Lectrogram - Issue 7
Author: Lectromec
Published: 07-01-2005
Lectromec
LECTROGRAM©
July
2005 Newsletter
A Resource for your Wire Maintenance Program
The amount of wiring 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.
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 BMS 13-58)
Trade
names: Kapton™, Apical™
Identification
codes: BMS 13-51, MIL-W-81381/7 through /14 and /17 through /21,
DMS 7007, and others
Aircraft
used in:
• Space Shuttle • Airbus
• B727
• B737
• DC-10
• F-14
• F-16
• P-3
(Not all-inclusive)
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.
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.
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 OasisTM insulation will be addressed in a future
Lectrogram.
Polyvinyl Chloride (Constructed with Nylon
topcoat)
Trade name: Quad 4™ (Cross Section Photo of
Quad 4™
)
Identification codes: BMS 13-13, MIL-W-5086/1 and
/2
Aircraft used in:
• B707 • B727 •
B737
• DC-8 • DC-9 •
KC-135
• A-10 • C-130
(Not all-inclusive)
Advantages:
• Good resistance to chafing (thicker
insulation)
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)
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.
For more information on these and other
wire types, and their implications for safety, contact Lectromec.