Understanding aircraft wire differentiation to improve EWIS

The amount of aircraft wire on a civil aircraft has increased in recent decades such that today’s typical aircraft has hundreds of miles of wire connecting all of its subsystems. The increase in the amount of wire on an aircraft, coupled with weight concerns, has lead to progressively thinner and lighter insulation materials. Wires used on today’s aircraft are 70% lighter and occupy 20% of the space of their 1950’s counterparts. When considering that the insulation on today’s wires can be as thin as three human hairs, we should realize that modern aircraft wiring is a fragile, yet critical component of the overall aircraft electrical interconnect system. Each insulation material has advantages and disadvantages that make it perform well under some circumstances and unacceptably in others. Some wires are especially resistant to fire, others are resistant to chafing. Before selecting a type of wire to install on an aircraft, one should understand the properties of the insulating material and the environmental conditions to which the wire is exposed (Lectromec’s testing services) can provide the data you need to select the right wire for your application).

Additionally, identifying types of wire can be extraordinarily difficult because wires come in a variety of colors and may not have any identifying markings. It is also difficult to identify wire based on the type of aircraft in which it is found and even experts on the subject are often unable to identify wire types with only a visual inspection. We have presented two types of wire and summarized their properties to illustrate the wide range of advantages and disadvantages to different types of insulation.

Aromatic Polyimide (Cross Section Photo of M81381-11)

1. Trade names: Kapton™, Apical™
2. Identification codes: BMS 13-51, MIL-W-81381/7 through /14 and /17 through /21, DMS 7007, and others
3. Aircraft used in:
• Space Shuttle
• Airbus
• B727
• B737
• DC-10
• F-14
• F-16
• P-3 (Not all-inclusive)
4. Advantages:
• Lightweight, typically 4.6lbs./1,000 ft. (6.8 kg/km)(20 AWG)
• Good abrasion and cut-through resistance
• Passes standard low temperature chemical flame tests
• Excellent thermal and electrical properties
5. Disadvantages:
• Will crack or delaminate when it ages
• Prone to wet and dry arc tracking
• Will deteriorate when exposed to heat or stress
• The only insulation type mentioned by the Federal Aviation Administration (FAA) as not to be mixed with other insulation types because of its propensity to cut through softer insulations
6. Overall: Widely used in commercial, aerospace and military vehicles because of its light-weight and resistance to abrasion. Not found on new aircraft as much because of its propensity to sustain high temperature electrical arcs. Note: the preceding information applies only to the H-film build seen in the left column photo. The DuPont Oasis^TM insulation will be addressed in a future Lectrogram.

Polyvinyl Chloride (Constructed with Nylon topcoat)

1. Trade name: Quad 4™ (Cross Section Photo of Quad 4&trade)
2. Identification codes: BMS 13-13, MIL-W-5086/1 and /2
3. Aircraft used in:
• B707
• B727
• B737
• DC-8
• DC-9
• KC-135
• A-10
• C-130 (Not all-inclusive)
4. Advantages:
• Good resistance to chafing (thicker insulation)
5. Disadvantages:
• Loss of dimension and weight over time
• Discolors with moderate heating
• Produces harmful gasses and smoke when it burns
• Comparatively low temperature rating measured against aromatic polyimide
• Heavy, about 6.8 pounds/1,000 ft (10 kg/km) (20 AWG)
6. Overall: Will not pass the FAA’s 60º flame test. Gasses from burning are extremely harmful if inhaled. Although this wire does not meet current standards, it is still found on some older aircraft.

It is not just insulation that protects wires, but also a trained maintenance staff. When maintaining or replacing wires, it is vitally important to understand their properties. The wrong wire placed in the wrong environment can significantly impact aircraft airworthiness. Proper electrical systems maintenance and care can both improve aircraft readiness and help retain the resale value of your aircraft.