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newSchoolWorks.com › VoTechWorks › Transportation › Aviation Maintenance › Avionics: Aircraft Electrical, Instrument, and Communication Systems › Lesson 2
TRN-AVIA22-04 · Lesson 2 · 45 clock hours · lab

Aircraft Wiring, Connectors, and Fabrication

Objective. Students will fabricate a four-circuit aircraft wiring harness segment — including correct wire gauge selection, splicing, terminal crimping, and connector assembly — that passes continuity and insulation-resistance tests to MIL-SPEC and manufacturer tolerances.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Aircraft Wiring, Connectors, and Fabrication

Aircraft wiring is safety-critical. A poorly selected wire gauge can overheat and start a fire; a bad crimp can cause an intermittent open that shuts down an instrument at the wrong moment. Every step in fabricating a wiring harness — gauge selection, stripping, splicing, crimping, and connector assembly — must follow AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair) and the applicable manufacturer data.

Wire Gauge Selection

Aircraft electrical wire is sized by two independent limits: ampacity (how much current the wire can carry continuously without overheating) and voltage drop (how much resistance the wire adds to the circuit). Both must be checked; the larger gauge (smaller AWG number) that satisfies either limit governs.

AC 43.13-1B provides wire-sizing tables organized by current load (amps) and circuit length. To use them:

1. Identify the continuous current draw of the load.

2. Identify the one-way wire length in feet.

3. Read the minimum wire gauge from the bundle or open-air table, whichever applies to your installation (wires in a bundle run hotter and may need to be derated).

4. Cross-check that the selected gauge is also adequate for the allowable voltage drop (typically 1 V for essential bus circuits and 2 V for non-essential circuits in 28 V systems, though always verify against the specific aircraft data).

Always document the rationale — note the current, length, table used, and final gauge chosen — so another technician can audit your decision.

Aircraft-Grade Wire

Do not substitute automotive wire in aircraft. Aircraft wire such as MIL-W-22759 uses high-strand-count conductors for vibration resistance and insulation rated for the aircraft environment (temperature, fluids, abrasion). Identify wire by the part number and specification printed on the insulation before cutting.

Stripping and Splicing

Use a wire stripper calibrated for the exact gauge. Nicks in the conductor strands are cause for rejection — a nicked strand concentrates current and can fail under vibration. Strip length is determined by the terminal or splice manufacturer; follow it exactly. For splices, use an environmentally sealed, crimp-type splice sleeve rated for the wire gauge. Never use solder as the primary means of making an aircraft electrical connection unless specifically called out in the aircraft maintenance manual, because solder wicks into the stranded wire, creates a rigid zone, and can crack under vibration.

Crimping

A ratchet crimper is required for aircraft terminal work. The ratchet mechanism prevents a partial crimp — the tool will not release until the dies have fully closed to the designed geometry. Using a non-ratcheting tool is not acceptable.

After crimping, perform a pull test with a calibrated pull-test gauge. Pull-force minimums are published by the connector manufacturer (often also listed in MIL-DTL-22992 for D-sub style connectors). A properly crimped terminal on a typical 20 AWG pin will require significant force to pull free; consult the table and record the actual reading against the minimum.

Apply heat-shrink tubing over splices and any exposed conductor areas. Use a heat gun — not an open flame — and verify the tubing has fully recovered without scorching.

D-Sub Connector Assembly

D-sub connectors use individual crimp pins or sockets inserted into a plastic insert with a pin-retention tool. Insert each pin until you feel the retention clip engage (a positive click or resistance to rearward movement). Use the correct extraction tool to remove any misplaced pin; never force it out by pulling on the wire. After assembly, perform a pin-retention check: push gently rearward on each pin and verify it does not back out.

Continuity and Insulation Resistance Testing

Continuity testing with a multimeter verifies that each circuit is complete (low resistance from end to end) and confirms there are no shorts between adjacent circuits. Measure from pin to pin with the harness disconnected from any aircraft system. Record the reading; a good conductor will show near-zero ohms.

Insulation resistance (IR) testing with a megohmmeter checks that the insulation between conductors (and between conductors and the harness shield or ground) is intact. AC 43.13-1B specifies minimum IR values; readings below the minimum indicate damaged or contaminated insulation. Apply the megohmmeter test voltage appropriate for the wire rating — always verify voltage setting before connecting, because applying excessive voltage to low-voltage-rated wire can itself damage insulation. Record every reading on the inspection worksheet.

Chafing Protection and Routing

Wires must not contact sharp edges, hot surfaces, or moving parts. Secure the harness with adel clamps or tie-wraps at intervals consistent with AC 43.13-1B routing guidelines. Inspect for any location where the outer jacket could rub against structure and add spiral wrap or a grommet as needed.

Safety in the Lab

Completing all three pass criteria — continuity, insulation resistance, and visual inspection — is required because each addresses a different failure mode. A harness that passes electrically but has an unprotected chafe point is still a fire risk.

Key terms10

AmpacityThe maximum continuous current a wire can carry without exceeding its rated temperature, used as one criterion in wire gauge selection.
AC 43.13-1BFAA Advisory Circular 43.13-1B, the primary reference for acceptable aircraft inspection and repair methods, including wire sizing, routing, and splicing.
Ratchet crimperA hand tool with a mechanism that prevents release until the dies have fully closed, ensuring every crimp reaches the required geometry.
Pull testA quality check in which a calibrated gauge applies a specified tensile force to a crimped terminal to verify the crimp will not separate in service.
MegohmmeterAn instrument that applies a high DC test voltage and measures insulation resistance in megohms, used to detect damaged or contaminated wire insulation.
Insulation resistance (IR)The resistance measured between conductors or between a conductor and ground through the insulating jacket; low values indicate degraded insulation.
D-sub connectorA multi-pin electrical connector with a characteristic D-shaped metal shell, widely used in aircraft avionics and instrument wiring.
ESD wrist strapA grounding strap worn on the wrist to equalize the technician's electrostatic potential with the work surface, preventing discharge damage to electronic components.
Voltage dropThe loss of voltage across a wire's resistance under load; excessive drop can cause equipment malfunction and is a second constraint in wire gauge selection alongside ampacity.
MIL-DTL-22992A military specification covering circular threaded and D-sub style electrical connectors, including crimp pin pull-force requirements used to qualify connector assemblies.

Worked examples4

1. A landing-light circuit carries 15 A continuously. The one-way wire run is 12 feet, routed in an open-air (single wire) installation on a 28 V system. Using AC 43.13-1B Table 11 logic, determine the minimum acceptable wire gauge and verify the voltage drop is within 2 V.
  1. Step 1 — Ampacity check: From AC 43.13-1B, a single wire in open air rated at 15 A continuous falls within the capacity of 20 AWG for some conditions, but the table typically requires 18 AWG to carry 15 A with adequate margin in aircraft applications. Select 18 AWG as the ampacity candidate.
  2. Step 2 — Voltage drop check: Use the formula V_drop = (conductor resistance per foot × 2 × length × current). 18 AWG copper has approximately 0.00640 Ω per foot. Round-trip length = 2 × 12 ft = 24 ft. V_drop = 0.00640 × 24 × 15 = 2.304 V. This exceeds the 2 V limit for a non-essential circuit.
  3. Step 3 — Upgrade to 16 AWG: 16 AWG has approximately 0.00403 Ω per foot. V_drop = 0.00403 × 24 × 15 = 1.45 V. This is within the 2 V limit.
  4. Step 4 — Select and document: Choose 16 AWG. Record: load = 15 A, one-way length = 12 ft, ampacity check passed at 18 AWG, voltage drop at 18 AWG = 2.30 V (exceeds limit), voltage drop at 16 AWG = 1.45 V (within limit). Governing criterion: voltage drop. Final gauge: 16 AWG.
Answer: 16 AWG is the minimum acceptable gauge for this circuit; 18 AWG would exceed the 2 V voltage-drop limit for this run length and current.
2. A 20 AWG crimp pin in a D-sub connector is pull-tested. The manufacturer's table specifies a minimum pull force of 8 lbf for 20 AWG. The technician reads 6.5 lbf on the pull-test gauge. What is the outcome, and what is the next action?
  1. Step 1 — Compare measured value to minimum: Measured = 6.5 lbf; minimum required = 8 lbf. 6.5 < 8, so the crimp fails the pull test.
  2. Step 2 — Determine next action: A failed pull test means the crimp is defective. The pin must be extracted from the connector with the proper extraction tool, the terminal must be cut off and discarded (do not re-crimp a used pin), and a new pin must be crimped onto freshly stripped wire.
  3. Step 3 — Root-cause check: Inspect the ratchet crimper die size and verify it matches the pin series. Check that the wire was inserted to the correct depth before crimping. Recrimp with the correct tool and die, then re-test.
Answer: The crimp fails. Extract and discard the defective pin, cut back the wire, install a new pin with the correct die setting, and re-perform the pull test to confirm ≥ 8 lbf.
3. During insulation resistance testing, a 28 V aircraft wiring harness segment reads 8 MΩ between two adjacent conductors. AC 43.13-1B requires a minimum IR of 1 MΩ for serviceable aircraft wiring. Is this harness segment acceptable?
  1. Step 1 — State the minimum: AC 43.13-1B minimum IR = 1 MΩ.
  2. Step 2 — Compare: Measured IR = 8 MΩ. 8 MΩ > 1 MΩ.
  3. Step 3 — Record and conclude: The segment passes the insulation resistance requirement. Record the value, the test voltage used, ambient conditions, and the pass/fail result on the inspection worksheet.
Answer: The harness segment is acceptable; 8 MΩ exceeds the 1 MΩ minimum. Record all values on the inspection worksheet.
4. A technician needs to strip 0.25 inch of insulation from a 22 AWG aircraft wire to seat it in a crimp pin. After stripping, two of the seven visible strands appear nicked (reduced in cross-section). What is the correct action?
  1. Step 1 — Evaluate the damage: Nicked strands are cause for rejection per AC 43.13-1B because they reduce ampacity, concentrate current, and are prone to fatigue failure under vibration.
  2. Step 2 — Determine whether there is enough wire to re-strip: If wire length allows, cut back past the nicked area by at least the strip length plus a small margin, then re-strip cleanly.
  3. Step 3 — Inspect again: After re-stripping, examine all strands under good lighting or magnification. All strands must be undamaged before proceeding to crimp.
  4. Step 4 — If wire length is insufficient: Splice in a new segment using an approved, environmentally sealed crimp splice at an approved location, or replace the wire run entirely.
Answer: Reject the nicked strip; cut back and re-strip if sufficient wire remains. All conductor strands must be undamaged before crimping.

The math in this lesson2

Learners compute voltage drop as ohms per foot × round-trip length × current to pick 16 AWG over 18 AWG.
A-SSE.1N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Use an aircraft electrical load analysis chart and the FAA wire-sizing guidelines in AC 43.13-1B to select correct wire gauges for four different current loads, documenting the selection rationale in writing. · Fabricate a wiring harness segment using aircraft-grade wire, heat-shrink tubing, D-sub connectors, and a ratchet crimper; verify each crimp with a pull-test gauge to manufacturer-specified minimum pull force. · Perform continuity testing with a multimeter and insulation resistance testing with a megohmmeter on the completed harness, recording all readings on an inspection worksheet and identifying any wire that falls outside limits.
Materials: Aircraft-grade wire (various gauges), wire stripper, ratchet crimper, pull-test gauge, heat-shrink tubing, heat gun, D-sub connectors and pins, multimeter, megohmmeter, AC 43.13-1B, MIL-DTL-22992 reference sheet, safety glasses, ESD wrist strap and mat
Check: All four circuits pass continuity check (no opens, no shorts); insulation resistance meets minimum threshold per AC 43.13-1B; harness visually passes instructor inspection for chafing protection and secure terminations — all three criteria required for pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Aircraft Electrical Systems
1. When selecting wire gauge for an aircraft circuit using AC 43.13-1B, a technician must satisfy which TWO independent criteria?
Aircraft Electrical Systems
2. Why is a ratchet crimper required for aircraft terminal crimping rather than a standard plier-style crimper?
Aircraft Electrical Systems
3. After crimping a 20 AWG pin into a D-sub connector, the pull test reads below the manufacturer's minimum pull force. What is the correct corrective action?
Aircraft Electrical Systems
4. A megohmmeter is used instead of a standard multimeter for insulation resistance testing because it:
Aircraft Electrical Systems
5. A wire strip is examined and two strands of the 22 AWG conductor are found to be nicked. According to AC 43.13-1B practices, this wire should be:
Safety and Regulations
6. Which personal protective measure is MOST important when handling individual D-sub connector pins during harness fabrication?
Aircraft Electrical Systems
7. A completed four-circuit harness passes the continuity test but shows insulation resistance of 0.4 MΩ between two conductors against an AC 43.13-1B minimum of 1 MΩ. How should this harness be dispositioned?
Aircraft Electrical Systems
8. When routing a fabricated wiring harness segment past an aircraft structural bracket with a sharp edge, the BEST protection method is to:
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