Aerospace Cable Assemblies

Aerospace avionic systems have a modular design for easy removal and installation, allowing the majority of repairs to be completed at the test bench. Multi conductor cables with quick disconnect plugs connect various communication and navigation components together, along with the cockpit. Flight controls, landing gear, interior and exterior lighting, and most gauges and instruments are also electrically controlled.

The standards for cable assembly design and manufacture are specific for both civil and military use. These standards are not only intended to maintain a high level of quality, but also consistency among assemblies produced by different manufacturers. Military specification (mil-spec) standards are applied to the actual aircraft itself, as well as for individual replacement cables and harnesses. Separate mil-specs cover the cable, the wires making up the cable, and the connectors.

Mil-Spec Cables

Mil-spec MIL-C-27500 covers avionic, airframe, and miscellaneous system aerospace cable assemblies. A typical cable part number would be MIL-C-27500A20MD5F02, for example. Charts call out the codes on the part number: following “MIL-C-27500”, the “A” denotes the identification method, “20” is the AWG conductor size, “MD” is the wire insulation type, “5” is the total number of conductors in the assembly, “F” is the shield description, and “02” is the jacket style.

Different types of cables cover different applications. Power cables require larger grade wires and connectors that are insulated and separated to prevent arcing. Signal cables are made up of smaller gauge wires, and are shielded to prevent high frequency leakages, while other cables are designed with shock and strain resistance.

Mil-Spec Wire

A cable is only as effective as its wiring. Every wire in a mil-spec cable has its own mil-spec:  like the cable assembly example above, wire specification “MD” is indexed for MIL-W-81044/5 with the following criteria:

1. Silver coated copper conductor
2. Extruded XL Polyalkene insulation
3. Extruded XL Polyvinylidene Fluoride jacket
4. 150 degrees C. 600 Volt rating.

If a hazard is present, such as contamination by hydraulic fuel, then a resistant wire should be used. MIL-W-5086, for example, is a wire resistant to high heat and abrasion. It has a compounded PVC insulation, an impregnated glass braid jacket and a clear nylon coating. The conductor is stranded tinned copper.

Mil-Spec Connectors

Circular connectors are often used in aerospace cable assembly applications. Commonly known as “Cannon Plugs” due to a popular manufacturer, these multi-conductor plugs are available with threaded, push-pull, cam-lock, and breach-lock disconnects. Single or multiple keyways assure proper polarization of the multiple pins: the male side has pins, while the female side has sockets. Some types have o-ring seals to keep moisture and corrosives out, while others are hermetically sealed to withstand great pressures and vacuums. Individual pins and sockets are replaceable on circular connectors, which are crimped or soldered onto the individual wires.Connectors come in a variety of shell diameter sizes, number of contacts, profile angles, and shell materials. As in cable assemblies, they are labeled largely by numbers detailing their specifications. Connectors in the same mil-spec come in different configurations for installation on cables, equipment, and bulkhead walls.

Two common mil-specs for connectors are MIL-C-1505 and MIL-C-38999. The mil-specs cover not only the connectors themselves in terms of materials and dimensions, but also parameter requirements such as durability, resistance to vibration and shock, and corrosion resistance.

MIL-C-5015 Connectors

MIL-C-5015 connectors are the oldest type of mil-spec connectors in use. Commonly called “MS” connectors, these circular units are for high voltages and large cables and wire sizes. Each connector can handle a maximum of over one hundred separate contacts. The 5015 connectors come in five classes of materials and finishes, five classes of applications, and five different voltage configurations ranging up to 7,000 volts.

MIL-C-5015 connectors are also specified to withstand shock, high and low temperature ranges, immersion testing, and repeated connecting and disconnecting. Different application classes include general duty power supply in non-critical areas, and pressurized bulkhead connections where pressure leakage must not exceed one cubic inch per hour at 30 PSI.

MIL-C-38999 Connectors

MIL-C-38999 connectors are made up of smaller gauge wires and are used in conveying smaller voltages and signals. Three general types are available for different applications.

Type one is a bayonet-locking type. These high-density connectors can contain up to one hundred and twenty-eight contact pins, each one with conical risers that seal the mating surface for corrosion resistance. They are also shielded for leakage of electromagnetic interference, both incoming and outgoing.

Type two is similar to type one, but is lower in profile due to a shorter socket and is used where small spaces prohibit a type one connector. Because of the shorter socket, type two plugs can be erroneously plugged in crookedly, bending the pins.

Type three connectors have triple start threaded connections for advanced vibration protection. As in types one and two, type three can handle up to one hundred and twenty-eight contact pins per connector. It has a longer socket, as in type one, preventing the pins from bending during connecting. In cable assembly lingo, this is known as a “Scoop Proof” design.

To read more on aviation standards, visit the FAA's Website.