5.3 Locking Devices, Safety Wiring & Torque Procedures

5.3 Locking Devices, Safety Wiring & Torque Procedures

Dynamic vibration, cyclic thermal expansion, and alternating flight loads continually threaten the integrity of bolted aircraft joints. If a structural fastener loosens in service, clamping preload drops to zero, subjecting the fastener shank to severe bending and shear fatigue that can culminate in catastrophic joint separation. To guarantee flight safety, aerospace engineering mandates redundant positive locking systems, standardized safety wiring (lockwiring), and precise torque application protocols.

Under EASA Part-66 Module 06, certifying technicians must master the operational criteria for locking hardware, execute safety wiring to exact airworthiness tolerances, and perform torque adjustments including extension adapter calculations.

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Classification of Aircraft Nuts

Aircraft nuts are broadly categorized into non-self-locking nuts (which require external locking devices such as cotter pins or lock washers) and self-locking nuts (which incorporate integral prevailing-torque locking mechanisms).

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Cotter Pin (Split Pin) Installation & Hole Alignment

Castellated nuts (AN310) paired with drilled-shank bolts and corrosion-resistant steel (CRES) cotter pins (MS24665) provide absolute, positive mechanical locking.

                  Cotter Pin (MS24665) Installation Methods

            [ Wrap-Around Method: Preferred ]        [ Over-The-Top Method: Optional ]

                   ┌──────────────┐                         ┌──────────────┐
                   │ ┌───┐  ┌───┐ │                         │ ┌───┐  ┌───┐ │
                   │ │   │  │   │ │                         │ │   │  │   │ │
            ───────┴─┘   └──┴───┴─┴───────           ───────┴─┘   └──┴───┴─┴───────
            ◄── Prong bent down flat                     ▲ Prong bent up over
                against nut flat                         │ bolt end (max dia)
            ──────────────────────────────           ──────────────────────────────
            Prong bent down over bolt end ──►            ▼ Prong bent down flat
            (does not exceed bolt dia)                   │ against nut flat

Installation Methods

1. Wrap-Around Method (Preferred): One prong is bent downward flat against the side flat of the nut. The other prong is bent over the end of the bolt and trimmed so that it does not extend beyond the bolt's outer diameter. Prongs must never project beyond the nut profile where they can snag wiring, hoses, or technician hands.
2. Over-The-Top Method (Alternative): One prong is bent straight up over the top of the bolt end (trimmed flush with the center of the bolt). The other prong is bent straight down flat against the nut flat.

The Mandatory Alignment Protocol: The Never-Back-Off Rule

When torquing a castellated nut, the castellations in the nut crown must align precisely with the cross-drilled hole in the bolt shank to accept the cotter pin.

• STRICT RULE: NEVER LOOSEN (BACK OFF) A NUT TO ACHIEVE PIN-HOLE ALIGNMENT.
• Loosening the nut destroys the established preload tension and clamping force.
• Correct Alignment Procedure:
  1. Torque the nut to the minimum specified torque in the Aircraft Maintenance Manual (AMM).
  2. Inspect alignment between the nut slot and the bolt hole.
  3. If not aligned, gradually increase torque while searching for alignment, continuing up to the maximum specified torque limit.
  4. If the hole does not align within the allowable torque range, remove the nut and add an AN960 thin washer (AN960L light series), change the washer thickness, or try a different castellated nut.

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Self-Locking Nuts: Operational Limits & Inspection

Self-locking nuts maintain clamping force through internal frictional interference (prevailing drag torque) between the bolt threads and the locking element.

Fiber / Nylon Insert Stop Nuts (AN365 / MS20365)

• Operating Temperature Boundary: The resilient nylon collar softens and permanently loses its gripping elasticity when exposed to elevated temperatures. Nylon stop nuts are strictly limited to an operating temperature of 121°C (250°F).
• Strict Prohibitions: Prohibited in engine compartments, nacelles, thrust reverser linkages, landing gear brake assemblies, and exhaust shroud fairings.
• Continuous Rotation Prohibition: Prohibited on any bolt or joint subject to continuous relative rotation (e.g., flight control cable pulley axles, bellcrank pivots, landing gear torque-link hinges). Relative rotation acts directly against the locking friction, spinning the nut off.
• Bolt Thread Projection Rule: The bolt threads MUST protrude at least 1.5 to 2 full threads (or the complete chamfer of the bolt end plus one full thread) through the top of the locking insert, subject to a maximum of 3 threads limit (no more than 3 threads protruding) to avoid excess unengaged bolt weight and interference. An unexposed thread indicates insufficient engagement, risking thread stripping and loss of locking action.
• Reusability and Friction Drag Test: A nylon stop nut can be reused ONLY if it passes a prevailing torque test. If the nut can be spun onto the bolt using bare fingers past the nylon locking insert, the nut is worn beyond airworthiness limits and must be scrapped immediately.

All-Metal Stiffnuts (MS21042 / NAS1291)

• Design: Manufactured entirely from carbon alloy steel, corrosion-resistant stainless steel, or high-temperature nickel superalloys (A286, Inconel 718). The locking action is achieved by crimping the upper threaded collar into an out-of-round elliptical geometry or slitting the crown into resilient slotted beam segments.
• Temperature Resilience: Retain full prevailing torque locking capability at temperatures ranging from 230°C (450°F) for cadmium-plated steel up to 650°C (1200°F) for Inconel.
• Applications: Powerplant engine mountings, turbine exhaust nozzles, turbocharger wastegates, and high-temperature firewalls.

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Auxiliary Locking Devices

1. Tab Washers (Locking Plates):
   • Thin sheet-metal washers featuring one or more external tabs. One tab is bent down into a mating hole, slot, or over the component edge to prevent washer rotation. After tightening the hex bolt or nut, another tab is bent upward flat against a wrenching flat.
   • Strict Airworthiness Rule: SINGLE-USE ONLY. Bending work-hardens the sheet metal. Straightening and re-bending a tab induces micro-fissures that cause the tab to fracture in flight. Tab washers must be discarded and replaced upon removal.
2. Spring Lock Washers (Split-Ring Washers):
   • Split helical spring washers provide locking tension through spring deflection and sharp cutting edges that bite into mating surfaces.
   • Aircraft Proscription: Prohibited on primary flight structures, critical joints, or thin aluminium skins where the biting edges cause stress corrosion cracking. Permitted only on non-critical, non-structural accessories.
3. Pal Nuts (Stamped Lock Nuts):
   • Lightweight, single-thread stamped spring-steel lock nuts spun on top of a standard plain nut with the concave side facing the plain nut. Once seated, it is tightened an additional 1/3 to 1/2 turn with a wrench.

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Safety Wiring (Lockwiring) Standards

Safety wiring is the universal positive locking method used to secure two or more adjacent threaded fasteners, electrical connector shells, and fluid fittings.

                    Aircraft Double-Twist Safety Wiring

         Fastener 1                                   Fastener 2
       ┌────────────┐                               ┌────────────┐
       │ (Drilled)  │                               │ (Drilled)  │
       │     ○══════╪═══════════════════════════════╪══════○      │
       └────────────┘  6 to 8 Twists Per Inch (TPI) └──────┬─────┘
             ▲                                             │
             │                                             ▼
       Wire tension pulls                            Pigtail Termination:
       Fastener 1 CLOCKWISE                          3 to 6 twists (1/4" - 1/2"),
       (Tightening direction)                        bent UNDER toward head

1. Safety Wire Materials and Sizing

Aircraft safety wire is manufactured from high-ductility, annealed corrosion-resistant steel (CRES, MS20995C / ASTM A580), Monel (for magnetic compass isolation), or Inconel (extreme powerplant heat):

• 0.020 inch (0.5 mm): Small screws, electrical connectors, closely spaced fasteners ($< 2\text{ in}$ apart), and emergency shear wire on fire handles.
• 0.032 inch (0.8 mm): The universal standard aircraft safety wire. Used on structural bolts, nuts, engine casing studs, and high-pressure fluid couplings.
• 0.041 inch (1.0 mm): Heavy-duty fasteners: propeller hub bolts, landing gear pivot retention pins, wheel axle nuts, and large fluid valves.

2. Double-Twist Method Rules

• Direction of Pull: The safety wire must always be routed so that the tension of the wire pulls the fastener in the TIGHTENING DIRECTION (clockwise for standard RH threads). Any tendency of the bolt to loosen must be actively resisted by increasing wire tension. An over-the-head or around-the-head pull must be used.
• Twist Rate: Safety wire must be twisted at a uniform rate of 6 to 8 twists per inch (TPI). Under-twisting leaves excess slack that allows the bolt to back out slightly before tension develops. Over-twisting work-hardens the wire, creating micro-cracks that snap under engine vibration.
• Maximum Fasteners in a Run: In a single wire run, a maximum of three fasteners may be secured together in a series. If fasteners are spaced widely ($> 6\text{ in}$ apart), individual pairs (two fasteners) must be wired.
• Pigtail Termination: The safety wire terminates in a pigtail consisting of 3 to 6 twists (approximately 1/4 to 1/2 inch long). The pigtail must be cleanly clipped and bent downward and inward under the bolt head. Exposed, straight wire ends are severe puncture and laceration hazards for maintenance personnel and constitute dangerous Foreign Object Debris (FOD).

3. Single-Wire Method

Permitted only on small, closely spaced fasteners in a closed, compact geometric pattern (e.g., round electrical connector shells, instrument bezel rings, or emergency access panels), with a maximum wire length of 24 inches.

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Fastener Torque Mechanics: Tension versus Friction

The primary purpose of torquing a structural fastener is to induce a controlled axial tensile preload ($F_p$) in the bolt shank. This preload stretches the bolt elastically, clamping the joint sheets together with immense compressive force. Proper clamping prevents cyclic fatigue separation, joint fretting, and bolt shear loading.

The Friction Breakdown

When a technician applies torque ($T$) with a torque wrench, only a small fraction is converted into clamping preload:

$T_{\text{applied}} = T_{\text{thread friction}} (\approx 40\%) + T_{\text{under-head friction}} (\approx 45\%) + T_{\text{bolt preload tension}} (\approx 10\% - 15\%)$

Because approximately 85% to 90% of applied torque is consumed overcoming friction, any alteration in surface condition drastically alters bolt clamping force.

Dry Torque versus Lubrication Correction

• Standard Torque Tables: Manufacturer maintenance manual (AMM) and FAA AC 43.13-1B torque tables are calculated for clean, degreased, dry cadmium-plated threads unless explicitly stated otherwise.
• The Lubrication Hazard: If anti-seize compound, engine oil, hydraulic fluid, or thread locking sealant is applied to threads rated for dry torque, friction drops by 50% or more. Applying specified dry torque to a lubricated fastener converts excessive energy into bolt elongation, stretching the bolt past its elastic yield point, stripping threads, or snapping the shank. Never lubricate dry-torque fasteners unless authorized by the AMM; if lubrication is specified, torque must be reduced by 20% to 50% according to AMM charts.

Tare Torque (Prevailing Drag Torque) Addition

When installing self-locking nuts, the friction generated by the nylon collar or crimped metal crown must be overcome before the nut seats against the washer:

1. Run the self-locking nut onto the bolt until the locking element is fully engaged, but before the nut contacts the bearing washer.
2. Rotate the nut with a calibrated torque wrench and read the running friction value: this is the tare torque (prevailing drag torque, $T_{\text{drag}}$).
3. Add this drag torque to the specified AMM assembly torque ($T_{\text{spec}}$):

$T_{\text{final}} = T_{\text{spec}} + T_{\text{drag}}$

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Torque Wrench Extensions and Adapters

In confined engine compartments or structural bays, a torque wrench cannot always engage a bolt head directly. Technicians must attach a drive extension, crowfoot adapter, or box-end adapter. When an adapter extends the centerline distance beyond the wrench drive square, the effective lever arm is increased, multiplying the torque applied to the fastener.

                      Torque Wrench Extension Geometry

      ◄───────────────────── L ─────────────────────►◄───── E ─────►
      ○═════════════════════════════════════════════■═════════════●
      ▲                                             ▲             ▲
      │                                             │             │
   Hand Pivot                                  Drive Square   Fastener Center
   (Point of Force)                                            (Actual Torque Ta)

The Torque Extension Formula

To ensure the actual torque delivered at the fastener ($T_a$) matches the AMM specification, the technician must lower the setting on the torque wrench scale ($T_w$) using the formula:

$T_w = T_a \times \frac{L}{L + E}$

Where:

• $T_w$ = Torque setting indicated on the torque wrench barrel
• $T_a$ = Desired actual torque to be applied at the fastener center
• $L$ = Length of the torque wrench measured from the center of the handle grip (pivot) to the center of the drive square
• $E$ = Center-to-center length of the extension adapter measured from the drive square to the center of the fastener socket, aligned parallel to the wrench axis

Practical Extension Example

An AMM requires an actual torque ($T_a$) of 360 in-lb on an engine cylinder hold-down nut. The technician uses a torque wrench with an effective length ($L$) of 15 inches and an in-line crowfoot adapter ($E$) measuring 3 inches:

$T_w = 360 \times \frac{15}{15 + 3} = 360 \times \frac{15}{18} = 360 \times 0.8333 = 300\text{ in-lb}$

Setting the torque wrench scale to 300 in-lb delivers exactly 360 in-lb of actual torque at the nut.

The 90° Adapter Exception ($E = 0$)

If the crowfoot adapter is positioned at an exact 90-degree right angle relative to the longitudinal axis of the torque wrench, the effective length along the lever arm remains unchanged ($E = 0$). Therefore:

$T_w = T_a$

Positioning adapters at 90° eliminates mathematical calculations and setup errors, making it standard aerospace line maintenance practice.

Torque Wrench Care and Storage

• Micrometer Clicker Wrenches: After completing maintenance, micrometer clicker torque wrenches must always be wound down to their lowest scale marking (or zero). Leaving the wrench set at high torque keeps internal compression springs compressed, causing spring relaxation, permanent calibration drift, and severe under-torquing on subsequent airframes.
• Calibration Cycles: Torque wrenches must be calibrated every 6 to 12 months (or sooner if dropped) and stamped with a valid calibration expiry sticker.

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Aircraft Maintenance Scenarios & Common Exam Traps

Maintenance Scenario: A technician installs a new CRES fuel pump flange using NAS1291 all-metal locknuts. The AMM calls for a final assembly torque of 85 in-lb. Before the nut touches the flange, the technician measures a prevailing drag torque of 12 in-lb on the torque wrench. If the technician stops tightening at 85 in-lb on the wrench dial, the actual clamping torque delivered to the joint is only $85 - 12 = 73\text{ in-lb}$, resulting in inadequate joint preload and in-service fuel leaks. The correct setting is $85 + 12 = 97\text{ in-lb}$.

Exam Warning / Common Trap:

• Trap 1: Nyloc Temperature Ceiling: Exams frequently test the maximum service temperature for nylon stop nuts. It is strictly 121°C (250°F). Selecting 230°C (which applies to metal stiffnuts) is a common failure.
• Trap 2: Never Back Off a Nut: If a castellated nut slot does not align with the cotter pin hole, NEVER loosen it. Tighten within the allowable torque range, or swap washers.
• Trap 3: Pigtail Orientation: Safety wire pigtails must be bent DOWN and UNDER the bolt head. Leaving pigtails sticking out fails regulatory safety audits and Module 06 exams.
• Trap 4: 90° Extension Rule: When an adapter is mounted at 90° to the wrench handle, $E = 0$ and wrench setting equals fastener torque ($T_w = T_a$).