Getting Wise to the EWIS: Wire Insulation

So many wires, so little maintenance. This seems to be the unfortunate state of affairs for many planes. Technology, especially in the form of computers, has evolved in leaps and bounds yet many of today’s aircraft are designed as if they are stuck fighting Hitler. No surprise then that wiring is still configured in ways that are incompatible with newer systems, communications or otherwise. Not to mention such configurations are inefficient, non-ergonomical, and dangerous.

Modern aircraft consist of an Electrical Wiring Interconnection System (EWIS). This system is complex, to say the least. Depending on the type of plane, this system can include anywhere from 10-200 miles of wire. That’s a lot of string that needs periodic investigation.

When dealing with your EWIS you have wire but you have other related components to consider as well: connectors, circuit breakers, and conduits.

One of the major issues when installing or replacing your EWIS is wire insulation. Of course, the purpose of insulation is quite obvious. If you’re running an electric current through a wire, you don’t want to get electrocuted and you also want the current to get to the right destination. Insulation thus provides both a barrier for protection and a means to guide electricity; that way it doesn’t spread in all directions along the wire path or transfer to surrounding areas. Thus, a circuit’s integrity is maintained and personnel are able to handle wire.

Selecting the right wire insulation is not easy since there are many trade-offs by choosing one over the other. Advantages and disadvantages need to be carefully considered and often costs and time may trump all. Still, it’s important that we at least supply you with some information to guide you through the process.

INSULATION TYPE

Without getting too complicated, there are generally four types of insulation material commonly found in aircraft: (1) Aromatic Polyimide, (2) ETFE, (3) PTFE, and (4) TKT.

Aromatic Polyimide – Sometimes polyimide is abbreviated PI; it is also called Kapton. The ‘aromatic’ refers to its molecular structure that offers great thermostability, hence its usage in insulating wiring. This material possesses great abrasion and cut-through resistance. It is also low smoke and non-flammable while lighter in weight compared to others. Its disadvantages: low arc-track resistance and limited flexibility.

ETFE – Short form of ethylene tetrafluoroethylene; aka Tefzel. Its ease of use makes it desirable. It also excels in chemical and abrasion resistance. Beware: ETFE tends to soften at higher temperatures and does not offer much in cut-through resistance. Because of its softness, it is well to avoid when bundling with other wire types.

PTFE – Stands for polytetrafluoroethylene but is often referred to as Teflon. PTFE offers a slew of advantages: superb high temperature properties, non-flammability, great flexibility and chemical resistance. Unfortunately, PTFE has poor cut-through resistance and is the heaviest form of insulation.

TKT – A composite of plastics; it stands for Teflon-Kapton-Teflon. It possesses a high temperature rating (260°C) and contains a solid level of cut-through and arc-track resistance. However, TKT is prone to outer layer scuffing.

For an equally useful and more thorough presentation of the pros and cons of insulation type, you can check out NASA’s Wire Insulation Selection Guideline Chart.

INSULATION PROPERTIES

Ten characteristics are often used when comparing one insulating material over another.

Weight – This is a major issue. Heavy wire multiplied by the amount of wire can add several hundred pounds of weight to your plane, especially if there are miles of wire required in your EWIS. The lighter the plane the less fuel is needed to overcome the weight burden. This translates to savings on costs associated with fuel consumption.

Temperature – Flight exposes wire to wide temperature variances. You want to ideally aim for insulation that offers the widest range or the highest temperature resistance.

Creep (at temperature) – This refers to the insulating material’s susceptibility to deformation, in this case how temperature warps the integrity of the material.

Flammability – When you deal with electrical wire there’s always the possibility of fire. Safety concerns have you aim for insulation that offers added protection against flames.

Smoke Generation – With flames comes smoke. Once a fire exists and the insulation starts to burn will it generate a great deal of toxic fumes?

Flexibility – Try to wire an airplane and you will quickly find configurations that use only straight lines are impossible. Wires have to bend during installations where point A to point B are not direct.

Resistance to:

Abrasion – Friction wears down materials. Wires are sometimes bundled but can still run up against adjacent surfaces. This leads to chafing and deterioration of insulation. No question, you don’t want insulation to wear down. If a wire is exposed, there’s the potential for electrocution, arc and spark creation, and possible fires.

Cut-through – This refers to the pressure exuded by a mechanical force, like the sharp edge of a wire cutter. You want the insulation material to be durable and be able to resist heavy loads or forces acted on it without disrupting the wires functionality.

Chemical – Planes, amongst other things, operate on the interaction of various chemicals. Fuel and even cleaning supplies will over time corrode insulation.

Arc Propagation – The causes of arc propagation are numerous; chafing, faulty installation, exposure to nearby water and fluids. Electric arcs can cause fires and pose a serious safety threat.

SIDE-BY-SIDE COMPARISON

FAA-wire-insulation

Comparison among wire insulation material based on desirable properties. Source: FAA Aircraft EWIS Best Practices Job Aid (Click to enlarge)

The FAA sure does have a lot of documentation. And that’s a good thing. Especially when dealing with the ins and outs of airplanes. The accompanying chart provides a useful guide in selecting wire insulation. Based on the chart, you can see that PI and PTFE offer the most desirable combination of traits. But TKT and ETFE still maintain certain advantages (stated earlier). By matching the above mentioned properties with the above types of insulating material, you can get an idea of how to improve your EWIS.

TERMINATION…FOR NOW

In the weeks and months ahead we will revisit issues related to EWIS as it is a growing concern and requires constant vigilance. Hopefully you found this post informative. Remember, EWIS is really just the word “wise” jumbled up. Don’t get tangled up with your wires. Get wise to the EWIS. Keep checking in from time to time and we will make sure you don’t.

References

http://www.eaa.org/experimenter/articles/0903_aircraft_wiring.pdf
http://www.vision.net.au/~apaterson/aviation/wire_types.htm
http://www.wirefacts.com/web/Insulations.aspx
http://www.tc.faa.gov/its/worldpac/techrpt/artn06-17.pdf
http://en.wikipedia.org/wiki/Electrical_wiring_interconnection_system
http://www.faa.gov/training_testing/training/air_training_program/job_aids/media/EWIS_job-aid_2.0_Printable.pdf
http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%20120-102A.pdf
http://www.mitrecaasd.org/atsrac/meeting_minutes/2002/2002_04_FAA_Wire_Degradation_Study.pdf
http://www.easa.eu.int/agency-measures/docs/agency-decisions/2008/2008-007-R/Decision%202008-007-R%20-%20Annex%20II%20-%20AMC%2020-21.pdf