Polyimide Aircraft Wiring
- An RAF TriStar aircraft was undergoing maintenance with the APU
running when suddenly circuit-breakers popped, aircraft strobe lights
flashed and the ground warning horn operated. Finally there was
a loud bang and flash from the Left Hand side of the aircraft. After
the excitement was over, the investigation revealed that a wiring
loom, consisting of 168 wires carrying a variety of supplies had
69 wires completely severed!
- What could have caused this sequence of events? The investigation
revealed that this highly dangerous situation was the result of
a phenomenon known as carbon arc tracking. This devastating destruction
of wiring is a fault that can occur with a type of cable insulation
produced with a polyimide tape (trade name Kapton). How could a
cable capable of this destruction be installed on an aircraft?
BACKGROUND TO THE INTRODUCTION OF
- Notwithstanding the advent of data-bus and fibre-optic links,
the increased number of electronic and electrical equipments being
fitted to modern aircraft, like the Tornado, has resulted in a proportionate
increase in the number of interconnecting wires and cables. Whilst
equipment manufacturers have been able to take advantage of microchip
technology to reduce the size of their products, wire manufacturers
have had to develop smaller lightweight wires having a performance
equal to, or better than, previous wire types like Nyvin or Minyvin.
- Wire manufacturers sought new insulation materials which would
possess qualities commensurate with more stringent requirements.
Some manufacturers turned to a polyimide material in a tape form
made by Dupont, possessing exceptionally good dielectric properties
and high physical strength. These qualities allowed the thickness
of the insulating material to be greatly reduced, in some cases
down to six thousandths of an inch, which in turn improved heat
dissipation, thus permitting smaller conductors to be used to carry
the same current. The resultant wires were therefore thinner and
lighter than previous types and showed a vastly improved mechanical
performance in laboratory tests.
- Polyimide insulation with silver-plated copper conductors were
approved for use on UK military aircraft in 1972 with the issue
of Specification EL 2124, followed by EL 3001 for tin-plated conductors
in 1981. Only the Lynx Helicopter and the (then) new-build Harriers
had cabling to these specifications for airframe wire. Other polyimide
types gained approval under PAN Standards as the main airframe wire
for all marks of Tornado and Mil W 81381 for Harrier GR5. Polyimide
has also been introduced on many older aircraft by the installation
of modifications. Invariably, wire insulated with Polyimide has
been hidden under an aircraft manufacturers specification,
as opposed to the cable specification or NSN which made it unrecognizable
to aircraft EAs (Electrical Artificers?). As guidance was
not sought from the equipment EAs at the time, this type of
insulation proliferated amongst mod kits issued over the last eight
years as Design Authorities became pro-polyimide.
REALISATION OF THE PROBLEM WITH POLYIMIDE
- The problem of fitting more wire into less space now appeared
to be solved until the US Navy reported that they were experiencing
excessive chafing, cracking and damage to the cable looms, especially
in severe wind and moisture-prone areas (SWAMP). As these problems
appeared to be attributable to a breakdown of the insulation material
the US Navy conducted preliminary tests which revealed three potential
problems which appeared to be unique to polyimide wire types:
a. Hydrolysis Hydrolysis is a phenomenon characterised
by cracking and breakdown of the insulation material through exposure
to moisture; the speed of breakdown depending on both temperature
b. Wet Arc Tracking Carbon arc tracking occurs when
contaminating moisture or aircraft fluids create a short circuit
between an exposed conductor and the aircraft structure or an adjacent
exposed conductor at a different potential.
c. Dry Arc Tracking Carbon arc tracking occurs in
dry conditions when one or more conductors are shorted as a result
of abrasion from the aircraft structure, wire to wire abrasion,
installation error or battle damage.
THE THEORY OF CARBON ARC TRACKING
- The phenomenon of carbon arc tracking has been known for decades
in the Electrical Distribution Industry and many test methods have
been devised to classify the resistance of installation materials
to failure by this mechanism. Fig 1 shows a strip of insulation
material with an electrode attached to its surface at each end;
if there is a suitable potential difference between the electrodes
and if they are now bridged by a film of wet conducting contaminant,
a current of a few milliamps will flow through the moist layer and
cause slight heating.
- This heating will lead to the formation of an occasional very
narrow "dry band" in the film (see Fig 2). When one of
these bands is formed, most of the voltage between the electrodes
is concentrated across the tiny dry gap and a small flashover will
occur. This tiny arc cannot be sustained because of the high resistance
of the moisture but in a typical situation such dry bands and sparks
will occur continually and randomly over the surface in an effect
known as scintillation. (ticking fault?)
- The micro arcs have a temperature around 1000 degrees Celsius
and so cause intense heating of the insulation surface on a micro
area basis sufficient to pyrolyse (chemically decompose by the action
of heat) any organic polymer. The pyrolysis products of the particular
polymer will determine its tracking behaviour.
- In the case of a "tracking" polymer (i.e. Kapton insulation)
each scintillation will deposit a micro-spot of carbon char (see
Fig 3) with a thermally stable conducting graphitic structure. There
is no change in leakage current at this stage and the formation
of spots will continue, often with a characteristic tree
pattern until a sufficiently complete path has formed to enable
the next flashover to be sustained as a power arc (see
Fig 4) through the newly formed low resistance graphitic carbon
track. At this point there is an electrical and thermal avalanche
effect which will have a magnitude governed by circuit and power
source impedance and by any circuit protection devices.
- In the case of a "non-tracking" polymer i.e. Ethylene
Tetrafluorethylene (ETFE) and Polytetrafluorethylene (PTFE) insulation,
under the same conditions the intense micro heating of the surface
gives gaseous pyrolysis products so that a minute quantity of polymer
evaporates away leaving the composition of the insulation surface
unchanged. There is therefore no char or track formation and no
thermal or electrical runaway. The loss of tiny quantities of polymer
(as gas) gives rise to erosion (see Fig 5) and the rate of erosion
will depend upon the type of polymer. In the case of a non-tracking
fluoropolymer the fluorinated gases given off have the additional
property of suppressing electrical arcing.
- In summary, many aromatic polymers (compounds with carbon rings)
are literally converted from insulators to conductors when subjected
to very high temperatures (as in pyrolysis). It is this feature
which appears to control the susceptibility of aromatic wire
insulations to tracking at relatively low voltages (e.g. 16
- Examples of predominantly aromatic tracking polymers
include Kapton*, Peek* and Ultern*. Examples of predominantly aliphatic
non-tracking polymers (compounds with carbon chains)
include Polyethylene (PE), Polyvinylidene (PVDF), Polyvinylchloride
(PVC), Polytetrafluorethylene (PTFE) and Tefzel*, a form of ETFE.
DRY ARC TRACKING
- More recent work shows that bundles of wire insulated with an
aromatic severe tracker can exhibit total bundle destruction
via carbon arc tracking under completely dry conditions such as
impact damage or when vibration leads to a conductor making direct
contact with a metallic structure or a wire at a different voltage.
The small arcs involved emanating from intermittent contact again
convert adjacent wire installations to graphitic conductors and
lead to catastrophic failure of the wire bundles. Where the wires
are insulated with a non-tracking polymer, in an otherwise identical
setup, there is no avalanche effect and no extension of damage beyond
the initial fault.
TYPE OF DAMAGE BEING EXPERIENCED
- Polyimide insulated wires can be found in many guises with a variety
of part numbers and specifications; a selection of the most common
is as follows:
liquid H lacquer topcoat
||Mil W 81381
Fluorinated Ethylene Propylene (FEP) lacquer topcoat
PTFE tape lacquer topcoat
||PTFE tape. Polyimide
insulation. FEP or PTFE topcoat
- Polyimide insulation can be recognised by its bright translucent
copper colour. This is often misinterpreted as the conductor being
exposed when the topcoat cosmetic layer has been removed due to
damage. Although this has not degraded the cables insulating properties,
this mottling of the cable does lead to actual chafes being harder
to detect amongst flaking lacquer. Complacency can also creep in
with tradesmen assuming it is actually flaking lacquer. A considerable
amount of flaking is now being exhibited on the Tornado and BAE
146 and many man-hours are being expended by electrical tradesmen
re-evaluating damage that has already been examined on a previous
- Cracking and splitting of the polyimide insulation is normally
found where cable bend radii have been exceeded or excessive flexing
of the wires allowed. This form of damage has been found on numerous
occasions within Tornado weapons pylons. This damage has been accelerated
by hydrolysis action within the SWAMP areas. Once this type of damage
has occurred and the conductors are exposed, the looms are in a
primed condition for a wet or dry carbon arc track event. There
are already occurrences of inadvertent weapons release from the
Tornado due to this type of damage.
- Polyimide is a tape-wrapped insulation with, in some cases, a
PTFE tape-wrap as a topcoat. In manufacture these tapes are sintered
together to seal them in place. Experience has shown that flexing
and damp conditions allow these tapes to unwrap, once again exposing
- Polyimide insulated wires are stiffer than other types of wire
and this has proved to be its weakness in fighter aircraft. It is
reluctant to move with areas of vibration and so chafe damage
is inflicted on the insulation. Due to the volume of wiring on current
fighter aircraft, where space is at a premium, cable is susceptible
to inadvertent damage by tradesmen removing or fitting equipments.
If this minor scuffing is ignored then catastrophic consequences
could result. If we are to avoid a high maintenance bill then husbandry
of wiring installations must be improved.
MAINTENANCE OF POLYIMIDE INSULATED
- The following paragraphs set out guidelines for the correct maintenance
of polyimide insulated wiring.
- Routing and Tying Harnesses should be routed and
supported well so that they will stay in position and not contact
installed equipments or structure. Looms should not be readjusted
during servicing and the build standard maintained by ensuring correct
lengths between cleating and equipments. Ties should be spaced close
enough to hold the harness together, preventing splaying of individual
wires where the harness bends to avoid snagging when maintenance
is carried out in the area.
- Harness Twist Harness flexibility is greatly improved
if the wire are twisted (one to two turns per foot) prior to tying.
Twisting allows the harness to bend by rotational motion of the
wires, rather than trying to stretch the wires. This is carried
out in initial build and should be maintained during rewire and
- Hinge areas Harnesses which must cross areas of relative
motion (eg wing fold mechanisms, landing gear and hinged access
panels) should be mounted so that the loom will twist rather than
bend when the joint moves. Care must be taken to ensure that the
loom does not bind or pinch during motion.
- Connector Strain Relief It is important that
strain relief supports be reinstated after maintenance and that
the wire is properly routed and supported by them. This is especially
relevant with the single tie-wrap post back shells which do not
have the same grip on the cables as the older saddle clamps. Accurate
cutting to length of the loom should be carried out especially on
90 degree clamps, in order to avoid single wires being placed under
- Bend radius Harnesses should be installed with bends
as generous as possible to avoid strain on the wires as the structure
flexes. The minimum bend radius allowed is six times the diameter
of the loom or ten times the diameter of the original wire
whichever is the greater. The minimum bend radii have been exceeded
on some fly leads at the rear of connectors even during aircraft
build. This practice should not be repeated on re-wires.
- Use of Test Prods Piercing of the insulation for
testing purposes is STRICTLY FORBIDDEN.
- Stripping and Crimping Great care must be taken to
avoid any insulation damage with stripper and crimping tools. Any
scuffing of the insulation from blades or grips is to be considered
unserviceable and the cable re-prepared.
- Wire Marking All marking must be carried out using
the tape dwell time, temperature and pressure recommended by the
wire manufacturer. Where hot stamp identification marking is involved,
a mandatory high voltage spark test must be carried out after marking.
A badly printed cable, which burnt through to the conductor, was
the cause of six feet of loom being destroyed in a carbon arc on
a civilian airliner. Inter-connect and Equipment
Wires should not be hot stamp printed.
- Inspections Every opportunity should be taken
to inspect installed wiring for signs of damage or chafing. This
should be done when equipments or panels are removed and access
can be gained to normally obscure areas. Inspection is especially
important after servicing has been carried out and is essential
prior to panelling up. All trades should be aware of the dangers
of even minor damage to Polyimide and an electrical tradesman with
10X magnification should be called to inspect all instances of suspected
damage. Cable looms should be maintained clean and dry and contamination
of cables by toilet and galley waste should be rigorously prevented.
- Damaged wire If any flats are detected on the
translucent copper polyimide insulation, apart from missing lacquer,
then it should be assumed that some of the insulation has been removed
and the cable should be repaired or replaced. Repairs to the cable
should be carried out in accordance with AP100B-01 Order 4901 using
the environmentally sealed in-line splices.
- Wire Installation The practice of pulling through
cables during replacement should be avoided. If cables are laid
in then damage to the top-coat lacquers would be avoided and
the snagging of PTFE topcoats, causing the tapes to separate, would
also be prevented.
- As a reminder that these precautions are essential for flight
safety, it is worth emphasizing that polyimide wiring was implicated
as a contributing factor in the loss of two Tornados. Furthermore,
numerous ground fires have been recorded as a result of damage to
this type of wiring. Laboratory tests have shown that the power
of the Carbon Arc not only can destroy wiring looms, but can severely
- Without doubt, complacency cannot be allowed to creep in when
handling polyimide wiring. It is no longer a fit and forget
component or a suitable place for hanging your torch in confined
areas. To maintain our aircraft in an airworthy condition, it is
essential that cables be continually inspected, cleaned and re-cleated.
It should be noted that this form of inspection and maintenance
will be with us for some time as Harrier GR5, Sentry AEW and Tristar
all contain a polyimide construction of cable that is even more
susceptible to carbon arc tracking than Tornado wiring!