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Aerospace Cable Considerations

Cable installations in virtually any aerospace system including interference friend or foe (IFF) transponders and ground moving target indicators (GMTI) equipment will experience both high and low temperatures as well as humidity. At 30,000 feet, atmospheric temperatures can range from -40°F to -70°F. Aircraft travel globally and must also therefore function in desert and arctic conditions. Moreover, heat conduction from a powerful front bulkhead and engines on the aircraft's metal shell can reradiate heat for further contribution to high temperatures. Cables are protected from much of this stress within a sealed aircraft that contains various heat exchanging avionics but must still have a wide operating temperature that can range from -55°C up to 165°C.

Cable contamination can occur through accidental spillage, leakage, or spray or during installation and routine maintenance with poor handling. These contaminants include fuel, turbine fuel, hydraulic fluid, lubricants, and coolant. Typically, jacketing exposed to these type of contaminants will cause migration and dispersion of the plasticizers in the material leading to melting, swelling, or cracking. This continual exposure alongside ambient mechanical stressors, the cable shielding can be exposed and corrode or a lack of conformity can occur in the cross sectional dimensions of the coax, thereby degrading performance or causing a failure. High-reliability cables must then be able to resist these contaminants through the use of employing specific plasticizers or by using materials that are inherently more robust.

Mechanical vibration will almost certainly be applied to cabling in aircraft during flight. This can occur during normal operation or from gunfire vibration. Mechanical shock can be induced through heavy landing or sharp aircraft maneuvers. This can cause connectors to un-mate or become dislodged. It is therefore valuable to assess connectors with vibration and shock tests as specified in MIL-PRF-39012 to ensure that there are no electrical interruptions exceeding 1-microsecond during test and no damage to the connector has occurred.

Figure 3. Triboelectric induced noise from vibrational strain on coax assemblies can cause additive noise in highly sensitive connected circuitry.

There is also a mechanically induced signal generation that occurs during vibration between the shield and dielectric material as shown in Figure 3. This tribo-electric induced noise can cause additive noise on sensors for in-flight testing or other highly sensitive equipment. It is important that this noise stay below a certain threshold for the proper performance of these systems.

Precautions must also be considered for the risk of fire where cables are often a source of toxic emission, or worse, flame spread. This is especially true for enclosed spaces such as aircraft, train cars, ground-based vehicles, or indoor facilities. Parameters such as flammability, flame propagation halogen content, and toxic/smoke index are all relevant in ensuring that no toxic fumes are dispersed when the cable comes into contact with heat. Fluorinated Ethylene Propylene (FEP), for instance, has a high resistance to flames as well as chemicals and could be ideal in these applications. The design considerations for aerospace high-reliability cabling has many overlaps with other military applications as well.

Ground-based Cable Considerations

Ground-based military applications such as Land Mobile Radios (LMR), Unmanned Ground Vehicles (UGV) and ground-based radar (e.g.: phased array radar) experience both high and low temperature extremes as well as humid conditions. Mechanical vibration and shock can be induced in mobile vehicles and train transports. Enclosed spaces will require that any toxic fumes released by the cabling be minimal. And a similar type of fluid contamination can occur with UGVs causing the need for robust jacketing material. Land vehicles may also require a level of crush and abrasion resistance to prevent the coax from breaking during vehicle run over. This can be accomplished through the use of armored cables or a suitable elastomer jacket.

Ground-based radar systems will often have high transmit powers and, therefore, parameters such as dielectric withstanding voltage, corona extinction voltage, and insulation resistance become very valuable as they assess the quality of the dielectric as the insulation also becomes key in transferring heat away from the high-power carrying conductors. For instance, partial discharges in high-voltage electrical wiring generates noise that can be conducted to connected low-level circuits that produce a corona of ozone, light, acid, and ultimately the deterioration of di-electrics[3]. This is also true in high-voltage avionics as circuits are densely packed. As an additional note, the rate of deterioration of the dielectric due to partial discharges is proportional to the operating frequency, so higher frequency radar systems will especially require the use of insulating materials with a high dielectric strength.

What to Look For

Conclusion

The interconnect between mission-critical equipment in military applications must be highly robust to withstand the myriad of environmental, electrical, and mechanical stressors that it can endure. The specific construction and testing of a coax can vary greatly depending on the component materials specified and the process standardization applied during manufacture. Measures should therefore be taken against any potentially damaging situation that could cause failures. Cables that abide by military standards offer a level of reliability that would not be found in many commercial-grade assemblies.

This article was written by Dan Birch, Product Manager, Pasternack (Irvine, CA). For more information, visit here .

References

  1. Moir, Ian, and Allan G. Seabridge. Design and Development of Aircraft Systems. John Wiley & Sons, 2013.
  2. Defense Technical Information Center report.
  3. Defense Technical Information Center report.