3.2 Terminations, Crimping, and Environmental Splicing

3.2 Terminations, Crimping, and Environmental Splicing

Core Aviation Standard: Wire terminations are a leading source of intermittent avionics faults. AC 43.13-1B prefers pre-insulated crimp-type ring-tongue terminals installed with calibrated ratcheting tools. Soldering is still used where the connector (for example, a solder-cup connector), a solder-sleeve shield termination, or the manufacturer's data calls for it.

A quality termination must establish two simultaneous conditions: a gas-tight electrical connection of near-zero resistance and a mechanically ductile joint capable of enduring thousands of hours of high-frequency airframe vibration.

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Mil-Spec Crimping Tooling: The M22520 Architecture

Aviation wire crimping relies on the standardized military specification MIL-DTL-22520 (now SAE AS22520) tooling family. These tools replace operator discretion with calibrated mechanical precision.

Primary Tool Frame Classifications

1. M22520/1-01 (Standard Large Frame / Daniels AF8): Accommodates wire sizes AWG 12 through AWG 26. Utilizes interchangeable multi-contact turret heads. It creates an 8-indent / 4-point radial crimp pattern.
2. M22520/2-01 (Miniature Frame / Daniels AFM8): Accommodates wire sizes AWG 20 through AWG 32. Specifically engineered for high-density miniature circular connectors (MIL-DTL-38999) and D-subminiature pins. Creates an 8-indent radial crimp using dedicated bayonet-mount positioners (K-series).
3. M22520/7-01 (Intermediate Frame / Daniels MH860): Intermediate-range tool covering AWG 16 through AWG 28, common in commercial transport and business aviation retrofits.

THE M22520/1-01 (AF8) CRIMP SYSTEM:

 +---------------------------------------------+
 |  MICROMETER SELECTOR KNOB (Settings 1 to 8) |
 +----------------------+----------------------+
                        |
             +----------v----------+
             |  FULL-CYCLE RATCHET |  <-- (Will not release
             |  INTERNAL MECHANISM |       until stroke complete)
             +----------+----------+
                        |
             +----------v----------+
             |  TURRET HEAD / DIE  |  <-- (Color-coded positioner
             |  (4 Indenters)      |       aligns pin & wire)
             +---------------------+

The Full-Cycle Ratcheting Mechanism

A critical safety feature of all M22520 crimp tools is the internal full-cycle ratcheting mechanism. Once the technician initiates handle closure, the internal ratchet pawl locks the mechanism. The handles cannot be reopened until the crimping dies have travelled to their 100% fully closed position. This engineering interlock completely prevents partial or under-crimped contacts caused by operator hand fatigue or haste.

Go / No-Go Pin Gauge Calibration

AC 43.13-1B paragraph 11-178 says suitable Go/No-Go gages shall be used prior to any crimping operation and whenever possible during the operation:

• Tool Setup: Set the selector to the position the tool's gaging instructions specify and close the handles until the ratchet completes its full stroke.
• The "Go" Test: The Go gage must enter the closed indenter opening freely.
• The "No-Go" Test: The No-Go gage must not enter.
• Action: If the Go gage binds or the No-Go gage enters, reject the tool. The AC says calibration and adjustment are made only by the manufacturer or an approved calibration laboratory.

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Crimping vs. Soldering in Modern Aviation

Historically, soldering was standard in electrical assembly. Today, crimping is the preferred termination for stranded harness wire, and many OEM wiring practices allow soldering only where their data specifies it.

The Solder Wicking Failure Mechanism

1. Capillary Action: When molten solder is applied to a stranded wire terminal, capillary action draws liquid solder upward underneath the insulation jacket—a phenomenon termed solder wicking.
2. The Rigid Boundary: As the solder cools and solidifies, it fuses the flexible, finely stranded copper conductor into a rigid, solid copper rod.
3. Stress Concentration: Immediately adjacent to the point where solder wicking ends, the wire transitions abruptly from an inflexible rod to flexible stranded wire. This boundary creates an extreme mechanical stress concentration point.
4. Vibration Fatigue: Under normal flight vibration and engine acoustic resonance, the wire flexes repeatedly against this rigid boundary. The outer copper strands work-harden, develop micro-cracks, and suffer fatigue fracture, causing sudden open-circuit failures.

The Physics of Calibrated Crimping

In contrast, precision crimping creates a ductile mechanical cold-weld:

• Plastic Deformation: Radial crimp dies exert tons of pressure per square inch, plastically deforming the contact barrel and copper strands.
• Gas-Tight Interface: The metal flows together, eliminating microscopic air voids between strands. This creates a gas-tight seal that prevents atmospheric moisture and oxygen from penetrating the joint, eliminating galvanic and fretting corrosion.
• Retained Flexibility: Because heat is not used, no wicking occurs. The stranded conductor retains 100% of its mechanical flexibility directly up to the entrance of the contact wire barrel.

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Mechanical Pull-Test Tensile Strength Requirements

AC 43.13-1B paragraph 11-174 sets the goal: the tensile strength of the wire-to-terminal joint should be at least equal to that of the wire itself, and its resistance negligible compared with the wire. Shops verify this with periodic destructive pull tests on sample crimps. The required pull values come from the contact, terminal, or splice specification (for example, MIL-T-7928 for terminal lugs and splices) or the shop's process specification, so use the value for the exact part and wire size.

Failure Mode Analysis

• Conductor Tensile Break (Pass): If the conductor snaps outside or at the throat of the contact barrel at or above the minimum force, the crimp compaction is optimal.
• Conductor Pull-Out (Fail): If the intact wire slips cleanly out of the barrel below the rated pull force, the crimp was under-compressed (incorrect tool setting or oversized contact barrel).
• Conductor Shear (Fail): If the conductor shears cleanly at the crimp indentation at low pull force, the tool was set too tight, severing the conductor strands during crimp closure.

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Terminal Lugs and Terminal Block Hardware Stacking Rules

For power distribution, grounding, and heavy-duty bus tie points, wiring terminates in Pre-Insulated Diamond Grip (PIDG) ring or spade terminals mounted to barrier terminal blocks (MIL-T-55164 / MS27212).

PIDG TERMINAL CROSS-SECTION:

                 Outer Nylon Insulation Sleeve
               +-------------------------------+
   Wire =====> |  Copper Sleeve  [Serrations]  | ===> Ring Tongue
               +-------------------------------+
                 Insulation Support Sleeve (Gripping outer jacket)

• PIDG Construction: Features a tin-plated copper body with internal serrations that bite through conductor oxide films, an outer copper sleeve that supports the wire insulation jacket to prevent sharp bending, and a color-coded nylon or PVDF insulator:
  • Red: AWG 22–16 (some ranges stop at 18)
  • Blue: AWG 16–14
  • Yellow: AWG 12–10

Terminal Block Stud Stacking Rules (AC 43.13-1B Paragraph 11-174)

Connecting terminal lugs to threaded studs requires strict adherence to physical geometry and hardware sequence:

1. Maximum Four Lugs: No more than four terminal lugs, or three lugs and a bus bar, should be connected to any one stud; the common bus bar counts toward the total. When more than four terminals must be joined, mount a small metal bus across two or more adjacent studs.
2. Largest Lug on the Bottom: When the lugs on a stud vary in size, AC 43.13-1B says the greatest diameter goes on the bottom and the smallest on top. Lug stud holes should match the stud diameter.
3. Direct Face-to-Face Contact: Terminal tongues must mate directly against each other. AC 43.13-1B says spacers or washers should not be used between the tongues of terminal lugs. A washer between lugs adds contact resistance and a hot spot.
4. Lug Orientation: Position lugs so that removing the nut does not require bending a lug, and so that movement of the lugs tends to tighten the connection. Tightening must not deform the lugs or the stud.
5. Hardware Sequence:
   • Terminal Board Insulator
   • Bus Bar or Lower Stud Base
   • Terminal Lugs (1 to 4 max, stacked flat face-to-face)
   • Wide Flat Washer (to distribute clamping pressure across lug tongues)
   • Split-Lock Washer (or use an all-metal self-locking nut)
   • Plain Nut, torqued to specification

Paragraph 11-179 adds that locknuts used on electrical terminals should be all-metal, and that when a spring lock washer is used, a plain washer goes between it and the terminal to prevent galling. Paragraph 11-176 notes that torque values for terminal boards, studs, and posts normally come from the manufacturer's maintenance instructions, so use those instead of a generic table.

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Environmental Inline Splicing Techniques

In aircraft harness fabrication and battle-damage repair, cutting out a damaged conductor section and inserting an environmental inline splice is an acceptable repair under AC 43.13-1B paragraph 11-167 when it does not affect the wiring's reliability.

Splice Types

AC 43.13-1B paragraph 11-167 prefers self-insulated splice connectors. A non-insulated splice may be used if it is covered with plastic sleeving secured at both ends, or with dual-wall shrink sleeving. Environmentally sealed splices conforming to MIL-T-7928 are a reliable choice in severe wind and moisture problem (SWAMP) areas. A typical sealed heat-shrink splice (for example, the SAE AS81824 type) has:

• A plated seamless copper crimp barrel.
• A transparent, radiation-crosslinked heat-shrink outer sleeve.
• Meltable sealant rings at each end of the sleeve.

AS81824 ENVIRONMENTAL SPLICE HEATING PROFILE:

                 +-----------------------------------------+
                 |     Heat-Shrinkable PVDF Sleeve        |
+-----------+    |  +--------+                 +--------+  |    +-----------+
| Wire #1   |==> |  | Melt   |  [CRIMP BARREL] | Melt   |  | <==| Wire #2   |
| (Stripped)|    |  | Ring   |  (Plastically   | Ring   |  |    | (Stripped)|
+-----------+    |  +--------+   Deformed)     +--------+  |    +-----------+
                 +-----------------------------------------+
                                     ||
                     APPLY HOT AIR PER SPLICE MAKER (Reflector Nozzle)
                                     \/
                 +=========================================+
                 | Adhesive Melt Rings Flow & Seal Ends    |
                 | Transparent PVDF Shrinks to Form Cable  |
                 +=========================================+

Splice Installation Procedure

1. Strip conductors to the barrel depth the splice maker specifies and crimp with the calibrated tool listed for that splice.
2. Center the heat-shrink sleeve over the crimped barrel.
3. Apply uniform heat using a hot-air gun equipped with a curved ceramic or stainless-steel reflector nozzle. The reflector directs hot air 360° around the splice, preventing one-sided blistering.
4. Heat until the PVDF sleeve contracts smoothly and the colored thermoplastic sealant rings melt and flow, forming a visible 360° bead around each wire entrance. This seals the joint against moisture and fluids.

Staggering Rules for Wire Bundles

AC 43.13-1B paragraph 11-167 sets these rules:

• Stagger splices in a bundle so they minimize any increase in bundle size, which could keep the bundle from fitting its designated space or crowd maintenance access (Figure 11-18). The AC does not give a fixed stagger distance.
• Keep splices out of the 12 inches nearest a termination device, except when attaching to the spare pigtail of a potted connector, splicing multiple wires to a single wire, or adjusting wire size to fit a contact crimp barrel.
• One splice per wire segment: there should not be more than one splice in any one wire segment between two connectors or other disconnect points, except in those same cases or as part of an approved repair.
• Avoid splicing in high-vibration areas, locate splices where they can be inspected, and get engineering approval to splice individual wires in a group or bundle.
• Approved data is required to splice power wires, coaxial cables, multiplex buses, and large-gauge wire.