4 aspects of wiring selection for special aircraft applications

Not all wires are created equal. Through the years, Lectromec has learned the best practices for wire selection that would ensure the Electrical Wire Interconnect Systems’ (EWIS) longevity.

There are thousands of wire standards for the aerospace industry. These include: the National Electrical Manufacturers’ Association (NEMA) standards, the Society of Automotive Engineers (SAE) standards, as well standards from military organizations commonly referred to as Military Specifications or MIL-Specs. Counting only those from the aerospace industry, there are more than a thousand standards, which do not even include the thousands of standards for the supporting equipment (connectors, splices, etc.). Each has a particular use, application, and properties. Each standard is intended to help manufacturers meet section 3.8.10 of the AS50881 requirements, for the purposes of wire selection for special applications within the aircraft. There are several areas of an aircraft that require special attention and equipment. Here we begin a discussion on four aspects of wiring selection for special aircraft applications.

SWAMP

Severe wind and moisture-prone (SWAMP) areas are areas of the aircraft, in which the electrical system has the greatest exposure to the elements. For these areas, most modern wire insulation types are acceptable, but consideration should be given to those wire constructions that offer better humidity resistance as well as mechanical strength.

Additional thought should be given to the protection of EWIS components. The FAA’s EWIS Job Aid provides some guidance as to what should be considered within these areas:

Environmentally‐sealed splices that conform to MIL-T-7928 provide a reliable means of splicing in SWAMP areas. However, a non‐insulated splice connector may be used, provided the splice is covered with dual wall shrink sleeving of a suitable material.

Flexing

One of the shortcomings of flex testing is that a simple continuity check will not be able to identify any issues, even when a wire may have several conductor strands broken due to flexing. This is due to the fact that a continuity check only looks for a conductive path for low current and voltage, which is well below the energy level that can cause damage to a single strand of conductor. Even a handheld multimeter is not sensitive enough to detect when most of the conductor strands are broken.

What this means is that, not only is it necessary to consider the fatigue on a wire, but also on the other EWIS components. For example, some of the components that may be overlooked in a flex prone application are the connectors. If not properly mounted and supported with strain relief the flexing of wire harnesses can place significant strain at the terminal locations. Regular flexing at these locations can dramatically reduce the component service life.

Tests like the AS4373 Method 704, look closely at the longevity of wires in flexing environments. By selecting those wire/cables with the best performance for this test, it is possible to extend the component service life.

Electromagnetic Interference (EMI)

(For an introduction to EMI, check out Lectromec’s article on how to deal with EMI and Electromagnetic Vulnerability (EMV) on Radio Frequency (RF) Cables for Aerospace Applications.)

There are some applications, in which an unshielded wire is acceptable for signal transmission. However, given the high voltages and signal rates for many applications, there is often a need to protect certain systems from EMI. These situations can be resolved with shielded cables such as those that are defined by several varieties of NEMA27500 cables or AS85485 wire standards. The type of cable shielding (coverage, number of shields) should be dependent on the signal sensitivity and system criticality.

Thermocouple Wiring

Thermocouple wiring is used as part of temperature monitoring circuits. Because these wires are specially made from two metals that produce a temperature dependent voltage, additional care must be taken with the routing and maintenance. For installation, the FAA’s Job Aid (referenced above) recommends that the bend radius of thermocouple wiring be not less than 20 times the diameter of the wire. Note that this recommended bend radius is significantly larger than the 6x for RF cables, and 10x for harnesses.

With regard to thermocouple wiring maintenance, splicing of thermocouple wiring is not recommended as this may impact the temperature reading. If splicing is necessary, there are splices in the AS85485 specification that are ideal for the task.

Summing it Up

While these part selection recommendations may sound straight forward, it took the combined effort of the aerospace industry to make them as comprehensive and useful as possible to the industry. When evaluating the best solution for a given installation, most of the time there is an existing solution. Using the right part for the right application will yield superior system performance and improved system reliability.