Custom Cable Assemblies & Wire Harnesses:
The Aerospace & Defense Design-to-Qualification Guide

The Connector Is a Component. The Assembly Is a System.

A cable assembly is where the electrical design stops being an abstraction and becomes a physical part that has to survive a temperature swing, a vibration profile, a salt fog chamber and a technician installing it in the dark. Buyers who specify the connector carefully and leave the assembly to chance are the ones who find intermittent opens at the crimp, chafed insulation at the clamp, and a shield that terminates through a drain wire long enough to be an antenna.

Every cable assembly is the same eight decisions in a different order. Get them wrong and the failure shows up somewhere else entirely — which is why the troubleshooting starts at the connector and the root cause is usually in the harness.

Step 1 — Choose the Wire Before the Connector

Wire specification drives the contact size, the backshell, the bend radius and the price. In aerospace and defense work the choice is almost always a member of a small, well-defined family.

Two rules survive every programme. First, the wire temperature rating must exceed the hottest steady-state ambient at the harness, not the ambient of the compartment it starts in. Second, size the conductor for the current at the worst-case bundled condition and altitude — a wire that is comfortable alone in free air is not comfortable in a 40-conductor bundle at 55,000 ft. Bundle derating, altitude derating and connector contact rating are three separate limits, and the lowest one governs.

Step 2 — Termination: Crimp Wins, and It Is Not Close

Crimp termination dominates aerospace harness work because it is the only method that is simultaneously repeatable, inspectable and heat-free. A calibrated crimp tool with the correct positioner produces a defined deformation of the barrel into the conductor strands — a cold weld at the strand level — and the result can be verified by a pull test and by micro-section on a sample.

• Crimp — use the tool and positioner specified for the contact, verify with the go/no-go gauge, and pull-test per the applicable wire and contact size. Never mix tooling between contact families.
• Solder — still correct for some legacy and mixed-technology joints, but it wicks into the strands, creates a stiff point right where the wire flexes, and cannot be un-done in the field.
• Welded / ultrasonic — for high-current and specialised terminations where a splice or bus joint must carry hundreds of amps without a mechanical interface.

The two workmanship defects that end up in the field are a crimp made with the wrong positioner (looks correct, tests intermittently) and a stripped length that puts the insulation either inside the barrel or too far from it. Both are caught by inspection — which means the inspection criteria must be written into the drawing, not left to the operator's judgement.

Step 3 — Backshells, Strain Relief and Shield Termination

The backshell is what makes the difference between a harness that lasts the life of the platform and one that fails at the connector. It does three jobs: it anchors the cable so no tensile load reaches the crimps, it provides the environmental seal at the rear of the connector, and it is the ground path that terminates the shield.

• Strain relief is a load path, not a clamp. Tensile load must transfer from the cable jacket into the shell through a mechanical grip, so the conductors behind the contacts never see tension. A backshell that clamps the shield braid but not the jacket will fail the first time someone pulls the harness out of a rack.
• Terminate the shield 360°, not through a pigtail. A circumferential termination through a band or compression ring keeps the return path impedance low at high frequency. A long drain wire raises transfer impedance and turns the shield into a radiator at the very frequencies it was meant to suppress.
• An M85049 accessory family covers the geometry. A shield-termination backshell, a 90° band-lock backshell and a shrink-boot adapter are three different answers to the same shell size: the accessory range covers straight, 90° and 45° exits with the correct strain relief and shield termination, and choosing the wrong exit angle forces the bend radius up against the connector — the single most common mechanical defect in a finished harness.
• Boot or overmold? A heat-shrink boot with adhesive lining is repairable and adds little mass; a moulded transition is more robust to abrasion and flexing and is the right answer for a high-vibration run or a harness that is handled daily. Decide it before the drawing is released, not after the first article.

Bend radius is a specification, not a preference. General practice for a bundled harness is a minimum bend radius of roughly six times the bundle outside diameter, and shielded or coaxial cable typically sits at the higher end of the six-to-ten-times range because a tight bend distorts the dielectric and changes the impedance. Always confirm against the cable's own specification — and remember the bend starts at the backshell, so the accessory and the clamp spacing set the real limit.

Step 4 — Routing Rules That Are Enforced After the First Failure

Wiring in an aerospace vehicle is governed by a vehicle wiring standard — in the civil world, SAE AS50881, formerly MIL-W-5088. The parts of it that cause real harness problems are the boring ones:

• Clamp spacing and support — a bundle supported at wide intervals has a natural frequency inside the vibration profile; the clamp spacing is a structural decision.
• Separation — keep power, signal and RF runs physically separated, or accept the coupling and pay for it with filtering and shielded cable.
• Chafe protection — sleeving, grommets and edge protection at every penetration and tie-down. Most field harness failures are chafe, not electronics.
• Service loops and drip loops — enough slack to disconnect and re-terminate without cutting the harness; a low point that drains fluid away from the connector rather than into it.
• Identification — wire and bundle marking to the applicable identification standard so a fault can be traced to a circuit without ringing out the whole vehicle.

Step 5 — Sealing and Environmental Protection

Sealing happens at three levels and all three have to work: the contact and interfacial seal inside the connector, the rear seal at the backshell or boot, and the jacket or overmold that keeps the bundle dry along its length. A sealed connector on an unsealed bundle simply moves the ingress point 200 mm down the harness.

For off-highway, marine and engine-compartment work, an IP67/IP68 sealed harness combines grommeted connectors, adhesive-lined boots and a jacket that resists fuel, oil, hydraulic fluid and salt water. Where the harness enters a pressurised or potted assembly, the transition is potted rather than taped — a taped transition is a wicking path.

Step 6 — Workmanship: Which Standard Do You Build To?

Harness quality is defined by a workmanship standard, and the class you build to is a commercial decision that should appear on the purchase order.

The Qualification Test Matrix

A cable assembly is qualified before it is shipped, and the tests map directly onto the failure modes above. Not every programme needs every row — but the first four are the ones that should never be waived.

The RFQ Checklist: Ten Things to Send

An RFQ for a custom cable assembly is answered in a day or turns into three weeks of questions. These ten inputs make the difference:

1. Connector part numbers at every end, including keying variant and finish.
2. Wire type, AWG and colour code for each circuit.
3. A dimensional drawing with leg lengths, tolerances and branch angles — not a photograph of a hand sketch.
4. Shield termination requirement: 360° backshell, band, or drain wire.
5. Backshell style and exit angle, plus boot or overmold.
6. Sealing requirement: IP class, immersion, fluid exposure.
7. Environmental profile: temperature, vibration, altitude, salt spray.
8. Marking and documentation: wire identification, harness label, revision, certificate of conformance.
9. Qualification evidence required: test report, first article inspection, sample quantities.
10. Prototype quantity and annual volume — this is what sets tooling, fixture and price.

An experienced harness house will run a design-for-manufacture review on that package and come back with the three or four places where the drawing will make the part expensive or unreliable: a bend radius that is not achievable at the backshell, a shield termination that cannot be banded at that diameter, a splice buried where it cannot be inspected.

Manufacturer Cross-Reference: Cable Assemblies and Harness Hardware

Airoadcon Cable Assembly & Wire Harness Lines

Airoadcon builds both halves of the problem: the connectors and the finished assembly. Relevant product lines:

• Wire Harness & Cable Assembly — the full custom build-to-print range, from single-leg assemblies to branched harnesses.
• Engine Cable Assemblies — fire-resistant, oil-resistant construction with stainless steel overbraid for turbine engines and firewall passthroughs.
• High-Temperature Cable Assemblies — PTFE / FEP / PFA insulation with nickel-plated copper conductors.
• Extreme Thermal Cable Assemblies — mineral-insulated and ceramic-beaded constructions to +260 °C continuous.
• Sealed Harnesses (IP67 / IP68) — sealed harnesses for off-highway, marine and engine-compartment service.
• Ground Power Cable Assemblies — 400 Hz aircraft GPU cable and ship-to-shore power.
• High-Frequency Cable Assemblies — low-loss PTFE, phase-matched pairs, SMA / 2.92 mm / SMP.
• Bomb-Borne Cable Assemblies — high-shock, lightweight ordnance harnesses.
• Helmet Communication Leads and Lightweight Measurement Assemblies — tactical audio leads and low-noise instrument cables.
• J599 Series III connectors plus M85049 shield-termination backshells and shrink-boot adapters — the shell and accessory families the assemblies are built on.

Frequently Asked Questions

What is the difference between a cable assembly and a wire harness?

A cable assembly is a length of cable terminated with connectors at one or both ends to a defined electrical performance; a wire harness is a branched bundle with multiple legs, breakouts and often several different connector types. In practice the two overlap — a harness is a cable assembly with a topology. The distinction matters commercially because a branched harness with breakouts and splices is a more complex build than a single-leg assembly, and it needs a dimensional drawing rather than just a length and two part numbers.

Can you build to my drawing, or do you design the harness?

Both. Build-to-print is the normal route: send the connector part numbers, wire schedule, dimensional drawing and qualification requirements and the assembly is built and tested to that package. Where the harness is not yet designed, the same inputs plus the routing envelope and environmental profile are enough to produce a drawing for approval, including a design-for-manufacture review that flags the bend radii, shield terminations and splice locations that would make the part unreliable.

Which wire should I specify for an aerospace harness?

Start with an AS22759 / MIL-W-22759 XL-ETFE wire — 600 V, rated to +200 °C, lightweight and resistant to fuels and hydraulic fluid. Move to PTFE (MIL-W-16878/4) where temperature or chemical exposure is more severe, to M27500 shielded and jacketed multi-conductor cable where the harness itself has to contain EMI, and to mineral-insulated or ceramic-beaded construction inside a fire zone. Then check the connector contact rating and derate the bundle for current, altitude and ambient temperature.

What testing do you perform on a finished cable assembly?

100 % continuity and wiring verification, low-level contact resistance, dielectric withstand (DWV / Hipot) and insulation resistance on every assembly. Sample-based or type testing then covers cable pull-out, contact retention, vibration and shock, thermal cycling, salt spray and fluid immersion, with first article inspection to AS9102 on the first production unit where the programme requires it.

How do I cross-reference an existing cable assembly?

Send the incumbent assembly or connector part numbers, the wire schedule, the drawing (or the routing envelope if there is no drawing) and the qualification requirements. Airoadcon maps Amphenol, TE / Deutsch, Souriau, Glenair, ITT Cannon, Gore and Carlisle hardware onto the equivalent J599 circular, ZH83723 bayonet, marine or subsea connector and the matching M85049 accessory, then quotes the assembly with its test report.