11.3 Pulleys, Fairleads, Cable Inspection & Broken Strand Limits

11.3 Pulleys, Fairleads, Cable Inspection & Broken Strand Limits

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.

Routing flight control cables through the structural labyrinth of an aircraft fuselage, wing, and empennage requires precision hardware to change cable direction with minimal mechanical friction while preventing abrasive contact with airframe bulkheads, frames, and stringers. Over flight hours, control cables suffer from continuous cyclic bending fatigue, abrasive friction, vibration fretting, and atmospheric corrosion. The EASA Part-66 maintenance engineer must master the installation and inspection criteria for pulleys, guard pins, and fairleads, and strictly apply airworthiness reject standards for broken wire strands and cable wear.

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Flight Control Pulleys: Construction & Alignment

Pulleys (sheaves) are used to change the direction of flight control cables. They are manufactured from phenolic laminate (Micarta), ultra-high-molecular-weight nylon, or anodized aluminium alloy, mounted on sealed, pre-lubricated anti-friction ball bearings (MS20219 and MS20220 series).

+-------------------------------------------------------------------------+
|                    AIRCRAFT PULLEY GROOVE GEOMETRY                      |
|                                                                         |
|      CORRECT GROOVE                 TOO NARROW             TOO WIDE     |
|   (120° to 150° Support)          (Cable Pinched)      (Cable Flattens) |
|          _       _                    _     _             _           _ |
|         / \     / \                  / \   / \           / \         / \|
|        |   '---'   |                |   '-'   |         |   '-------'   |
|        |   ( O )   |                |  ( O )  |         |    (  O  )    |
|         \         /                  \       /           \             /|
|          '-------'                    '-----'             '-----------' |
|                                                                         |
|   - Correct: Cable seated snugly; loads distributed evenly.             |
|   - Too Narrow: Pinches cable; high friction; shears outer wires.       |
|   - Too Wide: No lateral support; cable flattens into an ellipse.       |
+-------------------------------------------------------------------------+

1. Pulley Groove Geometry & Contour

The pulley groove must match the specific cable diameter:

• Proper Seating: The bottom of the groove must be contoured to support 120° to 150° of the cable circumference.
• Groove Too Narrow: If a cable is routed through an undersized pulley, the cable cannot seat in the groove root. The groove flanks pinch the sides of the cable, generating extreme friction and rapidly shearing the outer wires.
• Groove Too Wide: If the groove is oversized, the cable lacks lateral support. Under operating tension, the circular cable cross-section flattens into an ellipse across the flat bottom of the groove, inducing severe wire distortion, internal strand chafing, and premature fatigue.

2. Maximum Cable Fleet Angle & Alignment

Pulleys must be aligned squarely with the cable run. The angle between the cable entry path and the pulley center plane is known as the fleet angle (misalignment angle):

• The maximum allowable fleet angle between a flight control cable and its pulley is $2^\circ$.
• Misalignment exceeding $2^\circ$ causes the cable to rub continuously against the side flanges of the pulley, wearing a deep knife-edge groove into the flange and rapidly fraying the outer strands of the cable.

3. Anti-Jam Pulley Guard Pins

Every aircraft flight control pulley must be fitted with rigid guard pins (or formed metal guard brackets):

• Purpose: During flight turbulence, zero-g maneuvers, or temporary cable slackening, the cable can lift out of the pulley groove. Without a guard pin, the cable would jump the pulley rim and lodge in the narrow wedge between the pulley flange and the structural mounting bracket. Once wedged, the flight controls jam solidly, preventing all surface movement.
• Guard Pin Clearance: Guard pins (typically hardened steel cotter pins or through-bolts sleeved with aluminium tubing) must be positioned close to the pulley rim. Standard specifications require the radial clearance between the guard pin and the pulley rim to be less than the diameter of the cable (typically 1/10 to 1/8 of cable diameter, or maximum 0.020" to 0.030" / 0.5 to 0.8 mm) so that the cable cannot physically slip between the pin and the pulley.
• Guard pins must never touch or rub against the rotating pulley.

4. Pulley Bearing Inspection & "Flat Spotting"

During routine maintenance, technicians must relieve cable tension and spin each pulley by hand:

• Bearing Roughness: The pulley must rotate smoothly without axial play, wobble, or ratcheting resistance.
• The Seized Pulley "Flat Spot": If a pulley bearing seizes, the cable slides across the stationary sheave rather than rolling over it. Within a few flight cycles, the continuous steel cable sawing action cuts a deep flat spot into the phenolic or aluminium rim. A flat-spotted pulley must be scrapped immediately, and the cable must be subjected to a 100% inspection for abrasion wear across that contact zone.

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Fairleads: Bulkhead Protection & The 3-Degree Deflection Rule

Fairleads are non-metallic guides installed where control cables pass through structural aircraft bulkheads, frames, ribs, and pressure seals.

+-------------------------------------------------------------------------+
|                    FAIRLEAD INSTALLATION & LIMITS                       |
|                                                                         |
|   Bulkhead Structure           Split Phenolic / Teflon Fairlead Bushing |
|       |                                |                        |
|       |                          .-----+-----.                  |
|       |==========================|     (o)   |==================|
|       |                          '-----+-----'                  |
|       |                                |                        |
|                                        v                        |
|                    Cable Deflection Angle (theta) <= 3°                 |
|                                                                         |
|   TRAINING EXAMPLE — APPROVED ROUTING DATA CONTROL:                     |
|   Some legacy examples use 3 degrees; the actual installation varies.   |
|   Use a pulley whenever the approved design requires it.               |
+-------------------------------------------------------------------------+

Construction & Alignment

• Materials: Machined from split phenolic laminate (Micarta), Teflon (PTFE), or ultra-high-molecular-weight polyethylene (UHMW-PE), enclosed in a cadmium-plated steel retaining bracket.
• Function: To prevent the moving steel cable from contacting and sawing into the aluminium airframe structure, and to dampen lateral cable vibration harmonics (whip).
• Alignment Rule: In the neutral control position, the cable should ideally pass straight through the center of the fairlead bore with zero contact against the fairlead hole under static conditions.

The Mandatory 3-Degree Maximum Deflection Rule ($3^\circ$)

Fairleads are NOT load-bearing directional devices:

• Standard aeronautical regulations (FAA AC 43.13-1B and EASA Part-66) dictate that the maximum allowable cable deflection angle through a fairlead is strictly $3^\circ$.
• Consequences of Excessive Deflection: If an airframe modification or improper rigging imposes a deflection angle greater than $3^\circ$ on a fairlead, the high lateral side-load causes the steel cable to saw rapidly through the phenolic or Teflon bushing. Once the fairlead is breached, the cable saws into the aluminium fuselage bulkhead frame, compromising structural integrity and inducing rapid cable failure.
• Installation control: Use a fairlead or pulley only within the alignment and direction-change limits in the approved routing data.

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Cable Inspection Protocols: Visual, Tactile & Reverse Bending

Flight control cables must be thoroughly inspected at scheduled intervals (typically during 100-hour, Annual, and Phase maintenance checks) using three complementary non-destructive methods:

1. The Tactile "Rag Wiping" Technique

Visual inspection alone cannot detect microscopic wire fractures hidden beneath grease and dirt:

• Procedure: The technician saturates a clean, lint-free cotton cloth or rag with approved solvent or light mineral oil and slowly wipes the cloth along the entire accessible length of the cable.
• Mechanism: When individual wires suffer fatigue failure, the broken ends spring outward from the strand like tiny needle-sharp "fishhooks". As the rag passes along the cable, the fibers of the cloth snag immediately on these protruding wire ends, instantly flagging the exact location of broken strands.
• Safety Caution: Technicians must never run their bare hands along a control cable. The razor-sharp fishhooks of broken high-tensile wires will easily puncture the skin, creating deep puncture wounds and severe risk of infection.

2. Optical Magnification Inspection

Whenever a snag is flagged by the rag wipe (or at known critical fatigue zones), clean the cable thoroughly with a soft bristle brush and solvent. Inspect the surface under 7x to 10x optical magnification under strong oblique lighting to count broken wires and evaluate strand wear.

3. The Reverse U-Loop Technique for Internal Corrosion Detection

Cables rarely fail solely from visible external wire fractures. The most insidious and catastrophic cable failures originate from internal core corrosion and fretting wear occurring deep inside the strands where external wiping cannot detect it:

• Procedure: The technician relieves cable operating tension by disconnecting a turnbuckle or bellcrank. The cable is removed from its pulleys and bent by hand into a reverse loop (U-shape).
• Mechanical Separation: Bending the cable backwards forces the outer strands to spread apart, opening the interstitial spaces between the strands and exposing the internal core wires.
• Inspection Sign - "Red Bleed": The technician examines the exposed core for dry lubrication, broken interior wires, and the presence of "red bleed" (a fine reddish-brown powder or sludge consisting of ferric oxide / iron rust). Red bleed indicates fretting corrosion caused by unlubricated internal wire strands rubbing against each other under cyclic flight loads. If internal corrosion or broken core wires are detected, the cable is compromised and must be scrapped immediately.

+-------------------------------------------------------------------------+
|               REVERSE U-LOOP INTERNAL CABLE INSPECTION                  |
|                                                                         |
|   Straight Cable (Relaxed):                                             |
|   [===================================================]                 |
|   (Outer strands tightly closed; internal core is invisible)            |
|                                                                         |
|   Reverse U-Loop (Hand-Bent):                                           |
|                         .-'''''-.                                       |
|                       .'  .---.  '.                                     |
|                      /   /     \   \                                    |
|                     |   |   O   |   |                                   |
|                     |   | (Core)|   |                                   |
|                      \   \     /   /                                    |
|                       '.  '---'  .'                                     |
|                         '-.....-'                                       |
|                                                                         |
|   EFFECT: Outer strands spread apart!                                   |
|   INSPECT FOR: 1. Broken internal core wires                            |
|                2. "Red Bleed" (fretting rust powder)                    |
|                3. Severe lack of internal lubricant                     |
+-------------------------------------------------------------------------+

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Cable Airworthiness Rejection Criteria: Broken Strand Limits

Under EASA Part-66 Module 07, FAA AC 43.13-1B, and standard AMM Chapter 27 specifications, rigid mathematical thresholds govern when a worn or damaged control cable must be removed from service:

1. General Broken Wire Limits (Along Straight Runs)

A cable must be removed from service and replaced if:

• More than three (3) broken wires are found in any one strand in any one-inch (25.4 mm) length.
• More than six (6) broken wires are found across all strands combined in any one-inch (25.4 mm) length.
• Any individual outer wire is worn flat by more than 40% to 50% of its original cross-sectional diameter due to abrasion.

2. Critical Operating Fatigue Zones (Over Pulleys and Fairleads)

The section of a control cable that passes over a pulley, quadrant, or through a fairlead undergoes continuous reverse-bending and localized stress concentrations:

• Critical Fatigue Zone Standard: Because bending fatigue propagates rapidly across adjacent wires, many commercial transport Aircraft Maintenance Manuals specify a ZERO BROKEN WIRE TOLERANCE (or maximum of 1 broken wire) for any cable section operating within a pulley wrap zone.
• The Replacement Rule: If even a single broken wire is discovered in a section of cable that travels over a pulley or wraps around a quadrant during flight control surface deflection, the cable must be condemned and replaced.

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Flexible Push-Pull Control Systems: Bowden vs. Teleflex

In addition to closed-loop stranded cable systems, aircraft utilize flexible mechanical push-pull conduits for secondary mechanical operations:

+-------------------------------------------------------------------------+
|                   BOWDEN VS TELEFLEX CONTROL SYSTEMS                    |
|                                                                         |
|   1. BOWDEN CABLE:                                                      |
|      - Construction: Smooth high-tensile wire in flexible coiled outer. |
|      - Action: TRANSMITS TENSION ONLY.                                  |
|      - Return: Requires mechanical return spring at the actuated unit.  |
|      - Applications: Carburettor heat, emergency fuel shutoff, brakes.  |
|                                                                         |
|   2. TELEFLEX / PUSH-PULL CABLE:                                        |
|      - Construction: Inner flexible cable with continuous outer HELICAL |
|        winding, functioning as a flexible mechanical rack.              |
|      - Action: TRANSMITS BOTH TENSION (PULL) AND COMPRESSION (PUSH).    |
|      - Drive: Meshes directly with geared drive wheels / pinion teeth.  |
|      - Applications: Engine throttle, propeller governor, trim controls.|
+-------------------------------------------------------------------------+

1. Bowden Cable Systems

• Design: Composed of a single solid or stranded flexible steel wire moving inside an outer flexible casing made of helically wound spring steel wire covered with a protective vinyl sleeve.
• Kinematic Capability: Transmits tensile (pulling) force only. It cannot transmit compressive force because the flexible internal wire would buckle inside the conduit.
• Operation: When the cockpit control is pulled, tension transmits directly to the actuating arm. When the cockpit lever is returned, an external mechanical return spring mounted at the engine or valve pulls the wire back to its resting position.
• Applications: Carburettor heat controls, cabin heater valves, mechanical parking brake releases, and engine emergency fire shut-off valves.

2. Teleflex / Push-Pull Controls

• Design: Features a specialized inner cable composed of a high-tensile steel wire core wound with an external helical steel wire forming a continuous, raised spiral thread. This inner assembly slides within a rigid aluminium or flexible composite conduit lined with low-friction brass or Teflon.
• Kinematic Capability: Transmits BOTH tension (pull) AND compression (push) loads over complex curved routes with zero backlash.
• Operating Principle: The raised helical winding acts as a flexible gear rack. At each end of the control run, the cable engages matching geared drive wheels (pinions) inside rotary cockpit control units or actuator gearboxes. Rotating the cockpit handwheel drives the gear teeth against the helical cable winding, driving the cable in push or pull.
• Applications: Aircraft engine throttle linkages, propeller pitch controls, mechanical trim tab position indicators, cargo door locking mechanism position indicators, and thrust reverser interlocks.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Inspection Scenario

During an annual inspection on a twin-engine turboprop, an aircraft maintenance engineer inspects the primary elevator control cable circuit. The cable is a 3/16-inch $7 \times 19$ galvanized carbon steel assembly.

1. Rag Wipe Check: The engineer wipes a clean cloth dampened with mineral spirit along the fuselage belly run. In the mid-fuselage section, the cloth glides smoothly. Near the aft pressure bulkhead where the cable passes through a phenolic fairlead, the rag snags firmly.
2. Magnification & Measurement: Inspecting the flagged area with a 10x magnifying loupe, the engineer discovers 2 severed outer wires on one strand and 2 severed outer wires on an adjacent strand within a 0.75-inch span (total of 4 broken wires). Because the general limit is a maximum of 3 broken wires per strand and 6 across all strands in any 1-inch length, this straight section theoretically passes the general airframe limit.
3. Fairlead Angle Check: However, the engineer checks the cable deflection angle through the phenolic fairlead using an inclinometer: the cable enters at an angle of $5.5^\circ$, and the cable has sawed halfway through the lower phenolic bushing.
4. Pulley Wrap Inspection: Continuing aft, the engineer inspects the section of cable that wraps around the 4-inch elevator bellcrank quadrant pulley. Under 10x magnification, the engineer discovers one single broken outer wire right at the pulley tangent point.
5. Airworthiness Decision: The engineer condemns the cable assembly. While the 4 broken wires at the fairlead were marginally within straight-run limits, the existence of even 1 broken wire in a critical pulley wrap zone mandates immediate replacement. Furthermore, the engineer submits an airworthiness defect report regarding the improper 5.5° fairlead deflection angle, which requires an engineering order to install a redirect pulley in place of the overloaded fairlead.

Common Exam Traps

• Trap 1: Treating a training fairlead angle as universal. Multiple-choice questions frequently propose using a fairlead to deflect a cable by 5°, 10°, or 15°. Always remember: A fairlead is strictly limited to $\le 3^\circ$ of deflection. Any greater angle mandates a ball-bearing pulley.
• Trap 2: The Pulley Wrap Zero-Tolerance Trap. Candidates mistakenly apply the "6 broken wires in 1 inch" rule to cables passing over pulleys. In pulley contact zones, a single broken wire is standard grounds for rejection in primary flight controls.
• Trap 3: Bowden vs. Teleflex Push-Pull Trap. Remember: Bowden transmits pull only (requires a return spring); Teleflex transmits both push and pull using its external helical rack winding and geared pinions.
• Trap 4: Bare-Hand Cable Inspection Hazard. Never select an answer that suggests inspecting cables by sliding bare fingers or hands along the cable. Broken wire fishhooks cause severe lacerations and puncture injuries; the tactile rag wipe is the mandatory standard.