6.2 Crimp Terminations, Connector Pin Insertion & Extraction Tooling

6.2 Crimp Terminations, Connector Pin Insertion & Extraction Tooling

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 modern aerospace maintenance, electrical reliability depends directly upon the integrity of thousands of individual wire terminations and multi-pin connector assemblies. A single high-resistance contact or intermittent pin connection can disrupt flight-critical fly-by-wire signaling, navigation buses, or engine Full Authority Digital Engine Control (FADEC) channels. EASA Part-66 Module 7 demands deep practical and theoretical mastery of cold-weld crimping mechanics, calibrated tooling standards, connector assembly architecture, and non-destructive contact insertion and extraction techniques.

---

Principles of Crimp Terminations vs. Soldering in Aviation

Historically, soldering was the primary method for terminating electrical conductors. However, in modern aviation maintenance, crimping has almost entirely superseded soldering for structural wiring looms.

The Physics of Crimping: Gas-Tight Cold Welding

Crimping is the controlled mechanical compression of a contact barrel around a stranded wire conductor. Under precision mechanical force:

1. The cylindrical contact barrel and the multi-strand copper wire undergo severe plastic deformation.
2. High-pressure friction shears away microscopic surface oxide films from both the inner barrel wall and the individual wire strands.
3. Clean, virgin metal surfaces are pressed together under pressures exceeding the yield strength of the materials, establishing intimate metal-to-metal contact at the atomic level—a true cold weld.
4. All air voids between strands are eliminated, producing a gas-tight joint. Because oxygen, moisture, and corrosive atmospheric contaminants cannot penetrate the compressed bundle, internal corrosion cannot occur, ensuring permanent low electrical resistance (<0.005 Ω).

Why Soldering is Restricted in Aircraft Looms

Soldering introduces several severe failure mechanisms in airborne environments:

• Solder Wicking and Vibration Fatigue: When molten solder is applied to a stranded conductor, capillary action draws liquid solder up beneath the wire insulation jacket. Upon cooling, the flexible stranded wire becomes a solid, rigid metal bar. In high-vibration aircraft environments, the boundary where the solder terminates beneath the insulation acts as an intense mechanical stress riser. Normal engine and airframe vibration causes rapid work hardening, metal fatigue, and catastrophic fracture of the wire strands at this rigid-to-flexible transition point.
• Corrosive Flux Residues: Solder joints require chemical flux to dissolve surface oxides. Even mild rosin fluxes can degrade or trap moisture over years of thermal cycling, causing galvanic corrosion.
• Operator Sensitivity and Thermal Damage: Hand soldering introduces extreme variability in temperature, dwell time, and solder volume. Overheating damages wire insulation, degrades conductor plating, and embrittles copper.

Solder Termination Hazard:
[Flexible Stranded Wire] ===> [Rigid Solder Wicking Zone] ===> [Contact]
                                         ^
                                         |--- Stress Riser: Vibration Fatigue Breakage Point!

Cold-Welded Crimp Joint:
[Flexible Stranded Wire] ===> [Crimp Barrel: Cold-Welded Metal] ===> [Contact Pin]
  (Zero Wicking; Wire retains full flexibility up to the barrel entrance)

---

Calibrated Ratcheting Crimp Tools & Gaging Standards

Aviation crimp terminations must never be made using uncalibrated automotive pliers or non-ratcheting hand tools. Aircraft maintenance organizations utilize precision crimp tooling governed by military and international standards, most notably MIL-DTL-22520 and manufactured by the Daniels Manufacturing Corporation (DMC).

The Full-Cycle Ratchet Mechanism

Certified crimp tools—such as the standard DMC AF8 (M22520/1-01) for wire gauges AWG 12 to 26 and the miniature DMC AFM8 (M22520/2-01) for micro-contacts AWG 20 to 32—incorporate an internal precision ratchet mechanism:

• Operating Principle: Once the technician begins squeezing the tool handles, the ratchet pawl engages. The tool cannot be opened or released until the handles complete their full stroke and the internal indenters travel to their absolute, calibrated depth.
• Safety Rationale: This mechanism eliminates human error. A technician cannot perform a partial crimp due to fatigue or haste; every crimped contact receives 100% of the designed mechanical compression.

Turret Heads and Positioners

Different contact part numbers and wire diameters require specific crimping depths and precise axial positioning within the tool frame:

• Turret Heads (AF8): The M22520/1-02 turret head snaps onto the tool. Rotating the turret indexes the contact stop to match specific contact families.
• Positioners (AFM8): Bayonet-style positioners (K-series) lock onto the miniature tool frame, establishing exact crimp height and pin retention.
• Selector Knob: Both tools feature a spring-loaded selector dial (settings 1 through 8) that adjusts indenter travel. Technicians must cross-reference the data plate on the turret/positioner to match the exact wire gauge to the correct selector number.

Gaging with Go / No-Go Pin Gauges

Before issuing or using a crimp tool, technicians must verify internal indenter wear using a calibrated cylindrical Go/No-Go pin gauge (e.g., DMC G125 for the AF8 on selector setting 4):

1. The tool handles are closed until the ratchet reaches its final click before release.
2. The green "Go" pin is inserted into the indenter orifice: it must enter freely and rotate smoothly without forcing.
3. The red "No-Go" pin is presented to the indenter orifice: it must NOT enter the opening under light manual pressure.
4. If the Go pin binds, or the No-Go pin penetrates between the indenters, the tool is out of calibration tolerance, must be tagged out of service immediately, and quarantined for overhaul.

---

Visual Inspection Criteria for Crimped Contacts

Prior to inserting crimped contacts into connector cavities, 100% of terminations must undergo meticulous visual inspection under magnification.

Wire Stripping Precision

Conductors must be stripped using precision mechanical strippers with calibrated blades or thermal wire strippers (which melt insulation without contacting metal). Conductor strands must never be scored, nicked, flattened, or severed. For wire gauges AWG 18 and smaller, zero nicked or cut strands are permitted under any circumstances.

Four Critical Visual Inspection Checkpoints

1. Inspection Sight Hole: The contact barrel features a small drilled inspection window. The technician must visually confirm that the stripped copper conductor strands are fully visible through the window, proving that the wire was inserted to its full depth.
2. Conductor Integrity: Every single conductor strand must be enclosed within the barrel. Zero stray, splayed, or "bird-caged" strands outside the contact are permitted.
3. Insulation Gap Clearance: The exposed bare wire between the rear of the contact barrel and the edge of the wire insulation jacket must measure between 1/64 inch and 1/32 inch (0.4 mm to 0.8 mm)—roughly equivalent to the wire insulation thickness. An excessive gap risks short-circuiting with adjacent pins; zero gap indicates that the wire insulation was pushed inside the crimp barrel, preventing metal-to-metal bonding.
4. Axial Alignment: The contact mating pin or socket must remain straight within 1° of the contact barrel axis, free of bulging, cracks, or bending.

---

Destructive Tensile Pull Testing Requirements

To ensure crimping quality in harness manufacturing facilities and during periodic maintenance audits, sample crimps undergo destructive tensile pull testing on calibrated pull testers. The joint must withstand specific mechanical separation forces without the wire pulling out of the barrel or the strands rupturing below minimum limits.

---

Circular Multi-Pin Connectors & Shell Architecture

Multi-pin connectors allow electrical harnesses to disconnect rapidly for LRU replacement, engine removal, and structural breakdown.

Primary Aviation Connector Specifications

• MIL-DTL-38999 (Series I, II, III, IV): The premier modern aerospace circular connector standard. Available in composite, aluminium, and stainless steel shells.
• MIL-DTL-26482 (Series I & II): Widely used miniature circular connector family.
• EN 3645: European aerospace equivalent to MIL-DTL-38999 Series III with metric thread backshells.

Coupling Mechanisms

• Bayonet Coupling (MIL-DTL-38999 Series I & II, 26482): Mates with a quick 1/3-turn twist. Stainless steel pins ride in spiral ramps on the receptacle shell, locking into a detent with an audible click and visual alignment dots. Ideal for rapid disconnect and tight spaces.
• Threaded Coupling (MIL-DTL-38999 Series III / EN 3645): Utilizes a heavy-duty, self-locking triple-start ACME thread. Incorporates an internal ratcheting mechanism that prevents vibration back-off without requiring external lockwire. Preferred in extreme vibration zones (engine cowlings and landing gear).

Master Keyways and Clocking Positions

Where multiple identical connectors are installed on a single bulkhead or junction box, there is a severe danger of cross-connection (e.g., connecting a 115 V AC power harness into a 5 V DC sensor receptacle). To make cross-connection physically impossible, connectors incorporate master keys and alternate clocking positions:

• Connectors feature one fixed master keyway and secondary polarizing keys.
• In the standard Normal (N) orientation, keys are positioned at default angular spacings.
• Alternate orientations—designated by letters A, B, C, D, and E—rotate the secondary keyways by specific angular degrees relative to the master key. Plugs clocked to position "A" physically cannot engage receptacles clocked to position "B" or "N", even if the shell size and contact arrangement are identical.

Connector Polarization & Clocking Concept:
   [Position N: 0° Shift]       [Position A: 35° Shift]       [Position B: 75° Shift]
        | Master                      | Master                      | Master
     +--+--+                       +--+--+                       +--+--+   
   |       |                     |         |                   |           |
   +--+----+                     +----+----+                   +------+----+ 
      | Secondary                     | Secondary                     | Secondary
   (Physical mismatch prevents cross-connecting adjacent identical-shell connectors)

Contact Construction & Gold Plating

Contacts consist of high-conductivity machined copper alloy pins (male) and sockets (female). Sockets incorporate an internal beryllium-copper spring clip or closed-entry hood that exerts continuous mechanical gripping force on the mating pin. Contacts are plated with 50 micro-inches of gold conforming to MIL-G-45204 over a high-purity nickel underplate. Gold provides permanent corrosion immunity and exceptionally low contact resistance (<0.005 Ω); the nickel layer acts as a vital diffusion barrier that prevents copper atoms from migrating through the gold to the contact surface.

---

Contact Sizing & Universal Colour Coding

Connector contacts, wire gauges, and plastic servicing tools are universally categorized by contact size numbers. The contact size corresponds to the maximum wire gauge it can accommodate:

---

Insertion and Extraction Tooling & Techniques

Contacts are retained inside the connector's rigid dielectric insert (hard plastic or diallyl phthalate) by miniature internal beryllium-copper retention tines.

Tool Operation: Plastic vs. Metal Tools

• Plastic Servicing Tools (MIL-I-81969): Double-ended, colour-coded plastic tools are the standard for maintenance. One end is moulded in natural white plastic for insertion; the opposite end is colour-coded (Red, Blue, Yellow) for extraction.
• Insertion Procedure:
  1. The wire is laid into the longitudinal slot of the white insertion tool.
  2. The tool shoulder is slid forward against the rear retention shoulder of the contact.
  3. The contact is pushed squarely through the rear elastomeric grommet until an audible "click" signifies that the retention tines have expanded and snapped into the contact retention groove.
  4. The tool is withdrawn, and the wire is given a light pull (approx. 2 to 5 lbs) to confirm positive mechanical lock.
• Extraction Procedure:
  1. The wire is laid into the slot of the coloured extraction end.
  2. The tool is slid forward along the wire through the rubber grommet until the tubular tip expands the internal metal retention tines away from the contact shoulder.
  3. With the tines spread open, the wire and contact are gently withdrawn from the rear of the connector.
• Metal Precision Tools: Spring-loaded metal tweezers (e.g., DMC M81969/1 series) are used in high-density connectors, but require extreme care to avoid tearing the elastomeric sealing webs.

---

Backshells, Strain Relief, Grommets & Sealing Plugs

• Backshell Assemblies: Screwed onto the rear metric/ACME threads of the connector shell. Backshells provide 360° electromagnetic shielding termination (via cone clamps or tinel-lock rings) and house heavy-duty saddle clamps that absorb mechanical strain.
• Rear Environmental Grommet: The rear of the insert features a bonded fluorosilicone rubber grommet with concentric internal sealing rings that compress tightly around each wire insulation jacket, preventing moisture, fuel, and de-icing spray ingress.
• Mandatory Sealing Plugs (MS27488): In modern aircraft connectors, every unused cavity must be fitted with an approved Teflon or plastic sealing plug. The plug is inserted large head outward into the rear grommet until the shoulder seats. Leaving an unused cavity open allows water, hydraulic fluid, and atmospheric contaminants to pool inside the connector, resulting in electrolytic corrosion, pin bridging, and catastrophic phase shorts. In critical applications, an unwired contact is installed beneath the plug to support the retention clip.

---

Soldering Electrical Connections in Aviation (Where Permitted)

While crimping is standard, soldering remains authorized for specific applications: solder sleeves (heat-shrinkable shield termination sleeves with pre-fluxed solder rings), hermetically sealed relays, and legacy avionic instrument boards.

Aviation Soldering Rules

• Approved Fluxes: Only non-corrosive rosin flux (Type R, RMA, or ROL0/ROM0 conforming to J-STD-004) is permitted. Acid-based or active chloride fluxes are strictly forbidden on aircraft.
• Temperature Control: Soldering irons must be temperature-regulated (typically 315°C to 370°C / 600°F to 700°F).
• Thermal Shunts (Heat Sinks): Copper clip-on heat sinks must be attached to conductor leads between the joint and delicate electronic components or wire insulation. This dissipates excess heat, preventing component destruction and stopping solder wicking beneath the insulation.

---

Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

A certifying engineer is assembling a replacement 55-pin MIL-DTL-38999 Series III connector for an engine Electronic Engine Control (EEC) harness. The harness contains thirty AWG 22 wires, twelve AWG 20 wires, and eight AWG 16 wires, leaving five cavities unused.

The technician verifies the DMC AFM8 crimp tool using a G125 Go/No-Go pin gauge, sets selector position 4 for the Size 20 contacts, strips the wires with a thermal stripper, and checks that every wire strand is visible in the inspection sight hole. Using the red plastic tool (Size 20) and blue plastic tool (Size 16), all wired contacts are clicked into position and pull-tested. For the five unused cavities, the engineer installs unwired dummy contacts and inserts red and blue MS27488 sealing plugs into the rear grommet. Finally, the triple-start threaded coupling ring is torqued onto the EEC receptacle.

Common Exam Traps

• Trap 1: Forcing a crimp tool open when it locks mid-stroke. The ratcheting mechanism is designed to prevent release until full compression is achieved. If a tool jams, it indicates incorrect contact size or foreign debris; forcing it open with screwdrivers destroys calibration.
• Trap 2: Neglecting sealing plugs in unused cavities. Exam questions frequently ask what must be installed in spare connector cavities. Omitting sealing plugs allows moisture ingress that destroys the connector.
• Trap 3: Using extraction tools to push contacts in. The extraction tool expands internal retention tines. Using it to insert contacts bends the tines, permanently ruining the connector insert.