01
Frozen configuration
Every change of pin, length or part number goes through an ECN and creates a new revision; the previous one must be reconstructable. A loose file on a shared drive does not survive an AS9100 audit.
Aerospace
Aerospace wire harness design
Wire list, 1:1 formboard and manufacturing package for harnesses built on MIL-DTL-38999 connectors and M22759 / M27500 cable, under AS50881, EWIS and IPC/WHMA-A-620 class 3. harnessPRO documents the harness with traceability; certification stays with you.
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Challenges
What makes an aircraft or satellite harness different
It is not the electrical difficulty: it is that every decision has to be written down, justified and reproducible twenty years later.
02
Part numbers that must be decoded
D38999/26WD35SN, M27500-22SD2T23, M85049/38S15W: here the part number is the specification. One misread character changes the finish, the conductor count or the contact gender, and nobody notices until the build table.
03
EWIS separation and identification
Separating redundant routes, keeping wiring away from fluid lines and marking every wire with its identifier is decided in design and recorded on the drawing, not improvised at installation.
Technical guide
Wire harness design guide for aviation and space
Standards, connectors, cable, protection and process: what you need to have straight before drawing the first line of a harness that is going to fly, on an aircraft or a launcher.
In this guide
Regulatory framework: AS50881, EWIS, A-620 and AS9100 quality
Wiring installation in an aircraft is governed by SAE AS50881, the successor to MIL-W-5088: wire selection, bend radii, support, separation, identification and termination. In civil aviation the FAA and EASA added the concept of EWIS (Electrical Wiring Interconnection System): 14 CFR 25.1701 to 25.1733 and CS-25 Subpart H treat wiring as a system with its own requirements for separation (25.1707), identification (25.1711) and safety analysis (25.1709).
Acceptability of the finished assembly is set by IPC/WHMA-A-620, with class 3 as the high-reliability tier; the quality system is AS9100 and the first article is documented under AS9102. Where the programme requires it, MIL-STD-1353 and the old MIL-STD-454 (now MIL-HDBK-454) are still cited for connector selection, and MIL-STD-130 governs marking and IUID. In space: NASA-STD-8739.4 and, at ESA, ECSS-Q-ST-70-26 (crimping) and 70-08 (soldering).
| Standard | Scope | What it decides in your harness |
|---|---|---|
| SAE AS50881 | Aircraft wiring installation | Radii, supports, separation, marking, termination |
| 14 CFR 25.1701-1733 / CS-25 H | EWIS on transport aircraft | Separation, identification, inspectability, safety analysis |
| IPC/WHMA-A-620 class 3 | Acceptability of the built assembly | Crimping, sealing, allowable damage, marking |
| AS9100 / AS9102 | Quality system / first article | Traceability, change control, FAI report |
| MIL-STD-1353 / MIL-HDBK-454 | MIL-SPEC connector selection | Permitted families, finishes, density |
| REACH / RoHS | Restricted substances (EU) | Cadmium and chromium VI; aeronautical and space exemption in RoHS |
Two issues the standard does not solve for you: obsolescence (DMSMS), because a programme lasts decades and inserts go out of production, so the BOM carries manufacturer and qualified alternatives; and lot traceability, which ties each lot of wire or contacts to the harness serial number.
Connectors: MIL-DTL-38999 and the other families
The MIL-DTL-38999 circular connector is the de facto standard. Its series share insert arrangements and contacts but differ in coupling: Series I (LJT) is bayonet and scoop-proof; Series II (JT) is a low-profile bayonet, lighter and not scoop-proof; Series III has a self-locking triple-start thread and is the best under vibration, sealing and shielding, which is why it dominates new programmes. Shell sizes run from 9 to 25 in odd steps, with 22D, 20, 16 and 12 contacts plus cavities for coax, power and fibre. The finish sets the environment: olive drab cadmium (class W, 175 °C, best in salt fog), electroless nickel (class F, 200 °C, cadmium-free), black zinc-nickel (class Z, the REACH alternative) and composite shells that save weight.
Other families live alongside it, summarised in the table: from the MIL-DTL-5015 threaded power connector to the rectangular MIL-DTL-24308 D-sub and MIL-DTL-83513 micro-D, via ARINC 600 in avionics racks and, in Europe, EN3645 (the 38999 Series III equivalent) and EN2997 (fire-resistant threaded connector for engine zones).
| Family | Coupling | Sizes / contacts | Typical use |
|---|---|---|---|
| MIL-DTL-38999 Series I (LJT) | Bayonet, scoop-proof | Shell 9-25 · 22D, 20, 16, 12 | Avionics, mission systems |
| MIL-DTL-38999 Series II (JT) | Low-profile bayonet | Shell 8-24 · 22D, 20, 16, 12 | Tight spaces, lower weight |
| MIL-DTL-38999 Series III | Self-locking triple-start thread | Shell 9-25 · 22D, 20, 16, 12, 8, 10 | Current reference |
| MIL-DTL-5015 | Conventional thread | Shell 8S-48 · 16, 12, 8, 4, 0 | Power, land vehicles |
| MIL-DTL-26482 Series II | Bayonet | Shell 8-24 · 20, 16, 12 | Cost-conscious equipment |
| MIL-DTL-83723 Series III | Thread or bayonet | Shell 8-24 · 20, 16, 12 | Engine zones |
| MIL-DTL-24308 / 83513 | Rectangular | D-sub 9-50 way · micro-D 9-100 way | Avionics boxes, signal |
| ARINC 600 · EN3645 · EN2997 | Rack-and-panel · thread | Modules by size · per EN standard | Avionics racks · European programmes · engines |
Crimp contacts are M39029 (/56 pin and /58 socket for 38999 Series III) and the tools are M22520: /1-01 for sizes 20 to 12 and /2-01 for 22D and 20, each with its positioner and go/no-go gauge. The backshell (M85049 family) completes the connector: straight, 45° or 90°; EMI with 360° braid termination; strain relief; self-locking. Leaving it off the BOM is a classic that turns up at the build table.
Galvanic trap. An olive drab cadmium connector with a nickel backshell forms a galvanic couple that corrodes within months in a salt environment. Connector, backshell and jam nut carry the same finish, and it must appear as such in the full part number of every part.
Cable: M22759, M27500 and the Kapton lesson
The aircraft wire par excellence is MIL-W-22759, now SAE AS22759. Its detail sheets fix insulation, wall and conductor plating: /16 is extruded ETFE over tin-plated copper (150 °C, 600 V), /32 is light-wall cross-linked ETFE and /34 the same chemistry with a normal wall; /11 and /12 are PTFE over silver-plated copper for 200 °C, and sheets /41 to /46 are dual-wall cross-linked ETFE, also for 200 °C. The choice matters for weight: a fuselage harness sheds kilos by moving from normal to light wall, if abrasion allows.
For pairs, triples and shielded cables you move up to M27500 (NEMA WC 27500), built from those same basic wires. The part number reads in blocks: M27500-22SD2T23 is 22 AWG, basic wire SD (M22759/34), 2 conductors, shield T of tin-plated copper and jacket 23 of cross-linked ETFE. The codes are tabulated in the standard: look them up, do not guess.
The story of Kapton (polyimide, MIL-W-81381) explains today's specifications: light and thin-walled, it was used massively until its arc tracking was demonstrated, in which an arc started by chafing carbonises the polyimide, turns it conductive and propagates the fault through the harness. Its restriction gave rise to TKT composites (sheets /80 to /92) and the general use of cross-linked ETFE.
Misdecoded M27500 part number. Reading the conductor count as the gauge or confusing the basic-wire code is common when transcribing old drawings. In the wire list, every multi-conductor cable carries its full part number and its readable breakdown, so that purchasing and production do not each interpret it their own way.
Shielding, bonding and protection against EMI and lightning
Shielding is designed as a system. M27500 braids are terminated 360° at the EMI backshell, never with a long pigtail that acts as an antenna at high frequency; to gather several braids, conductive heat-shrink tubing or termination rings are used. The complete harness may carry an overall braid with bonding points to structure and a bond resistance recorded on the drawing. The HIRF and zonal lightning protection requirements translate into which cables are shielded, where the braid is grounded (one end or both) and which harnesses share a braid; all of it goes per conductor into the wire list.
Mechanical protection, routing and identification
The sleeving depends on the environment: Raychem DR-25, fluid-resistant elastomeric heat-shrink to 150 °C; Viton for 200 °C; PTFE convolute against abrasion in moving zones; expandable braids such as Roundit or Expando that fit over a finished harness; and Nomex braid where low smoke emission is required. SWAMP zones (severe wind and moisture problem: wheel wells, pylons, nacelles) call for rear-sealed connectors and drainage-aware routing that does not lead water to the connector.
AS50881 sets rules worth keeping on the formboard: MS21919 clamps at intervals no greater than 24 inches, minimum bend radius of 10 times the bundle diameter (3 at a supported termination), at least half an inch from fluid lines, and separation or a barrier between redundant routes and between power and signal (25.1707). Identification is marked on the insulation, today almost always by UV laser, at most every 15 inches, with heat-shrink labels at the ends if the wire is thin or dark. The identifier encodes the circuit function, wire number and gauge, and is the same on schematic, wire list and harness.
Forgotten EWIS separation. Two redundant channels sharing a harness and a connector satisfy the schematic but fail 25.1707: a single chafe takes both out. Separation is decided in design and written on the drawing; the installer cannot recover it afterwards.
Process: from ICD to first article
The flow that works in aviation and space has little magic and a lot of discipline:
- Requirements and ICD. Each equipment supplier provides its interface control document (pinout, connector, shielding, voltages); the designer consolidates them onto each connector before laying a single wire.
- Wire list and from-to table. Per conductor: identifier, gauge, wire, origin (connector and cavity), destination, contact at each end, shield and termination. It is the contractual document of the harness.
- 1:1 formboard. True-scale routing with branch points, segment lengths and service margin, including what the backshell consumes. It is what gets printed for the table.
- Manufacture and test. Crimping with calibrated tooling, A-620 class 3 inspection and electrical tests: continuity, insulation resistance (typically at 500 V DC) and dielectric withstanding voltage (DWV or hipot), at 100 % on critical programmes.
- First article and configuration. The FAI under AS9102 compares the built harness with the approved package. From then on, every change is an ECN with a new, traceable revision of wire list, formboard and BOM.
Unidentified wires. A harness with no marking passes the electrical tests and fails inspection: without an identifier there is no traceability to the schematic and no in-service maintenance. Marking is defined in the wire list and is part of the drawing, not of the shop's goodwill.
Mapping
What the sector demands, and what harnessPRO does about it
harnessPRO defines and documents the harness. It does not certify, simulate or calculate derating: the engineer decides and signs that. What it does is keep every figure in its place, under revision.
Wire list and from-to table with cavity, gauge, wire and contact per end.
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Schematic + Connections. The netlist is built as you draw; Connections presents it as a from-to table with conductor, gauge, length and both ends.
1:1 formboard with breakouts and segment lengths.
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Layout at true scale. Branch points, type-oriented boots, manual harness routing and segment lengths, traced over a background photo or drawing if needed.
Load the ICD pinout onto a 38999 connector without typing cavity by cavity.
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Manufacturer pinout import. Paste the datasheet or ICD table and the cavities come out named and described, with ranges and NC respected.
Controlled library of connectors, contacts, backshells and cables with full part numbers.
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Parts. Connectors with cavities and gender; contacts, backshells and seals as connector accessories; wire and cable. Integrated Digi-Key catalogue with part number, manufacturer and price.
Configuration control: rebuild the FAI revision and see what changed with each ECN.
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Revisions / snapshots. Every state of the harness is saved as a recoverable revision; the full JSON serves as backup and interchange.
BOM for purchasing and lot traceability.
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BOM to CSV/Excel. Connectors, contacts, backshells, wire and cable consolidated by part number and manufacturer, ready for the ERP and the lot certificates.
Manufacturing package with title block and mutually consistent tables.
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PDF drawings. Schematic, formboard, connection tables, lengths and gauges with title block (title, revision, author), generated from the same model.
Flow
An avionics harness in harnessPRO, from ICD to PDF
01 · SCHEMATIC
Connectors, pinout and netlist
Create the 38999 connectors from the library or Digi-Key, import the ICD pinout onto each one and connect cavities. Splices, twisted pairs and shields stay on the schematic; the netlist builds itself.
02 · LAYOUT 1:1
Formboard and lengths
Place each connector with its boot oriented, mark the branch points and route the branches at true scale, tracing over the zone drawing if you have it. Segment lengths, with margin, are recorded here.
03 · PARTS / BOM
Full part numbers and accessories
Assign each connector its M39029 contacts, M85049 backshell and seals; each conductor its M22759 wire or M27500 cable. The BOM goes out to Excel with manufacturer and part number.
04 · DRAWINGS
Manufacturing package and revision
Generate the PDF with title block, tables and formboard, save the revision that goes to FAI, and every later ECN becomes a new revision, comparable with the previous one.
FAQ
Frequently asked questions about aerospace wiring
Which standard governs aircraft wiring?
The installation standard is SAE AS50881 (successor to MIL-W-5088). In civil aviation the FAA and EASA EWIS rules apply on top (14 CFR 25.1701 to 25.1733 and CS-25 Subpart H), and the quality of the finished assembly is judged against IPC/WHMA-A-620 at class 3.
What is the difference between MIL-DTL-38999 Series I, II and III?
They share insert arrangements and M39029 contacts but differ in coupling: Series I (LJT) is bayonet and scoop-proof; Series II (JT) is a low-profile bayonet, lighter and not scoop-proof; Series III uses a self-locking triple-start thread, is the most robust under vibration and is the default choice on new programmes.
What is an M27500 cable?
M27500 (now NEMA WC 27500) is the specification for shielded and jacketed multi-conductor cables built from basic wires such as M22759. The part number encodes gauge, basic wire (two letters), conductor count, shield material and jacket type: M27500-22SD2T23 is a 22 AWG pair with M22759/34 wire, a tin-plated copper braid and a cross-linked ETFE jacket.
What does class 3 mean in IPC/WHMA-A-620?
IPC/WHMA-A-620 defines three acceptability classes. Class 3 is the high-reliability tier required in aviation and space: stricter criteria for crimping, sealing, marking and allowable damage, with 100 % inspection on many programmes. The class is set by the customer on the drawing or in the contract.
How is a formboard made for an aircraft harness?
The harness is drawn at 1:1 scale with the position of every connector, breakout and protection element and printed onto the build table; fixtures are pinned onto that drawing and the operator lays the wires along it. Lengths include service margin and what the backshell consumes. In harnessPRO the Layout view is that formboard, at true scale and exportable to PDF.
Is harnessPRO suitable for aircraft and spacecraft harnesses?
It is suitable for defining and documenting the harness: schematic, wire list, 1:1 formboard, a library of MIL-DTL-38999 connectors with contacts and backshells, BOM, PDF drawings with title block and revisions. It does not certify, simulate the circuit or calculate derating: those figures are entered and validated by the engineer.
Can I use cadmium-plated connectors if my programme requires REACH and RoHS?
Cadmium is restricted under REACH and banned under RoHS, but RoHS excludes aeronautical and space equipment from its scope, and on many programmes olive drab cadmium (class W) remains the specified finish. For civil or dual-use scope the alternative is electroless nickel or zinc-nickel, with the same finish on connector and backshell to avoid galvanic corrosion.
Start with the wire list of your next harness
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