • If shank fails to enter slot -> Under-swaged (Risk of cable pullout).
      • If shank is loose/undersized -> Over-swaged (Work-hardened, cracked).

3. Visual and Straightness Inspection

• Surface Cracks & Flash: Inspect the swaged fitting under $10\times$ magnification. There must be zero longitudinal splits, cracks, or severe die marks. Any hairline fracture requires instant rejection.
• Straightness: Swaging dies can bend the shank if misaligned. Maximum allowable runout or bend is typically $0.5°$ or $0.005\text{ in per inch}$ of length.

4. Witness (Slippage) Paint Mark & Proof Loading

• Witness Mark: Immediately after swaging, the technician must apply a narrow band of bright-colored lacquer or paint (typically red or orange) at the exact junction where the cable emerges from the terminal shank.
• Proof Testing: The completed cable assembly must be placed in a calibrated proof-loading test bed and subjected to 60% of the cable's rated minimum breaking strength for at least 3 to 5 seconds.
• Post-Test Assessment: Inspect the paint mark. Any gap, cracking of the paint film, or exposed bare cable indicates that the cable has slipped inside the terminal. If any slippage is detected, the assembly must be scrapped immediately.

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Nicopress Oval Sleeves & Field Splices

In general aviation, vintage aircraft, and non-primary emergency runs where rotary swaging equipment is unavailable, Nicopress oval sleeves (copper or aluminium) provide an airworthiness-approved mechanical splice.

   NICOPRESS THIMBLE LOOP AND COMPRESSION SEQUENCE

               Step 1: Center Crimp        Step 2: Terminal Crimp      Step 3: Loop Crimp
                     | |                         | |                         | | 
                     v v                         v v                         v v 
             .--------------------------------------------------.     .-----------.
     Tail -> |  [ 2nd Crimp ]      [ 1st Crimp ]     [ 3rd Crimp] |---|  AN100     \
     Min     |                     (Center First)                 |   |  Thimble    |
     1/8"    '--------------------------------------------------'     .-----------/
                                  Oval Sleeve                         '-----------'

Installation Rules & Tool Mechanics

1. The Thimble: When forming a cable terminal loop, an AN100 galvanized or stainless steel thimble must always be inserted into the loop to protect the cable from sharp-radius crushing and localized wear against attachment pins.
2. Crimping Procedure: Nicopress copper sleeves are compressed using a dedicated, calibrated Nicopress toggle tool. The tool features precision-machined die grooves designated by letters or numbers (e.g., Groove C, M, J). Multiple crimps are required per sleeve (typically 2 to 4 crimps depending on cable diameter):
   • Crimp Sequence: For a 3-crimp sleeve, compress the center groove first, followed by the outer end crimps. This allows the displaced metal to flow outward smoothly without bowing the sleeve.
3. Post-Crimp Gauge Check: Every completed Nicopress crimp must be checked with a stamped Nicopress Go gauge. The compressed sleeve must slip easily into the designated gauge slot. If the sleeve is too thick to enter the slot, the tool must be adjusted and re-crimped.
4. Cable Tail Protrusion: The loose dead-end tail of the cable must protrude at least $1/8\text{ inch}$ ($3.2\text{ mm}$) beyond the sleeve. This visible protrusion allows instant visual confirmation that the cable has not slipped.

The Galvanic Corrosion Hazard: Copper vs. Aluminium Sleeves

Critical Maintenance Warning / Galvanic Trap: Aircraft control cables operate across harsh, humid, and coastal environments. When selecting Nicopress sleeves, material compatibility is paramount:

• Aluminium Nicopress Sleeves are strictly prohibited on stainless steel (CRES) control cables! In the presence of atmospheric moisture or salt spray, the massive electrochemical potential difference between active aluminium and noble stainless steel produces violent galvanic corrosion. The aluminium sleeve rapidly corrodes into white powder, leading to catastrophic in-flight loss of cable retention.
• Approved Combination: For CRES stainless steel cables, only tin-plated or cadmium-plated copper oval sleeves are approved.
• For galvanized carbon steel cables, plated copper sleeves are standard, though zinc-plated copper is widely preferred to prevent galvanic reaction with the zinc coating.

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Turnbuckle Assemblies: Mechanics & Rigging Rules

A turnbuckle is a mechanical adjusting device inserted into a cable run to regulate rigging tension, compensate for airframe manufacturing tolerances, and eliminate operational slack. A standard aerospace turnbuckle consists of a central barrel and two threaded end fittings:

   AEROSPACE TURNBUCKLE ANATOMY (AN130 / MS21251)

    Right-Hand Threaded Fitting         Turnbuckle Barrel          Left-Hand Threaded Fitting
       (MS20669 / AN161)                    (MS21251)                  (Identified by Groove)
         .-------------.                 .--------------.                 .-------------.
         | Threaded    |=================| Center Hole  |=================| Threaded    |
    <--- | Shank (RH)  |    ||||||||     | (Safety Wire)|      ||||||||   | Shank (LH)  | --->
         '-------------'                 '--------------'                 '-------------'
                                                 ^               ^ 
                                                 |               |
                                          Inspection Hole    External Groove
                                          (Threads meet      identifies LEFT-HAND
                                           past hole)        internal threads

Turnbuckle Standards and Anatomy

• Standard Part Numbers: AN130 (turnbuckle with cable eye and fork), AN135 (cable eye and pin eye), AN140 (cable eye both ends), MS21251 (turnbuckle barrel), MS21252 (threaded clevis/fork), MS21255 (threaded eye).
• Thread Mechanics: The barrel has internal Right-Hand (RH) threads at one end and internal Left-Hand (LH) threads at the opposite end. Rotating the barrel in one direction draws both threaded terminal shanks inward simultaneously (tightening the cable); rotating it in the opposite direction drives both shanks outward (slackening the cable).
• Identifying the Left-Hand Thread End: The left-hand thread end of the turnbuckle barrel is positively identified by an external machined annular groove, notch, or knurled band machined around the outer circumference near that end. The smooth end contains standard right-hand threads.

The Critical 3-Thread Exposure Rule

When rigging an aircraft flight control system, technicians adjust turnbuckle barrels to establish the precise tension specified in the Aircraft Maintenance Manual (AMM). During this adjustment, the following airworthiness limitation is absolute:

The 3-Thread Engagement Rule (EASA Part-66 / FAA AC 43.13-1B): Under no circumstances may more than THREE complete threads be exposed outside the barrel at either end fitting!

• Exposing more than 3 threads means the threaded shank does not engage sufficient internal barrel threads to withstand ultimate flight tension loads. The threads could shear or strip out under high-G aerodynamic maneuvers.
• The Inspection Hole Test: Modern turnbuckle barrels feature an inspection hole drilled through the center. Thread engagement is adequate when the threaded shanks extend past the hole. A piece of safety wire or a wire probe must NOT be able to pass through the inspection hole. If the probe passes through, the shanks are insufficiently engaged.

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Turnbuckle Positive Safety-Locking Methods

Because turnbuckles are subject to continuous engine vibration, aerodynamic buffeting, and airframe structural flexure, an unlocked barrel will rapidly rotate and unscrew. Civil airworthiness regulations mandate that all turnbuckles must be positively safetied using approved wire or clip locking methods.

   TURNBUCKLE SAFETY LOCKING METHODS

      Double-Wrap Safety Wire Method             MS21256 / NAS1756 Spring Clip Locking
      (0.032" or 0.041" Stainless Wire)          (Keyway Interlock)

        4 wraps around shank                       Straight prongs engage barrel keyway
             | |                                         | |
             v v                                         v v
         .========.----.------------------.----.========.      .------------------------.
    <==  |  SHANK |(..)|      BARREL      |(..)| SHANK  | ==>  | ===[ CLIP PRONG ]====  |
         '========'----'------------------'----'========'      '------------------------'
                         ^              ^\                                 ^
                         |              |                                 |
                 Through center    Wrapped in opposite            Curved spring head locks
                 hole in barrel    directions; 4 wraps            into barrel center hole

1. Double-Wrap Safety Wire Method (Standard Aerospace Practice)

• Wire Material & Diameter: High-strength, annealed stainless steel safety wire (MS20995C). For cable sizes up to $1/8\text{ in}$ ($3.2\text{ mm}$), use $0.032\text{ in}$ ($0.8\text{ mm}$) wire. For cables larger than $1/8\text{ in}$, use $0.041\text{ in}$ ($1.0\text{ mm}$) wire.
• Installation Procedure:
  1. Take two separate lengths of safety wire. Thread both wires through the hole in the center of the turnbuckle barrel.
  2. Pull the wires through until approximately equal lengths extend from each side.
  3. Bend one wire from each pair along the barrel toward opposite ends. Spiral the companion wire around the barrel toward the same end in an opposing spiral.
  4. Pass the wire ends through the hole in the terminal fitting (eye or fork) in opposite directions.
  5. Wrap each wire around the shank for a minimum of FOUR (4) full turns, wrapping tightly and neatly without overlapping.
  6. Cut off the excess wire and tuck the sharp pigtail ends inward toward the shank to prevent snagging clothing or injuring maintenance personnel.

2. Single-Wrap Safety Wire Method

• Permitted only on smaller cable installations ($1/8\text{ in}$ or smaller) where low tension exists and physical clearance prevents double-wrapping. A single piece of wire is routed through the center hole, passed through the end fittings in opposite directions, and wrapped at least 4 full turns around each shank.

3. Clip-Locking Turnbuckles (NAS1756 / MS21256 Spring Clips)

Modern commercial airliners and high-performance aircraft utilize clip-locking turnbuckle assemblies (MS21251 barrel with MS21256 or NAS1756 spring clips). This system eliminates safety wire, saving substantial labor and reducing human error:

• Design: The turnbuckle barrel features longitudinal external keyway slots. The threaded terminal shanks also feature matching longitudinal keyway grooves. The shanks are screwed into the barrel until proper cable tension is achieved and the grooves align.
• Installation: A specially formed spring steel clip (NAS1756) is inserted into the barrel. The straight prongs slide into the aligned keyways of the barrel and terminal shank, while the curved spring head snaps into the center hole of the barrel, positively locking the components against relative rotation.
• The Strict Single-Use Rule:

Airworthiness Mandate — Single-Use Locking Clips: Turnbuckle locking clips (NAS1756 / MS21256) are STRICTLY SINGLE-USE HARDWARE. Whenever a turnbuckle is loosened or adjusted for rigging, the existing clips must be removed and discarded into the scrap bin. Locking clips must NEVER BE REUSED. The removal process permanently distorts the spring-tempered retaining tangs; a reused clip can back out under vibration, allowing the turnbuckle to unthread in flight.