Commercial Protocols & Aerospace Cabling: Finding the Right Balance

Commercial protocols in aerospace applications present designers with challenges of balancing the standard against the application's special needs. In the physical layer, the cables used for Gigabit and 10 Gigabit Ethernet -- USB, IEEE 1394, and others -- are a prime example of this balancing act. The standards for these protocols detail electrical and structural requirements for the cables. One goal of standards is to characterize cables with enough specificity that they can be confidently plugged into the application.

For commercial applications, this approach works admirably. The additional concerns for aerospace applications lead designers to reconsider this convenient plug-and-play approach. Their primary driver is reducing size and weight at every opportunity in the aircraft. As the amount of data generated on aircraft operation and passenger services increases dramatically, so does the number of conductors needed to carry the data. In aggregate, commercial cables make an attractive target for size and weight reductions. At the same time, industry-standard cables may not possess the desired mechanical or environmental performance, especially the demanding requirements for low smoke generation, toxicity, and flammability in closed spaces where safe exit may be difficult or impossible.

The case for Cat cable
Consider a Cat 5e cable for Gigabit Ethernet. Its typical commercial construction is a 24 AWG solid bare copper conductor with polyethylene insulation and PVC jacket. The ANSI/TIA 568-C.2 standard defines electrical requirements for attenuation, insertion loss, return loss, crosstalk, and a host of other criteria. For our purposes, we will focus on insertion loss. This characteristic largely determines allowable cable distances (assuming crosstalk goals are met).

Consider a typical progression in defining a Cat 5e cable for aerospace. Each step tends to increase both attenuation and insertion loss, effectively reducing the allowable cable length.

Stranded conductors give greater flexibility in installing and routing cables in space-constrained aircraft. Though the 568-C.2 standard recognizes stranded conductors for short patchcords, it specifies solid conductors for backbone needs, because of their lower insertion loss. The change from solid conductor to stranded conductor allows for a 20% increase in insertion loss, which would result in a 20% decrease in the maximum cable run length.

Many aerospace applications specify silver-coated copper alloy conductors, because of their high tensile strength. Moving from pure copper to a copper alloy can increase insertion loss by another 10%, depending on the cable design.

Smaller conductors save weight, which explains the trend toward 26 AWG and even 28 AWG. Figure 2 shows practical distances for different conductors. Additional size and weight reductions can also be obtained using thin-wall, lower-permittivity dielectrics and tougher jacket materials.

Cat 6A cable, to enable 10G Ethernet, presents similar tradeoffs.