Aircraft EWIS Practices Job Aid 2.

Wiring Investigation Findings

Wiring is affected by:
• Design •Environment
• Maintenance •Awareness
• Operation •Abuse
• Training •Time
• Repair
• Installation

As listed here, the investigation of the

aircraft wiring revealed there are several
factors, together with time, that play a role
in wiring degradation.

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Routing/Chafing In-Service
Examples

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Administration

These photos show examples of findings. These examples show

the potential for chafing and arcing.

Accumulation of Dirt and Lint

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Administration

Accumulation of dirt and lint create a potential for smoke and fire,
making inspection of the EWIS impossible.

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Coil and Stow

In-Service Example / Result

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Administration

This slide shows an example of improper termination of unused

wires. On the left side, there are no termination caps and it’s
improperly stowed.
On the right side is a photo of an improper maintenance action.
Coaxial cable was left attached to an antenna instead of being
removed in its entirety – at least it should have been detached
from the antenna. Consequently, lightning attached to the
antenna, traveled along the EWIS, and caused a fire in the cabin.

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Bend Radius Problem In-

Service Example

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Administration

This photo shows an improper bend radius installation of

electrical wire. The proper bend radius for wire on aircraft should
be 10 times the outside diameter of the largest diameter wire in
the bundle for one side supported (3 times for two sides
supported.) Standard industrial practice and is in AC 43.13-1b
and Standard Wiring Practices Manual (SWPM).
. Uçaktaki tel
Arcing Event için uygun
bükülme
yarıçapı,
desteklenen
bir taraf için
demetteki en
büyük çaplı
telin dış
çapının 10
katı
olmalıdır
(desteklenen
iki taraf için
3 kat).

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Administration

These photos illustrate an improper maintenance action caused when

the airplane’s airworthiness certificate was relocated. The attach screw
penetrated the power cables. The FAA maintenance inspector riding on
the jump seat caught his clothing on fire.
The arcing event, which originated from inside the electrical power
center, burned a hole through the left side of the hallway (looking
forward) between the cockpit and cabin.
Effect of Improper Maintenance

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Administration

The left side of this photo shows failure of wire due to chafing inside a
metallic conduit. Created an arc, which created holes in the fuel line
below the conduit. Caused by improperly replacing an existing power
feeder.

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Effect of Poor Design

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This shows arcing due to lavatory servicing lines leaking on

damaged EWIS, creating an arc that caused a fire that led to
extensive damage inside and outside of the airplane. Business
class lavatory drip shield not installed – water shorted cannon
plug in wire bundle below deck.
Maintenance, design, training, operations – all need
improvement.

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EWIS
Electrical
Wiring
Interconnection
System

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Wiring needs to be treated as an important system on airplanes.

Wiring is now referred to as the Electrical Wiring Interconnection
System (EWIS).

EWIS Definition
• An EWIS is [per new § 25.1701(a)]:
Any wire, wiring device, or
combination of these, including
termination devices, installed in
any area of the airplane for the
purpose of transmitting electrical
energy between two or more
intended termination points . . . .

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… EWIS is not [§ 25.1701(a)]

• Electrical equipment or avionics qualified to
acceptable environmental conditions and
testing procedures

• Portable electrical devices not part of airplane’s

type design

• Fiber optics

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 Aircraft EWIS Practices Job Aid 2.0

EWIS Degradation Factors

Physical
Age
Properties

EWIS
Degradation

Installation Environment

Maintenance

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EWIS degradation
• EWIS degradation is a process that is a function of several
variables; aging is only one of these. Other main factors
that influence EWIS degradation are the:
- Environment in which it is installed.
- Physical properties of the EWIS.
- Actual physical installation of the EWIS.
- Maintenance (cleaning and repair) of the EWIS.
Characteristics of aging EWIS
• The manner in which EWIS degrades is therefore dependent
upon the EWIS type, how it was originally installed, the
overall time and environment exposed to in service, and how
the EWIS was maintained.
• Service history shows that “how the EWIS is installed” has a
direct effect on EWIS degradation. In other words, EWIS that
is not selected or installed properly has an increased
potential to degrade at an accelerated rate. Therefore, good

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aircraft EWIS practices are fundamental requirements for

EWIS to remain safely intact.

Causes of EWIS Degradation

• Vibration

• Moisture

• Maintenance

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Vibration – High vibration areas tend to accelerate degradation over time, resulting
in "chattering" contacts and intermittent symptoms. High vibration can also cause
tiewraps, or string-ties to damage insulation. In addition, high vibration will
exacerbate any existing problem with wire insulation cracking.
Moisture – High moisture areas generally accelerate corrosion of terminals, pins,
sockets, and conductors. It should be noted that EWIS installed in clean, dry areas
with moderate temperatures appears to hold up well.
Maintenance – Unscheduled maintenance activities, if done improperly, may
contribute to long term problems and EWIS degradation. Repairs that do not meet
minimum airworthiness standards may have limited durability. Repairs that conform
to manufacturers recommended maintenance practices are generally considered
permanent and should not require rework if properly maintained.
• Metal shavings and debris have been discovered on wire bundles after
maintenance or repairs have been conducted. Care should be taken to
protect wire bundles and connectors during modification work, and to
ensure all shavings and debris are cleaned up after work is completed.
• As a general rule, EWIS that is undisturbed will have less degradation
than EWIS that is reworked. As EWIS become more brittle with age,
this effect becomes more pronounced.

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Causes of EWIS Degradation, cont.

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Administration

Indirect damage – Events such as pneumatic duct ruptures can cause damage that, while
not initially evident, can later cause EWIS problems. When such an event has occurred,
surrounding EWIS should be carefully inspected to ensure no damage is evident.

Chemical contamination – Chemicals such as hydraulic fluid, battery electrolytes, fuel,

corrosion inhibiting compounds, waste system chemicals, cleaning agents, deicing fluids,
paint, and soft drinks can contribute to degradation of EWIS. EWIS in the vicinity of these
chemicals should be inspected for damage or degradation. Recommended original
equipment manufacturer cleaning instructions should be followed.

• Hydraulic fluids, for example, require special consideration. Hydraulic fluid is

very damaging to connector grommet and wire bundle clamps, leading to
indirect damage, such as arcing and chafing. EWIS components that may
have been exposed to hydraulic fluid should be given special attention during
EWIS inspections.

Heat – EWIS components exposed to high heat can accelerate degradation, insulation
dryness, and cracking. Direct contact with a high heat source can quickly damage insulation.
Even low levels of heat can degrade EWIS over long periods of time. This type of
degradation is sometimes seen on engines, in galleys, and behind lights.

Installation – EWIS not installed properly can further accelerate the EWIS degradation
process. Improper routing, clamping, and terminating during initial installation or during a
modifications can lead to EWIS damage.

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Current FAA Guidance

Part 25 Part 26 AC 25.1701-1

EWIS Policy
AC 25.27A
Practices ANM-01-04

AC 43.13-1b AC 25-16 AC 25-10

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Administration

There are direct specific part 25 and part 26 EWIS practices-related to 14

CFR.

There are some specific electrical power wiring requirements, such as

§25.1353, but they do not specifically address all aircraft EWIS.
14 CFR 25.1729 requires that instructions for continued airworthiness are
developed using analytical procedures, which would include maintenance
tasks and intervals for EWIS. A large body of FAA general guidance for
wiring practices is in Chapter 11 of AC 43.13-1b.

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Part 26
• Affects continued airworthiness issues and/or
safety improvements for transport airplanes
addressed via operational rules
• Supports the ability of operators to comply
with the operational rule requirements
• EAPAS Part 26 Requires actions of Design
Approval Holders (DAHs), such as:
– instructions for continued airworthiness,
– distribution of information to affected operators

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Part 25

New part 25 requirements for certification of

electrical wiring interconnection systems
(EWIS)
1.Revised existing EWIS related certification
requirements and relocated some of them
2.Created new EWIS certification requirements and
placed them in a new subpart H
3.New EWIS ICA requirements

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Part 26 Model Applicability

• Existing airplanes with a maximum
typecertificated passenger capacity of 30
or more, or

• Existing airplanes with a maximum

payload capacity of 7,500 pounds or more
(reference § 119.3)

• Existing airplane models with a type

certificate issued on or after January 1,
1958.
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AC 25.1701-1
• This Advisory Circular (AC) provides guidance
for certification of electrical wiring
interconnection systems (EWIS) on transport
category airplanes
– 14 CFR part 25, subpart H, sections 25.1701 through
25.1733
– H25.4 and H25.5 of Appendix H to part 25.

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AC 25.27A
• Provides guidance for developing
maintenance and inspection instructions for
EWIS
• Uses an enhanced zonal analysis procedure
(EZAP).
• For airplane models whose maintenance
programs already include a zonal inspection
program, the logic described here provides
guidance on improving those programs.
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AC 25.27A, cont.
• For airplanes without a zonal inspection program,
use of this logic will produce zonal inspections
for EWIS that can be added to the existing
maintenance program.
• Contains information that can be used by
operators to Improve EWIS maintenance
practices.
• Stresses the importance of inspecting EWIS and
promotes a philosophy of “protect and clean as
you go” when performing maintenance, repair, or
alterations on an airplane.
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Policy ANM-01-04
• Design data should NOT leave the installation to
the discretion of the installer.
• Routing of EWIS should follow the criteria
established by the FAA in the certification basis,
as reflected in the holder’s original or
subsequently approved type design.

• Installation drawings / instructions should

completely define the required routing and
installation with sufficient detail to allow
repeatability of the installation.
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Guidance: AC 43.13-1b

• AC 43.13-1b: Acceptable Methods,

Techniques, and Practices Aircraft
Inspection and Repair
–Flight Standards AC
–Chapter 11- Aircraft Electrical Systems
NOTE: The guidance provided in AC 43.13-1b is
general in nature and is not to be referenced or used as
a substitute for EWIS installation drawings and/or EWIS
diagrams.

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Administration

AC 43.13-1b covers a fairly comprehensive wide range of basic

EWIS practices topics.

NOTE: The guidance provided in AC 43.13-1b is general

in nature and is not to be referenced or used as a
substitute for EWIS installation drawings and/or EWIS
diagrams.

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Guidance: AC 25-16

• AC 25 -16: Electrical Fault and Fire

Prevention and Protection (4/5/91)
– Provides acceptable means to address
electrically caused faults, overheat, smoke,
and fire in transport category airplanes

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AC 25-16 provides wiring practices guidance as it relates to

aircraft fire and smoke safety with emphasis on wiring
flammability, circuit breaker protection, wiring near flammable
fluids, and associated acceptable test methods.
This AC is currently being considered for updating.

Guidance: AC 25-10

• AC 25 -10: Guidance for Installation of

Miscellaneous, Non-required Electrical
Equipment (3/6/87)
– Provides acceptable means to comply with
applicable 14 CFRs associated with installation
of
electrical equipment such as galleys and
passenger entertainment systems

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Administration

AC 25-10 mainly covers non-required equipment installations

such as galleys, passenger entertainment systems, etc. From a
wiring standpoint, all systems should be treated equally,
regardless of the functions criticality because of potential fire and
smoke hazards.
This AC contains minimal wiring practices specifics, including
general load analysis requirements and circuit breaker protection
requirements, which are more thoroughly covered in AC 43.13-1b
and AC 25-16, so we are not going to be covering 25-10 in any
detail.

Electrical Load Determination

• Load analysis
– Ensure that total electrical load can be safely
controlled or managed within rated limits of
affected components of aircraft’s electrical system
(§ 25.1351)
– New or additional electrical devices should not be
installed without an electrical load analysis (AC
43.13-1b)

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Electrical load determination is to ensure each aircraft electrical

bus can safely support a predetermined amount of electrical load
that is based on the electrical capacity of the aircraft generators
and the aircraft’s overall electrical distribution system.
§25.1351 requirement: It must be determined through analysis
that all electrical devices can be safely controlled or managed by
the aircraft’s electrical system.
AC 43.13-1b: Whenever an electrical device is added, a load
analysis should be performed to ensure that the new load on the

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bus can be powered adequately such that there is adequate

electrical power margin to avoid overloading the bus.
Where necessary as determined by a load analysis, wire, wire
bundles, and circuit protective devices having the correct ratings
should be added or replaced.

Circuit Breaker Devices

Must be sized to open before

current rating of attached wire is
exceeded, or before cumulative
rating of all connected loads is
exceeded, whichever is lower (§
25.1357)

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Section 25.1357 requires that automatic protective devices be

used to minimize distress to the electrical system and minimize
hazard to the airplane in the event of wiring faults or serious
malfunction of the system or connected equipment.

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Circuit Breaker Protection

• “A circuit breaker must always open
before any component downstream can
overheat and generate smoke
or fire.” (AC 43.13-1b, para. 11-48)

• “Circuit breakers are designed as circuit

protection for the wire, not for protection
of black boxes or components . . .” (AC
43.13-1b, para. 11-51)

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Administration

AC 43.13-1b contains some conflicting statements. The bullets in

this slide are somewhat contradictory. The first bullet says that
the breaker must protect against any downstream component
failure. The second bullet says breakers are designed such that
they DO NOT protect components or LRUs.
In reality, breakers are sized to protect the aircraft wiring as
the main design constraint. Any further protection of
components or LRUs is desirable but not mandatory.
Ideally, circuit breakers should protect against any wiring fault that
leads to arcing, sparking, flames, or smoke. But as we will learn,
thermal circuit breakers do not always detect arcing events.

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Circuit Breaker Protection, cont.

• Use of a circuit breaker as a

switch is not recommended
– Repeated opening and closing of
contacts can lead to damage and
premature failure of circuit breakers
– Most circuit breaker failures are
latent

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Administration

Most circuit breakers, other than some remote control circuit

breakers (RCCB), are not designed as switches and should not
be used as a switch. Repeated opening and closing of the
contacts can lead to damage and premature failure of the circuit
breakers. Also keep in mind that circuit breaker failures are, for
the most part, latent in nature. So you won’t know they have
failed until you need them.

Wire Selection
• Size wires so they:
– Have sufficient mechanical strength
– Do not exceed allowable voltage drop levels
– Are protected by circuit protection devices
– Meet circuit current-carrying requirements

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Section 25.1357 requires that automatic protective devices be

used to minimize distress to the electrical system and minimize
hazard to the airplane in the event of wiring faults or serious
malfunction of the system or connected equipment.

Wire Selection, cont.

• Mechanical strength of wire sizes less than

#20
– Do not use wire with less than 19 strands
– Provide additional support at terminations
– Should not be used when subject to excessive
vibration, repeated bending, or frequent
disconnection
(ref. para. 11-66(a), page 11-21)

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Wire containing less than 19 conductor strands must not be used.

Consideration should be given to the use of high-strength alloy conductors
in small gauge wires to increase mechanical strength.
As a general practice, wires smaller than #20 should be provided with
additional clamps and be grouped with at least three other wires.
They also should have additional support at terminations, such as
connector grommets, strain relief clamps, shrinkable sleeving, or
telescoping bushings.
They should not be used in applications where they will be subjected to
excessive vibration, repeated bending, or frequent disconnection from
screw termination.

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Wire Selection, cont.

• Conductor stranding
– Minimizes fatigue breakage

• Platings for all copper aircraft wiring

– Plated because bare copper develops
surface oxide film — a poor conductor
• Tin < 150° C

• Silver < 200° C

• Nickel < 260° C

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Elevated temperature degradation of tin- and silver-plated copper

conductors will occur if they are exposed to continuous operation
at elevated levels.
• For tin-plated conductors, tin-copper intermetallics will
form, resulting in an increase in conductor resistance.
• For silver-plated conductors, degradation in the form of
interstrand bonding, silver migration, and oxidation of the
copper strands will occur with continuous operation near
rated temperature, resulting in loss of wire flexibility. Also,
due to potential fire hazard, silver-plated conductors shall not
be used in areas where they are subject to contamination by
ethylene glycol solutions.
• Both tin- and silver-plated copper conductors will exhibit
degraded solderability after exposure to continuous elevated
temperature.

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Determining Current-
Carrying Capacity
• Effect of heat on wire insulation
– Maximum operating temperature
– Single wire or wires in a harness
– Altitude

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Heating is an important factor affecting wire insulation. This must

be factored into proper selection of wire for each particular
application.
sign
Determining Wire System
Design

• AC 43.13-1b, Section 5: tables and figures

provide an acceptable method of determining
wire system de

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The applicant should ensure that the maximum ambient

temperature that the wire bundles will be subjected to, plus the

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temperature rise due to the wire current loads, does not exceed
the maximum conductor temperature rating.
In smaller harnesses, the allowable percentage of total current
may be increased as the harness approaches the single wire
configuration.
The continuous current ratings contained in the tables and figures
in AC 43.13-1b were derived only for wire application, and cannot
be applied directly to associated wire termination devices (e.g.,
connector contacts, relays, circuit breakers, switches). The
current ratings for devices are limited by the design
characteristics of the device. Care should be taken to ensure that
the continuous current value chosen for a particular system circuit
shall not create hot spots within any circuit element which could
lead to premature failure.

Wire Substitution for Repairs

and Maintenance
• When replacement wire is required, review
aircraft maintenance manual to determine
if original aircraft manufacturer (OAM) has
approved any substitution
– If not approved, then contact OAM for
an acceptable replacement

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Most aircraft EWIS designs are to specifications that require

manufacturers to pass rigorous testing of wires before they are
approved or added to a Qualified Products List. Aircraft
manufacturers who maintain their own wire specifications
exercise close control of their approved sources. Therefore, it is
important to review the aircraft maintenance manual or contact

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the original aircraft manufacturer (OAM) when wire substitutions

are necessary.
The OAM may have special concerns regarding shielding,
insulation, etc. for certain wiring on the aircraft that perform
critical functions or wiring that is chosen based on a set of unique
circumstances.

EWIS Routing
• Eliminate potential for chafing against
structure or other components
• Position to eliminate/minimize use as
handhold or support
• Minimize exposure to damage by
maintenance crews or shifting cargo
• Avoid battery electrolytes or other
corrosive fluids

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In general, EWIS should be routed in such a manner to ensure

reliability and to offer protection from the potential hazards shown
in this slide.

The next several slides are pictures illustrating these hazards.

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EWIS Riding on Structure

Power cables riding
on structure can
cause damage to
the power cables

Improper

Proper

Example of wire chafing.

EWIS Riding on Other EWIS

Wire bundles that
cross should be
secured together to
avoid chafing

Improper

Proper

Example of wire chafing.

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EWIS Riding on Lightening Hole Edge

If the grommet is too

short, then there is
wire bundle chafing

Improper

Proper

Example of wire chafing.

EWIS as a Handhold

Route EWIS so that it is not used as a handhold or as a support for

maintenance personnel.

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In addition, route EWIS so that it avoids:

• Damage by personnel moving within the aircraft.
• Damage by stowage or shifting cargo.
• Damage by battery or acidic fumes or fluids.
• Abrasion in wheel wells where exposed to rocks, ice, mud, etc.
• Damage from external events (zonal analysis/particular risks analysis
demands).
• Harsh environments such as severe wind and moisture-prone
(SWAMP) areas, high temperatures, or areas susceptible to significant
fluid or fume concentration.
EWIS should be routed to permit free movement of shock and vibration
mounted equipment, designed to prevent strain on wires, junctions, and
supports, and, the EWIS installation should permit shifting of EWIS and
equipment necessary to perform maintenance within the aircraft. In addition,
wire lengths should be chosen to allow for at least two reterminations.

EWIS Routing, cont.

• Protect EWIS in wheel wells and other

exposed areas
• Route EWIS above fluid lines, if practicable
• Use drip loops to control fluids or condensed
moisture
• Keep slack to allow maintenance and prevent
mechanical strain

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Ensure that EWIS components are adequately protected in

wheel wells and other areas where they may be exposed to
damage from impact of rocks, ice, mud, etc. (If re-routing of
EWIS is not practicable, protective jacketing may be
installed.) This type of installation must be held to a
minimum.

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Where practical, route EWIS s and cables above fluid lines.

Wires and cables routed within 6 inches of any flammable
liquid, fuel, or oxygen line should be closely clamped and
rigidly supported. A minimum of 2 inches must be maintained
between wiring and such lines or related equipment, except
when the wiring is positively clamped to maintain at least 1/2-
inch separation or when it must be connected directly to the
fluid-carrying equipment.
Ensure that a trap or drip loop is provided to prevent fluids or
condensed moisture from running into EWIS and other
components. EWIS installed in bilges and other locations
where fluids may be trapped are routed as far from the lowest
point as possible or otherwise provided with a moisture-proof
covering.

EWIS Above Fluid Lines

Path of exposed end

Broken wire shall not make

contact with fluid line

EWIS above fluid lines. The clamps should be a compression

type and should be spaced so that, assuming a wire break, the
broken wire will not contact hydraulic lines, oxygen lines,
pneumatic lines, or other equipment whose subsequent failure
caused by arcing could cause further damage.

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Wires improperly tied,

riding on hydraulic lines,
contaminated with
caustic fluid

This example shows a number of problems:

• Wires in the bundles are not tied properly.
• The wire bundle is riding hard on the hydraulic lines.
• The wire bundles appears to be contaminated with hydraulic
fluid residue.

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Y Type Wire Bundle Breakouts

Figure 8 loop may be
located before
Afte
or after B r Wire bundle
ef
tail of Y or breakout
e

Head of strap shall not

Wire be located in this area
bundles or touching anything
to cause chafing

Plastic mechanical strapping

Wire bundle breakouts. There are three basic wire bundle

breakout types used in routing aircraft EWIS. They are called the
“Y,” “T,” and Complex types.
The “Y” type of breakout is used when a portion of EWIS from
one direction of the wire bundle departs the bundle to be routed in
another direction.

 Care should be taken when plastic tie wraps are used to

provide wire containment at the breakout so that the tie wrap
head does not cause chafing damage to the wire bundle at
the breakout junction.

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T Type Wire Bundle Breakouts

Head of strap shall Wire bundle breakout

not be located in
this area or
touching anything
to cause chafing

Wire
Plastic mechanical strapping
bundle

The “T” type of breakout (also called 90° breakout) is used

when portions of EWIS from both directions in the wire bundle
depart the bundle to be routed in another direction.

Complex Type
Wire Bundle Breakouts

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A Complex type of breakout is generally used to route certain

wires out of a wire bundle to a terminal strip, module block, or
other termination.
For all types of breakouts, there should be sufficient slack in the
wires that are being broken out of the bundle to avoid strain on
the wire between the wire bundle and the termination.

Stand-offs

• Use stand-offs to maintain clearance

between EWIS and structure
– Employing tape or tubing is generally not
acceptable as an alternative

• Exception: Where impossible to install off-

angle clamps to maintain EWIS separation
in holes, bulkheads, floors, etc.

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The EWIS design should preclude wire bundles from contacting

structure. Stand-offs should be used to maintain clearance
between EWIS and structure.
Employing tape or protective tubing as an alternative to
standoffs should be avoided as a primary means of preventing
EWIS contact with structure.
Exception: Using tape or tubing is allowed in cases where it is
impossible to install off-angle clamps to maintain EWIS
separation in holes, bulkheads, floors, etc.

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Using Stand-offs

Improper

Proper

When using standoffs for additional clearance, clamps should not

be installed in a manner that defeats the standoff’s purpose of
providing additional clearance between EWIS and structure.

Bundle riding on structure

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One of the more common aircraft EWIS problems is chafing due

to wire bundles coming into contact with aircraft structure or other
aircraft equipment.

Wire bundle too

close to control
cable

This picture shows a wire bundle that is in close contact with a

control cable. Adequate distance between EWIS and control
cables should be maintained to account for movement due to
slack and maintenance.

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Clamping

• Support wires by suitable clamps, grommets,

or other devices at intervals of not more that
24 inches

• Supporting devices should be of suitable size

and type with wire and/or cables held
securely in place without damage to wire or
wire insulation

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Wire supports and intervals. Clamps and other primary support

devices should be constructed of materials that are compatible
with their installation and environment, in terms of temperature,
fluid resistance, exposure to ultraviolet light, and wire bundle
mechanical loads.
• Generally, clamps should not be spaced at intervals
exceeding 24 inches. In high vibration areas or areas
requiring routing around structural intrusions, the clamping
intervals may need to be reduced in order to provide
adequate support.

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Clamps

• Wire bundles should be snug in clamp

(no movement)
– Cable not able to move axially

• RF cables: do not crush

• Mount clamps with attachment hardware
on top
• Tying not used as alternative to
clamping

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Clamps on wire bundles should not allow the bundle to move through the
clamp when a slight axial pull is applied.

Clamps on RF cables must fit without crushing and must be snug enough
to prevent the cable from moving freely through the clamp, but may allow the
cable to slide through the clamp when a light axial pull is applied. The cable
or wire bundle may be wrapped with one or more turns of tape or other
material suitable for the environment when required to achieve this fit.

• Plastic clamps or cable ties must not be used where their failure could
result in interference with movable controls, wire bundle contact with movable
equipment, or chafing damage to essential or unprotected EWIS. They must
not be used on vertical runs where inadvertent slack migration could result in
chafing or other damage.

• Clamps must be installed with their attachment hardware positioned

above them, wherever practicable, so that they are unlikely to rotate as the
result of wire bundle weight or wire bundle chafing.

Clamps lined with nonmetallic material should be used to support the wire
bundle along the run. Tying may be used between clamps, but should not
be considered as a substitute for adequate clamping. Adhesive tapes are
subject to age deterioration and, therefore, are not acceptable as a clamping
means.

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Example of Correct Cable Slack

Appropriate slack

This is an example of an appropriate amount of cable slack

between clamps. Appropriate slack protects the wires from stress
and from contact with inappropriate surfaces.
• Too much cable slack can allow the cable to contact
structure or other equipment which could damage the wire
bundle.
• Too little slack can cause a pre-load condition on the cable
which could cause damage to the wire bundle and/or clamps
as well.
• Also, sufficient slack should be left between the last
clamp and the termination or electrical equipment to
prevent strain at the terminal and to minimize adverse effects
of shock-mounted equipment.

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Clamp Distortion
Proper clamp position

Improper clamp position

Distortion of rubber on
clamp is NOT acceptable

As is shown in the top graphic, the wire bundles are routed

perpendicular to the clamp.
• If wire bundles are not routed perpendicular to the clamp
(bottom graphic), stress can be created against the clamp
and clamp grommet which can distort the clamp and/or
clamp grommet. Distorted clamps/clamp grommets can
cause wire bundle damage over time.

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Clamp Orientation

90±5°

Correct Incorrect

90±5° Incorrect
Correct

This slide further illustrates correct and incorrect clamp

orientations. Incorrect clamp orientation can lead to wire bundle
damage.

Example - Clamp Distortion

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This photograph is a good example of clamp distortion. Note that

the wire bundle is not perpendicular to the clamp.

Plastic Snap-in Clamp (Tie Mount)

support
bracket

snap-in tie
mount

release
tail
tab

Some EWIS designs utilize plastic snap-in clamps sometimes

referred to as “tie mounts.” These types of clamps are not
suitable for large wire bundles and should not be used in high
temperature or high vibration areas.
• Any type of plastic clamp or cable tie should not be used
where their failure could result in interference with movable
controls, wire bundle contact with movable equipment, or
chafing damage to essential or unprotected EWIS.

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Typical Rubber Clamp

All wires contained
Rubber cushion
in rubber cushion

Clamp Wedge
tabs
No
pinching
Stand off

A common problem in aircraft EWIS is clamp pinching. This

occurs when the clamp is improperly installed or the clamp is too
small. Clamps on wire bundles should be selected so that they
have a snug fit without pinching wires.

Typical Nylon Closed-Face

Clamp Installation

Do not pinch
wire here

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It is important when adding wiring to an existing wire bundle to

evaluate the existing clamp sizing in order to avoid possible
clamp pinching. In some cases it may be necessary to increase
the size of the clamps to accommodate the new wiring.

Engage Clamp Tab in Slot

Improper

Clamp
tab

Clamp
slot
Proper

When using clamp tabs, make sure that the tabs are properly
engaged. Otherwise, the tab could become loose and cause
subsequent wire damage.
• During EWIS installation inspections, ensure that the clamp
is snapped before installing and tightening the bolt.

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Clamp Pinching
Improper

Do not pinch
wires here

Proper

This slide further illustrates how wires can be pinched and

damaged due to improper clamp installation.

Clamp Pinching, cont.

Improper

Proper

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Too much wiring in a clamp or improperly installed clamps can

lead to pinching of the wires.

Open-faced nylon clamp with cable

build-up (missing hardware)

This picture was taken during a general visual EWIS inspection of

a wide-body transport aircraft. Note the missing clamp hardware.
Also note that the black cable used a tape build-up at the clamp.
Some manufacturer’s EWIS specifications allow for wire cable
build-up under certain circumstances.

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Wire Bend Radii

• Minimum bend radius - 10 times the

outside diameter of the largest wire or
cable in the group — unsupported
– Exceptions
• Terminations/reversing direction in bundle (supported at
both ends of loop) -
3 times the diameter
• RF cables - 6 times the diameter

• Thermocouple wire - 20 times the diameter

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The minimum radius of bends in wire groups or bundles must

not be less than 10 times the outside diameter of the largest
wire or cable, except that at the terminal strips where wires break
out at terminations or reverse direction in a bundle.
Where the wire is suitably supported, the radius may be 3
times the diameter of the wire or cable.

Where it is not practical to install wiring or cables within the radius

requirements, the bend should be enclosed in insulating tubing.
The radius for thermocouple wire is 20 times the diameter. (This
is very delicate wire.)
Ensure that RF cables, (for example, coaxial and triaxial, are
bent at a radius of no less than 6 times the outside diameter of
the cable.

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Minimum Bend Radii

No support at Min. bend radius - 10 x

end of bend parameter of wire or cable

Min. bend radius

3 x diameter of wire

Diameter of Support at both

wire or cable ends of wire bend

This illustration shows the proper bend radii for three different
scenarios.

Bend radii okay-

Greater than 3 times diameter
(secured at both ends of loop
)

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This photograph shows a wire loop secured at both ends of the

loop. In this case, the bend radius should be no less than 3 times
the diameter of the largest wire in the wire bundle.

Bend radii problem-

Less than 3 times the diameter

Also supported at each end of the loop, this wire bundle does not
meet bend radius standards due to the large wires in the bundle.

Unused Wires

• Secured
– Tied into a bundle or secured to a permanent
structure

• Individually cut with strands even with

insulation

• Pre-insulated, closed-end connector or

1inch piece of insulating tubing folded and
tied back

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Ensure that unused wires are individually dead-ended, tied into

a bundle, and secured to a permanent structure.
• Each wire should have strands cut even with the insulation
and a pre-insulated closed end connector or a 1-inch piece of
insulating tubing placed over the wire with its end folded back
and tied.

Spare Connector Contact:

Preparing Single Contact

Tubing
Contact
Wire

3 times length of contact

This slide and the next two depict an acceptable method of

insulating and physically securing a spare connector contact
within a wire bundle.

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Spare Connector Contact: Folding

Tube and Tying Single Contact

0.75 ± 0.15
in.
Tying tape

Fold

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Spare Connector Contact: Single

Contact Attachment to Wire Bundle
Wire Tying tape
bundle

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Spare Wire Termination Using Endcap

Wire and end cap Install end cap over wire
in position end. Shrink in place.

Wire
Adhesive tape bundle

End caps
Fiberglass
tying tape

Installing prefabricated end caps are an effective method of

protecting unused wires with exposed conductors.

 This slide depicts a typical example of the use of a

prefabricated end cap.

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Unused wiring -
Improper termination with exposed conductor
(should be properly insulated and
secured to bundle)

Found during a general EWIS visual inspection, this example

shows two unused wires that have been cut and the conductors
are unprotected. In addition, the unused wires are not secured to
the wire bundle.

Coil and Stow Methods

Wire
bundle Wire
bundle
ties
Clamp

Coil and stow short wire bundles in low

vibration areas

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Coil and stow methods are often used to secure excess length of
a wire bundle or to secure wire bundles that are not connected to
any equipment, such as wiring provisioning for a future
installation.

Coil and Stow Methods

, cont.

Wire bundle ties

Clamp

Excess wire
Wire bundle

Coil and stow long wire bundles in low

vibration areas

The key objective to coiling and stowing wiring is to safely secure

the wire bundle to prevent excessive movement or contact with
other equipment that could damage the EWIS.

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Coil and Stow Methods

, cont.

Wire
bundle Teflon Wire
tape bundle
ties

Adjacent wire bundle

Coil and stow in medium and high

vibration areas

Coil and stow in medium and high vibration areas requires

additional tie straps, sleeving, and support.

Stowing Unused Wires

Improper

Proper

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These photos show improper and proper stowing of unused

wires.

EWIS Replacement
• EWIS components should be replaced when:
– Chafed or frayed
– Insulation suspected of being penetrated
– Outer insulation is cracking
– Damaged by or known to have been exposed to
electrolyte, oil, hydraulic fluid, etc.
– Evidence of overheating can be seen

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EWIS needs to be replaced under a number of circumstances:

• Wiring that has been subjected to chafing or fraying, that has
been damaged, or that primary insulation is suspected of
being penetrated.
• Wiring on which the outer insulation is brittle when slight
flexing causes it to crack.
• Wiring that has weather-cracked outer insulation. NOTE:
some wire insulation types appears to be wrinkled when the
wire is bent and may not be damaged.
• Wiring that is known to have been exposed to electrolyte or
on which the insulation appears to be, or is suspected of
being, in an initial stage of deterioration due to the effects of
electrolyte.
• There is visible evidence of insulation damage due to
overheating.

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Heat Discoloration

This photograph shows an example of heat discoloration on

protective sleeving which is part of the wire bundle. The large
clamp was moved to see the difference in color. In this case, the
wiring that is not covered in sleeving shows no signs of heat
distress. An adjacent light bulb was radiating enough heat to
cause discoloration over time to the protective sleeving. Although
this condition is not ideal, it is acceptable.

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Wire Replacement

• Wire should be replaced when:

– Wire bears evidence of being crushed or kinked
– Shield on shielded wire if frayed and/or
corroded
– Wire shows evidence of breaks, cracks, dirt, or
moisture in plastic sleeving
– Sections of wire have splices occurring at less
than 10-ft intervals

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Continuing, these are additional circumstances that warrant

replacing EWIS:
• EWIS that bears evidence of having been crushed or
severely kinked.
• Shielded EWIS on which the metallic shield is frayed and/or
corroded. Cleaning agents (which can cause wire damage)
or preservatives should not be used to minimize the effects of
corrosion or deterioration of wire shields.
• EWIS showing evidence of breaks, cracks, dirt, or moisture in
the plastic sleeves placed over wire splices or terminal lugs.
• Sections of wire in which splices occur at less than 10-foot
intervals, unless specifically authorized, due to parallel
connections, locations, or inaccessibility.

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Wire Replacement, cont.

• Shielding requirements
– Replacement wires must have the same
shielding characteristics as the original wire, such
as shield optical coverage and resistance per unit
length

– Replacement wires should not be installed

outside the bundle shield

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Replacement wires should have the same shielding

characteristics as the original wires, such as shield optical
coverage and resistance per unit length.
If any wires are going to be replaced inside a shielded wire
bundle, the replacement wires should not be installed outside the
bundle shield.
For more information on shielding, the Lightning/HIRF Video and
Self-study Guide is available. (To obtain, see your Directorate
training manager.)

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Adding or Replacing Wires

on a Bundle

Chafing
Improper
procedure

Proper
procedure

When adding or replacing wires on or in a wire bundle, the

replacement or added wire should be routed in the same manner
as the other wires in the wire bundle.
• Wire bundle clamps and/or ties may need to be loosened or
removed in order to properly add or replace wires.
• When the new wire is installed, the ties and clamps should
be opened one at a time to avoid excessive disassembly of
the wire bundles.

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Adding Wires on a Bundle

Improperly routed
outside of the tie wrap
that secures the clamp

Properly routed

Proper and improperly routed wires in a bundle

Wire Splicing
• Keep to a minimum
• Avoid in high vibration areas
• Locate to permit inspection
• Stagger in bundles to minimize
increase in bundle size
• Use self-insulated splice connector, if
possible

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Splicing is permitted on EWIS as long as it does not affect the reliability and
the electro-mechanical characteristics of the EWIS. Splicing of power wires,

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co-axial cables, multiplex bus, and large gauge wire should be avoided. If it
can’t be avoided, then the power wire splicing must have approved data.

• Splicing of electrical wire should be kept to a minimum and

avoided entirely in locations subject to extreme vibrations. Splicing of
individual wires in a group or bundle should have engineering approval
and the splice(s) should be located to allow periodic inspection.

Many types of aircraft splice connectors are available for use when
splicing individual wires.

• Use of a self-insulated splice connector is preferred; however, a non-

insulated splice connector may be used provided the splice is covered
with plastic sleeving that is secured at both ends.

• 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.

Staggered Splices

Splices in bundles should be staggered so as to minimize any

increase in the size of the bundle that would:
• Prevent bundle from fitting into designated space.
• Cause congestion adversely affecting maintenance.

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• Cause stress on the wires.

Overheated wire at the splice

Splices that are not crimped properly (under or over) can cause
increased resistance leading to overheat conditions.

Ganged
wire
splices

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If splices are not staggered, proper strain relief should be

provided in order to avoid stress on the wires. In this particular
installation, strain relief was applied to avoid stress on the wires.

Ganged wire splices, cont.

The top two wires in this photo are experiencing stress due to a
preload condition. Also note that the wire bundle is not properly
clamped.

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Terminals

• Tensile strength of the wire-to-terminal

joint should be at least the equivalent
tensile strength of the wire

• Resistance of the wire-to-terminal joint

should be negligible relative to the
normal resistance of the wire

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Tensile strength terminals are attached to the ends of electrical

wires to facilitate connection of the wires to terminal strips or
items of equipment. The tensile strength of the wire-to-terminal
joint should be at least equivalent to the tensile strength of the
wire itself.
Resistance of wire-to-terminal joint should be negligible,
relative to the normal resistance of the wire.
• Selection of wire terminals. The following should be
considered in the selection of wire terminals.
- Current rating.
- Wire size (gauge) and insulation diameter.
- Conductor material compatibility.
- Stud size.
- Insulation material compatibility.
- Application environment.
- Solder/solderless.

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Bending of Straight Copper

Terminals

Brazed
joint
Position of
tongue before
bending

If bending of a terminal is necessary, care should be taken to avoid over

bending the terminal which can cause damage to the terminal. Also, a terminal
can only be bent once since any additional bending can cause damage.

Pre-insulated crimp-type ring-tongue terminals are preferred. The strength, size,

and supporting means of studs and binding posts, as well as the wire size,
should be considered when determining the number of terminals to be attached
to any one post.

In high-temperature applications, the terminal temperature rating must be

greater than the ambient temperature plus current related temperature rise. Use
of nickel-plated terminals and of uninsulated terminals with high-temperature
insulating sleeves should be considered. Terminal blocks should be provided
with adequate electrical clearance or insulation strips between mounting
hardware and conductive parts.

Terminals are sensitive to bending at the junction between the terminal ring and
the terminal crimp barrel. Bending the terminal more than once or exceeding
pre-determined terminal bend limits will usually result in mechanical weakening
or damage to the terminal.

This slide is an example of limits established by the OAM with regard to bending
the terminal prior to installation.

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Terminal Strips

• Barriers to prevent adjacent studs from

contacting each other
• Current should be carried by terminal contact
surface and not by stud
• Studs anchored against rotation
• Replace defective studs with studs of same
size and material, mount securely, tighten
terminal securing nut

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Wires are usually joined at terminal strips. A terminal strip fitted

with barriers should be used to prevent the terminals on adjacent
studs from contacting each other.
• Studs should be anchored against rotation. When more than
four terminals are to be connected together, a small metal
bus should be mounted across two or more adjacent studs.
In all cases, the current should be carried by the terminal
contact surfaces and not by the stud itself.
• Defective studs should be replaced with studs of the same
size and material since terminal strip studs of the smaller
sizes may shear due to overtightening the nut. The
replacement stud should be securely mounted in the terminal
strip and the terminal securing nut should be tight.

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Terminal Strips, cont.

• Mount strips so loose metallic objects
cannot fall across terminal
– Provide spare stud for breaks and future expansion
– Inspect terminal periodically for loose connections,
metallic objects, dirt, and grease accumulation
• Can cause arcing, resulting in fire or systems
failure

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Terminal strips should be mounted in such a manner that loose

metallic objects cannot fall across the terminals or studs. It is
good practice to provide at least one spare stud for future circuit
expansion or in case a stud is broken.
• Terminal strips should be inspected for loose connections,
metallic objects that may have fallen across the terminal
strip, dirt and grease accumulation, etc. These conditions
can cause arcing which may result in a fire, or system
failures.

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Terminals on circuit breakers

Connectors and terminals in aircraft require special attention to

ensure a safe and satisfactory installation. Every possibility of
terminals not being torqued properly, due to misinstallation, poor
maintenance, and service life, should be addressed in the design.
• Electrical equipment malfunction has frequently been traced
to poor terminal connections at terminal boards.
• Loose contact surfaces can produce localized heating that
may ignite nearby combustible materials or overheat
adjacent wire insulation.
Note the green torque stripes painted on the terminal fasteners in
this picture. This is an excellent method to quickly determine if a
terminal fastener is still torqued to its original value.

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Power feeder terminals

Again, you can see the red colored torque stripes applied to these
high current power feeder terminations. High current terminals
are more sensitive to increased resistance due to a improperly
torqued terminal.
• As a side note, the power feeder cables should not be
touching each other without being suitably tied with spacers
or other securing device.

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Terminal Lugs
• Connect wiring to terminal block studs
• No more than 4 lugs, or 3 lugs and a bus bar,
per stud
• Lug hole size should match stud diameter
– Greatest diameter on bottom, smallest
on top
– Tightening terminal connections
should not deform lugs

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Wire terminal lugs should be used to connect wiring to terminal

block studs or equipment terminal studs. No more than four
terminal lugs or three terminal lugs and a bus should be
connected to any one stud.
• Total number of terminal lugs per stud includes a common
bus bar joining adjacent studs. Four terminal lugs plus a
common bus bar thus are not permitted on one stud.
Terminal lugs should be selected with a stud hole diameter that
matches the diameter of the stud. However, when the terminal
lugs attached to a stud vary in diameter, the greatest diameter
should be placed on the bottom and the smallest diameter on top.
Tightening terminal connections should not deform the terminal
lugs or the studs. Terminal lugs should be so positioned that
bending of the terminal lug is not required to remove the fastening
screw or nut, and movement of the terminal lugs will tend to
tighten the connection.

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Terminal Lugs, cont.

• Aluminum lugs
– Crimped to aluminum wire only
• Special attention needed to guard against excessive
voltage drop at terminal junction
– Inadequate terminal contact area
– Stacking errors
– Improper torquing

– Use calibrated crimp tools

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Aluminum terminal lugs should be crimped to aluminum wire only.

The tongue of the aluminum terminal lugs or the total number of tongues of
aluminum terminal lugs when stacked, should be sandwiched between two flat
washers (cadmium plated) when terminated on terminal studs. Spacers or
washers should not be used between the tongues of like material terminal lugs.
Special attention should be given to aluminum wire and cable installations to
guard against conditions that would result in excessive voltage drop and high
resistance at junctions that may ultimately lead to failure of the junction.
 Examples of such conditions are improper installation of terminals and
washers, improper torsion (“torquing” of nuts), and inadequate terminal
contact areas.
Note that aluminum wire is normally used in sizes of 10 gauge and larger to carry
electrical power in large transport category aircraft in order to save weight.
Although not as good a conductor as copper, aluminum is lighter when compared
to copper and the weight savings can be significant for a large aircraft that may
have several hundred feet of power feeder cable.
Because aluminum is used primarily for high current power applications, the
terminal junctions are more sensitive to conditions leading to increased
junction resistance which can cause arcing and localized heat distress.

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Termi nal
Stacking Nut
(like materials) Lock washer

Flat washer

Copper
terminal
lugs

Terminal stud

Terminal stacking materials and methods

• Multiple wires often terminate onto a single terminal stud.
Care should be taken to install the terminal properly. The
materials that the terminals are constructed of will impact the
type of stacking methods used. Dissimilar metals, when in
contact, can produce electrolysis that can cause
corrosion, thus degrading the terminal junction resistance
and causing arcing or hot spots.
• For stacking terminals that are made of like materials, the
terminals can be stacked directly on top of each other.

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Terminal Stacking Nut

(unlike materials) Lock washer

Flat washer

Copper terminal
Aluminum
terminals
Flat
washers

Terminal stud

When stacking unlike materials together (for example,

aluminum and copper), a cadmium-plated flat washer is usually
needed to isolate the dissimilar metals.

Terminal Nut
Stacking Lock
washer
Methods
Flat
washer
Crimp barrel
(belly up)
Crimp barrel
(belly down
)

One-Sided Entry With Two Terminals

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When two terminals are installed on one side of the terminal

strip, care should be taken to ensure that the terminal crimp
barrels do not interfere with one another. One method to avoid
this problem is to install the terminals with the barrels “back to
back.”

Terminal Nut
Lock
Stacking washer
Flat washer
Methods, cont.
Crimp barrel
(belly up) in
center of “V”
Crimp barrel
(belly down)
in “V” split

One-Sided Entry With 3 Terminals

This illustration depicts a terminal installation with three

terminals entering on one side.

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Terminal Nut
Lock washer
Stacking Flat washer
Methods, cont. Crimp barrel
(belly up) in
“V”split

Crimp barrel
(belly down)
in “V”split

One-Sided Entry With 4 Terminals

This illustration depicts a terminal installation with four terminals

entering on one side.
The stacking method used to connect terminals to terminal strips
should cause no interference between terminals that could
compromise the integrity of the terminal junction.

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Terminal Tightening Hardware

Improper
Proper
Space
Nut
Lock
washer
Flat
washer

Lock washer
not compressed
Lock washer compressed

Service history has shown that hardware stack up at terminals is

prone to human error. Omission of lock washers, incorrect
washers, improper sizing of washers, etc. has been a definite
problem.
It is important to use the correct tightening hardware and install it
correctly for a given installation. This illustration shows a typical
flat washer/lock washer/nut installation. It is important to ensure
the locking washer is fully compressed and is adjacent to the nut.
After the terminal is completely assembled, there should be a
minimum of two to three threads showing on the stud when
the nut is properly torqued.

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Washer Size Selection

Improperly sized

Raised portion of
terminal

Split lock
washer
Non-self locking nut

Steel washers Aluminum

terminal
Properly sized
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It is important to select and use the correct size washers in any

termination. Undersized or oversized washers can lead to
increased junction resistance and localized heat or arcing.
This illustration shows how an improperly sized washer can lead
to insufficient contact between the terminal and terminal lug.

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Lock Washers

In this photograph, the lock washer is missing from the terminal

on the left.

Grounding: Definition

• Grounding is the process of

electrically connecting
conductive objects to either a
conductive structure or some
other conductive return path for
the purpose of safely
completing either a normal or
fault circuit.

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Grounding. One of the more important factors in the design and

maintenance of aircraft electrical systems is proper bonding and

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grounding. Inadequate bonding or grounding can lead to

unreliable operation of systems, such as EMI, electrostatic
discharge damage to sensitive electronics, personnel shock
hazard, or damage from lightning strike.

Grounding
• Types of grounding
–AC returns
–DC returns
–Others

• Avoid mixing return currents from various

sources
–Noise will be coupled from one source to another and
can be a major problem for digital systems

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Grounding types: AC returns, DC returns, and others.

Mixing return currents. If wires carrying return currents from different
types of sources, such as signals or DC and AC generators, are
connected to the same ground point or have a common connection in
the return paths, an interaction of the currents will occur. This interaction
may not be a problem, or it could be a major non-repeatable anomaly.
• To minimize the interaction between various return currents, different
types of grounds should be identified and used. As a minimum, the
design should use three ground types: (1) AC returns, (2) DC
returns, and (3) all others.
• For distributed power systems, the power return point for an
alternative power source would be separated.
- For example, in a two-AC generator system (one on the right side and
the other on the left side), if the right AC generator were supplying
backup power to equipment located in the left side, (left equipment rack)
the backup AC ground return should be labeled “AC Right.” The return
currents for the left
generator should be connected to a ground point labeled “AC Left.”

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Grounding, cont.
• Design of ground path should be given as
much attention as other leads in the
system
• Grounding should provide a constant
impedance
• Ground equipment items externally even
when internally grounded
– Avoid direct connections to magnesium structure for
ground return
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Design of ground paths. The design of the ground return circuit should be
given as much attention as the other leads of a circuit.

Constant impedance. A requirement for proper ground connections is that

they maintain an impedance that is essentially constant.

• Ground return circuits should have a current rating and voltage drop
adequate for satisfactory operation of the connected electrical and
electronic equipment.

• EMI problems, that can be caused by a system’s power wire, can be

reduced substantially by locating the associated ground return near the
origin of the power wiring (e.g., circuit breaker panel) and routing the power
wire and its ground return in a twisted pair.

• Special care should be exercised to ensure replacement on ground return

leads. The use of numbered insulated wire leads instead of bare
grounding jumpers may aid in this respect.

External grounding of equipment items. In general, equipment items

should have an external ground connection, even when internally grounded.
Direct connections to a magnesium structure (which may create a fire
hazard) must not be used for ground return.

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Grounding, cont.

• Heavy current grounds

– Attach to individual grounding brackets attached to
aircraft structure with a proper metal-to-metal bond
– Accommodate normal and fault currents of system
without creating excessive voltage drop or damage
to structure
– Give special attention to composite aircraft

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Heavy current grounds. Power ground connections for

generators, transformer rectifiers, batteries, external power
receptacles, and other heavy-current loads must be attached to
individual grounding brackets that are attached to aircraft
structure with a proper metal-to-metal bonding attachment.
• This attachment and the surrounding structure must provide
adequate conductivity to accommodate normal and fault
currents of the system without creating excessive voltage drop
or damage to the structure.
• At least three fasteners, located in a triangular or rectangular
pattern, must be used to secure such brackets in order to
minimize susceptibility to loosening under vibration.
• If the structure is fabricated of a material such as carbon fiber
composite (CFC), which has a higher resistivity than aluminum
or copper, it will be necessary to provide an alternative ground
path(s) for power return current.

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Bonding
• Equipment bonding
– Low impedance paths to aircraft structure
required for electronic equipment to provide
radio frequency return circuits
– Facilitates reduction in EMI for most electrical
equipment
• Cases of components that produce EMI should be
grounded to structure

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Equipment bonding. Low impedance paths to aircraft structure

are normally required for electronic equipment to provide radio
frequency return circuits and for most electrical equipment to
facilitate reduction in EMI. The cases of components that
produce electromagnetic energy should be grounded to structure.
• To ensure proper operation of electronic equipment, it is
particularly important to conform the system’s installation
specification when inter-connections, bonding, and grounding
are being accomplished.

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Bonding, cont.

• Metallic surface bonding

– Electrically connecting conductive exterior airframe
components through mechanical joints, conductive
hinges, or bond straps
• Protects against static charges and lightning strikes

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Metallic surface bonding. All conducting objects on the exterior

of the airframe must be electrically connected to the airframe
through mechanical joints, conductive hinges, or bond straps
capable of conducting static charges and lightning strikes.
• Exceptions may be necessary for some objects such as antenna
elements, whose function requires them to be electrically
isolated from the airframe. Such items should be provided with
an alternative means to conduct static charges and/or lightning
currents, as appropriate.

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Bonding, cont.

• Static bonds
– Required for all isolated conducting parts with
area greater than 3 in2 and a linear dimension
over 3" subjected to appreciable electrostatic
charging due to precipitation, fluid, or air in
motion
• Resistance of less than 1 ohm when clean and dry
usually ensures static dissipation on larger objects

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Static bonds. All isolated conducting parts inside and outside

the aircraft, having an area greater than 3 in2 and a linear
dimension over 3 inches, that are subjected to appreciable
electrostatic charging due to precipitation, fluid, or air in motion,
should have a mechanically secure electrical connection to the
aircraft structure of sufficient conductivity to dissipate possible
static charges.
• A resistance of less than 1 ohm when clean and dry will
generally ensure such dissipation on larger objects. Higher
resistances are permissible in connecting smaller objects to
airframe structure.

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EWIS Identification

• Necessary for:
– Safety of operation
– Safety to maintenance personnel
– Ease of maintenance

• To identify performance capability, use

wire material part number and five
digit/letter code identifying
manufacturer

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Purpose. The proper identification of EWIS components with their circuits and voltages is
necessary to provide safety of operation, safety to maintenance personnel, and ease of
maintenance.

Common manufacturer marking process. Each wire and cable should be marked with a part
number. It is common practice for wire manufacturers to follow the wire material part number
with the five digit/letter C.A.G.E. code identifying the wire manufacturer. Using this code,
existing installed wire that needs replacement can be identified as to its performance
capabilities. This helps to prevent the inadvertent use of lower performance and unsuitable
replacement wire.

• NOTE: Special care should be taken when hot stamping wire. Service history has
shown problems associated with hot stamping due to insulation damage caused
during the process.

• The method of identification should not impair the characteristics of the EWIS.

• Original wire identification. To facilitate installation and maintenance, retain the

original wire-marking identification. The wire identification marks should consist of
a combination of letters and numbers that identify the wire, the circuit it belongs to,
its gauge size, and any other information to relate the wire to a EWIS diagram. All
markings should be legible in size, type, and color.

• Identification and information related to the EWIS diagrams. The wire

identification marking should consist of similar information to relate the wire to a
EWIS diagram.

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EWIS Identification, cont.

• Wire identification marks identify wire, circuit,
and gauge size
• Markings should be legible in size, type, and
color at 15-inch maximum intervals along the
wire (directly on wire or indirect
[sleeve/tag])
• Less than 3 inches needs no marking
– Readable without removing clamps, ties, or supporting
devices

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Marking EWIS in aircraft. Identification markings generally are placed at each end of
the wire and at 15-inch maximum intervals along the length of the wire.
• Wires less than 3 inches long need not be identified.
• Wires 3 to 7 inches in length should be identified approximately at the
center.
• Added identification marker sleeves should be located so that ties, clamps,
or supporting devices need not be re-moved in order to read the
identification.
• The wire identification code must be printed to read horizontally (from left to
right) or vertically (from top to bottom). The two methods of marking wire
or cable are as follows:
- (1) Direct marking is accomplished by printing the cable’s outer covering.
- (2) Indirect marking is accomplished by printing a heat-shrinkable sleeve and
installing the printed sleeve on the wire or cables outer covering.
Indirectmarked wire or cable should be identified with printed sleeves at each
end and at intervals not longer than 6 feet. The individual wires inside a cable
should be identified within 3 inches of their termination.
• The marking should be permanent such that environmental stresses during
operation and maintenance do not adversely affect legibility.

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Marking a Wire Bundle

No marking

Proper indirect
marking

There can be serious repercussions when there is a situation in

which a number of unmarked cables are disconnected. When the
cables reconnected, the chances are high that they will be
connected incorrectly, thus causing numerous problems.

Connectors
• Many types, however crimped contacts generally
used
– Circular type
– Rectangular
– Module blocks

• Selected to provide maximum degree of safety and

reliability given electrical and environmental
requirements
– Use environmentally-sealed connectors to prevent moisture
penetration

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Connectors. The number and complexity of EWIS have resulted in an increased use of electrical
connectors. The proper choice and application of connectors is a significant part of the aircraft EWIS
system. Connectors should be kept to a minimum, selected, and installed to provide the maximum
degree of safety and reliability to the aircraft. For the installation of any particular connector assembly,
the specification of the manufacturer should be followed.
Purpose and types. The connector used for each application should be selected only after a careful
determination of the electrical and environmental requirements. Consider the size, weight, tooling,
logistic, maintenance support, and compatibility with standardization programs.
• For ease of assembly and maintenance, connectors using crimped contacts are generally
chosen for all applications except those requiring a hermetic seal.
• A replacement connector of the same basic type and design as the connector it replaces
should be used.
• With a crimp type connector for any electrical connection, the proper insertion, or
extraction tool should be used to install or remove wires from such a connector. Refer to
manufacturer or aircraft instruction manual.
• After the connector is disconnected, inspect it for loose soldered connections to prevent
unintentional grounding.
• Connectors that are susceptible to corrosion difficulties may be treated with a chemically
inert waterproof jelly or an environmentally-sealed connector may be used.
 NOTE: Although not required by AC 43.13-1b, moisture-proof connectors should be used in all
areas of the aircraft, including the cabin. Service history indicates that most connector failures
occur due to some form of moisture penetration. Even in the pressurized,
environmentallycontrolled areas of the cockpit and cabin, moisture can occur due to “rain in the
plane” type of condensation that generally is a problem in all modern transport category aircraft.

Circular Connectors

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Although AC 43.13-1b does not address pin layout design aspects,

consideration should be given to the design of the pin arrangement to avoid
situations where pin-to-pin shorts could result in multiple loss of functions
and/or power supplies. For example, you would avoid 115 Vac, 400Hz being
located adjacent to low power wires, such as 28 and 5 Vdc.
A wide variety of circular environment-resistant connectors are used in
applications where they will probably be subjected to fluids, vibration, thermal,
mechanical shock, corrosive elements, etc. In addition, firewall class
connectors incorporating these same features should be able to prevent the
penetration of the fire through the aircraft firewall connector opening and
continue to function without failure for a specified period of time when
exposed to fire. Hermetic connectors provide a pressure seal for maintaining
pressurized areas.
• When EMI/RFI protection is required, special attention should be given to
the termination of individual and overall shields. Backshell adapters
designed for shield termination, connectors with conductive finishes, and
EMI grounding fingers are available for this purpose.

Circular Connectors , cont.

In medium or high vibration areas it may be necessary to provide

a locking device to keep the connectors from loosening.

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Improper Lock Wire Installation

This slide shows a lock wire improperly installed. The lock wire is
installed on the "loosening” side of the connector; it should be on
the “lightening” side.

Proper Lock Wire Installation

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This is an example of a properly installed lock wire.

Rectangular Connectors

Rectangular connectors are typically used in applications where

a very large number of circuits are accommodated in a single
mated pair. They are available with a great variety of contacts,
which can include a mix of standard, coaxial, and large power
types. Coupling is accomplished by various means.
• Smaller types are secured with screws that hold their flange
together.
• Larger ones have integral guide pins that ensure correct
alignment, or jackscrews that both align and lock the
connectors.
• Rack and panel connectors use integral or rack-mounted
pins for alignment and box mounting hardware for couplings.

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Module Blocks (Terminal Blocks)

Module blocks accept crimped contacts similar to those on

connectors. Some use internal busing to provide a variety of
circuit arrangements.
• Module blocks (or terminal blocks) are useful where a
number of wires are connected for power or signal
distribution. When used as grounding modules, they save
and reduce hardware installation on the aircraft.
• Standardized modules are available with wire-end grommet
seals for environmental applications and are track-mounted.

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Terminal Block Grommet Distortion

A wire

View A
Acceptable
  View A
Unacceptable

grommet

For complex wire breakouts that are terminated into terminal

blocks, care must be taken to allow enough slack to prevent
excessive forces from pulling the terminated wires that are
inserted into the terminal block.
• This condition can lead to terminal block grommet distortion,
which can lead to wire damage or a wire that will be pulled
free from the terminal block.

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Grommet Distortion
Improper: grommet
distortion due to tight
wires; not enough slack

Proper: no excessive
tension on wires;
enough slack to avoid
grommet distortion

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Conduits
• Purpose
– Mechanical protection of wires and cables
– Grouping and routing wires

• Standards
– Absence of abrasion at end fittings
– Proper clamping
– Adequate drain holes free of obstructions
– Minimized damage from moving objects
– Proper bend radii

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Purpose. Primarily the purpose of conduits is for mechanical protection of

cables or wires. Secondarily, conduits are used for environmental protection
and grouping of wires by signal type.
Standards
• Conduit should be inspected for: proper end fittings; absence
of abrasion at the end fittings; proper clamping; distortion;
adequate drain holes that are free of dirt, grease, or other
obstructions; and freedom from abrasion or damage due to
moving objects, such as aircraft control cables or shifting cargo.
• Size of conduit. Conduit size should be selected for a specific
wire bundle application to allow for ease in maintenance, and
possible future circuit expansion, by specifying the conduit inner
diameter (I.D.) about 25 percent larger than the maximum
diameter of the wire bundle.
• Conduit fittings. Wire is vulnerable to abrasion at conduit ends.
Suitable fittings should be affixed to conduit ends in such a
manner that a smooth surface comes in contact with the wire.
When fittings are not used, the end of the conduit should be flared
to prevent wire insulation damage. Conduit should be supported
by use of clamps along the conduit run.

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Conduit Installation Guidelines

• Do not locate conduit where service or
maintenance personnel might use as handhold
or footstep
• Provide inspectable drain holes at the lowest
point in conduit run — remove drilling burrs
carefully
• Support conduit to prevent chafing against
structure and avoid stressing end fittings

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Conduit installation. Conduit problems can be avoided by

following these guidelines:
• Do not locate conduit where service or maintenance
personnel might use it as a handhold or footstep.
• Provide inspectable drain holes at the lowest point in a
conduit run. Drilling burrs should be carefully removed.
• Support conduit to prevent chafing against structure and to
avoid stressing its end fittings.

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Conduit Covering

Damaged conduit
covering

Acceptable conduit
covering

FOD is a big problem with damaged conduit covering.

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Wire Insulation
Selection

• Chose characteristics base

environment
– Abrasion resistance d on
– Arc resistance
– Corrosion resistance– Cut- – Flame resistant
through strength – Mechanical strength
– Dielectric strength – Smoke emission
– Fluid resistance
– Heat distortion
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Environmental characteristics. As shown in this slide, there

are many insulation materials and combinations used in aircraft
wiring. Wire insulation characteristic should be chosen based on
meeting FAA flame resistance and smoke emission requirements
(25.869) and the environment in which the wire is to be installed.

13713
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Flame Resistant
Insulating Materials

Polymer
PTFE Mil Spec
ETFE 22759/12
Aromatic polyamide 22759/16
Composite 81381

22759/80-92
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These are the four most common types of insulation materials

used in aircraft today. All of the wire insulating materials in this
slide meet the minimum FAA smoke and flammability standards.

Selecting Insulating
Materials
FACT: There is no “perfect” insulation
system for aerospace wire and cable
The designer’s task:
• Consider trade-offs to secure best
balance of properties
• Consider influence of design,
installation and maintenance
.....for each application!
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How to Choose Wire Insulation

• Seek the best balance of properties:

– Electrical
– Mechanical
– Chemical
– Thermal

Plus
– Nonflammability and low smoke

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Comparative Properties of Wire Insulation

Systems
Most desirableLeast
Relative Ranking 1 2 3 4
Weight PI ETFE COMP PTFE
Temperature PTFE COMP PI ETFE
Abrasion resistance PI ETFE COMP PTFE Cut-through resistance PI
COMP ETFE PTFE Chemical resistance PTFE ETFE COMP PI
Flammability PTFE COMP PI ETFE Smoke generation PI COMP PTFE ETFE
Flexibility PTFE ETFE COMP PI
Creep (at temperature) PI COMP PTFE ETFE
Arc propagation resistance PTFE ETFE COMP PI

PI [Aromatic Polyimide (KAPTON)] - (mil spec 81381)

• Desirable properties: abrasion/cut-through, low-smoke/non-flame,
weight/space
• Limitations: arc-track resistance, flexibility
ETFE (TEFZEL) - (mil spec 22759/16)
• Desirable properties: chemical resistance, abrasion resistance, ease
of use
• Limitations: high temperature, cut-through, thermal rating (150C)
Composite (TKT) - (mil spec 22759/80-92)
• Desirable properties: high temperature rating (260C), cut-through
resistance, arc-track resistance
• Limitations: outer layer scuffing
PTFE (TEFLON) - (mil spec 22759/12)
• Desirable properties: 260C thermal rating, low-smoke/non-flame,
high flexibility
• Limitations: Cut-through resistance, “creep” at temperature

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Conclusion on Insulation

• Aircraft designer can choose among many

polymeric materials

• Physical and chemical properties are

equally important

• Safest system combines “balance of

properties” with inherent flame and/or
smoke resistance

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AC 25-16: Electrical Fault and

Fire Detection
• Supplements existing guidance provided in AC
43.13-1b
• Should apply to new airplanes, as well as
modifications
• Not intended to take the place of instructions
or precautions provided by aircraft/equipment
manufacturers

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The purpose of AC 25-16, “Electrical Fault And Fire Prevention,”

is to provide information on electrically caused faults, overheat,
smoke, and fire in transport category airplanes. Acceptable
means are provided to minimize the potential for these conditions
to occur, and to minimize or contain their effects when they do
occur. An applicant may elect to use any other means found to
be acceptable by the FAA.
This AC is currently being reviewed and will be revised based on
recent service history and ATSRAC recommendations.

AC 25-16: Circuit Protection

Devices (CPDs)
• Circuit breaker resets
– Can significantly worsen an arcing event
– Crew should only attempt to reset a tripped
breaker if function is absolutely required
• Information should be provided in AFMs or AFM
revisions or supplements

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Information should be provided in FAA-approved Airplane Flight

Manuals (AFM) or AFM revisions or supplements that the crew
should only attempt to restore an automatically-disconnected
power source or reset or replace an automatically-disconnected
circuit protection device (CPD) that affects flight operations or
safety.
• NOTE: It is strongly recommended that circuit breakers for non-
essential systems not be reset in flight.
Most transport OAMs and operators are revising their procedures
to not allow circuit breaker resets in flight following a circuit
breaker trip event. Service history has shown that resetting a
circuit breaker can greatly influence the degree of arcing damage
to the EWIS. Each successive attempt to restore an automatically-
disconnected CPD, can result in progressively worsening effects
from arcing.

Arc Tracking and Insulation Flashover

(Caused by multiple circuit breaker resets
)

This picture shows the effects of multiple circuit breaker resets.

In this case, the original arcing event was not able to be
determined due to the severe secondary damage following the
circuit breaker resets.
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EWIS Separation

• Regulatory requirements
– Sections 25.1707, 25.1709, 25.903(d),
25.631

• Manufacturers’ standards
– Power/signal wire separation
• EMI concerns

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EWIS separation/segregation is a fundamental design technique

used to isolate failure effects such that certain single failures that
can compromise redundancy are minimized. EWIS separation is
also used to control the effects of EMI in aircraft EWIS.
• From a regulatory standpoint, we have regulations in place that
may influence EWIS design with respect to
separation/segregation.
• In addition, manufacturers may have company design standards
which establish EWIS separation requirements with respect to
power and signal routing which are usually driven from a EMI
standpoint.
• The next few slides briefly present the primary regulations
associated with EWIS separation/segregation.

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System Separation: EWIS § between

25.1707 s condition, for

• Applies to each EWIS on airplane
• Requires adequate physical
separation
EWIS and certain airplane systems known to have
potential for creating a hazardou
example:
– Fuel systems
– Hydraulic systems
– Oxygen systems
– Water/waste systems

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System Separation: EWIS § 25.1707, cont.

• Adequate physical separation must be achieved by
separation distance or by a barrier that provides
protection equivalent to that separation distance
• “Hazardous” -- must perform a qualitative design
assessment of installed EWIS
– Use engineering & manufacturing judgment
– Evaluate relevant service history to decide whether an EWIS,
any other type of system, or any structural component could
fail so that a condition affecting the airplane’s ability to
continue safe operation could result

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System Safety: EWIS § 25.1709

1. EWIS be designed and installed so each
– catastrophic failure condition is extremely
improbable, and does not result from a single
failure, and
– each hazardous failure condition is extremely
remote

2. Both functional and physical failures of EWIS must

be assessed when demonstrating compliance with
this rule to fully assess effect of EWIS failures

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EWIS Separation from a

25.903(d) Standpoint
• Turbine engine installations: Minimize
hazards in case of rotor failure
– Project debris path through aircraft
• Determine vulnerable areas where redundancy can be
violated
– May need to separate certain critical systems components
including EWIS,
e.g., electrical power feeders, fly-by-wire control paths

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Recently, the JAA requirements with respect to uncontained engine

failure assessment were harmonized with the FAA and were
issued as AC 20-128A. AC 20-128A provides specific methods for
demonstrating compliance with 25.903(d).
• The primary requirement relative to uncontained engine failure is
to use practical design precautions to minimize the risk of
catastrophic damage due to non-contained engine rotor debris.
- An element of difficulty is introduced when the fuselage
diameter is exposed to the relatively large diameter fan
rotors of modern high-bypass-ratio turbofan engines.
- Separation of critical systems EWIS may be a primary factor
in establishing compliance.

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EWIS Separation from a

25.631 Standpoint
• Continued safe flight and landing after
impact with 8-lb. bird
– Consider protected location of control system elements
• If impact can effect redundant system EWIS, may need
additional physical protection of EWIS or wiring
separation
– E.g.: Impact brow area above windshield could affect
electrical power redundancy in some aircraft

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The birdstrike impact areas of the aircraft should be assessed for

their structural strength by test and/or approved analysis
methods. Any penetrations or deformations of the aircraft
structure should be further analyzed for the effect of systems
installation.
• For example, if a birdstrike test on the cockpit overhead eyebrow
area indicates an elastic deformation of 2 inches, then the
deformation should be analyzed or superimposed on whatever
systems may be installed in the overhead cockpit area at that
particular deformation location. In many aircraft, electrical and/or
hydraulic control panels and associated EWIS are installed in the
overhead cockpit area (this is a relatively small area).
• The effect of the birdstrike can be analyzed with respect to a
common cause failure standpoint. EWIS separation aspects of
the design may be an element in compliance to this rule.

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Post-TC EWIS Separation

• Maintain EWIS separation requirements throughout
life of aircraft
– STC applicants may not be aware of separation or other
EWIS requirements (i.e., do not have needed design
data)
– EWIS added or moved as part of the STC should satisfy
original separation requirements and EWIS standards
– FAA policy letter ANM-01-04

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A potential problem with STCs and other modifications to transport aircraft is that the
applicants may not analyze their proposed EWIS installation with respect to the OAM’s
EWIS separation requirements and other OAM EWIS design standards. Added or
modified EWIS could possibly defeat the OAM EWIS philosophy and create unsafe
conditions.
The FAA is currently in the process of drafting policy. The draft policy letter will clarify FAA’s
policy to require that type design data packages for multiple approvals include the following:
• A drawing package that completely defines the configuration, material, and production
processes necessary to produce each part in accordance with the certification basis of the
product.
• Any specifications referenced by the required drawings.
Drawings that completely define the location, installation, and routing, as appropriate, of
all equipment in accordance with the certification basis of the product.
• Examples of such equipment are wire bundles, plumbing, control cables, and other
system interconnecting hardware.
• If the modification being approved is a change to a type certificated product, the
modification must be equivalent to and compatible with the original type design standards.
Instructions for Continued Airworthiness (ICA) prepared in accordance with the
requirements of 21.50 (“Instructions for continued airworthiness and manufacturer’s
maintenance manuals having airworthiness limitations sections”).

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Instructions for Continued

Airworthiness (ICAs)
EWIS ICAs are developed using Enhanced Zonal
Analysis Procedure (EZAP):
• Each zone of airplane
• Each zone with EWIS
• Each EWIS zone with combustible materials
• Each EWIS zone close to both primary and back-up flight
controls and lines
• Tasks, intervals, and procedures to reduce combustibles
(i.e. clean-as-you-go)
• Instructions for protections & caution information

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14 CFR 25.1729 requires applicants to submit Instructions for

Continued Airworthiness, otherwise known as the maintenance
requirements, for the proposed EWIS installation as part of the
compliance data package. Historically, EWIS has been thought
of as “fit and forget” and typically has not been properly
addressed in the ICA data package submitted to the FAA for
approval.

In light of ATSRAC recommendations, the FAA requires applicants

to submit EWIS-related maintenance requirements to the FAA
ACO and AEG offices for approval to satisfy the intent of 25.1729.
This slide shows some of the issues that need to be addressed for
wire replacements instructions.

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Why EZAP?

• EZAP used to develop ICA to prevent the

possibility of smoke and fire by
– Minimizing accumulation of combustibles on and around
EWIS
– Detecting EWIS degradations

• This leads to fewer EWIS and other airplane

systems failures and to safer operation

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3 EWIS Inspection Types

• General Visual Inspection (GVI)

• Stand-Alone GVI

• Detailed Inspection (DET)

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General Visual Inspection (GVI)

• A visual examination of an interior or exterior area,

installation, or assembly to detect obvious damage,
failure, or irregularity. This level of inspection is
made from within touching distance unless
otherwise specified.

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For General Visual Inspection, a mirror may be necessary to

enhance visual access to all exposed surfaces in the
inspection area. This level of inspection is made under
normally available lighting conditions such as daylight,
hangar lighting, flashlight, or droplight and may require
removal or opening of access panels or doors. Stands,
ladders, or platforms may be required to gain proximity to
the area being checked.

Stand-Alone GVI

• A general visual inspection that is not performed

as part of a zonal inspection. Even in cases
where the interval coincides with the zonal
inspection, the stand-alone GVI remains an
independent step on the work card.

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Detailed Inspection (DET)

• An intensive examination of a specific item,

installation, or assembly to detect damage,
failure, or irregularity. DET is discussed in
greater detail in section 14b(1) of AC 25.27A.

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EWIS Inspection Focus Areas

• Clamping • points Connectors

– Improper installation – Worn seals

– Clamp/wire damage – Loose connectors

– Lack of strain relief
– Clamp cushion
migration – Drip loops
– Tight wire bends

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Clamping points - Wire chafing is aggravated by loose clamps,

damaged clamps, clamp cushion migration, or improper clamp
installations.
Connectors - Worn environmental seals, loose connectors,
excessive corrosion, missing seal plugs, missing dummy
contacts, or lack of strain relief on connector grommets can
compromise connector integrity and allow contamination to enter
the connector, leading to corrosion or grommet degradation. Drip
loops should be maintained when connectors are below the level
of the harness and tight bends at connectors should be avoided
or corrected.

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EWIS Inspection Focus Areas, cont.

• Terminations • Grounding points

– Lugs/splices – Tightness
– Cleanliness
• Backshells
– Corrosion
– Improper build-up
– Lack of strain relief

• Damaged sleeving and conduits

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Terminations - Terminal lugs and splices are susceptible to

mechanical damage, corrosion, heat damage and chemical
contamination. Also, the build up and nut torque on large-gauge
wire studs is critical to their performance.
Backshells - Wires may break at backshells, due to excessive
flexing, lack of strain relief, or improper build-up. Loss of
backshell bonding may also occur due to these and other factors.
Damaged sleeving and conduits - Damage to sleeving and
conduits, if not corrected, will often lead to wire damage.
Grounding points - Grounding points should be checked for
security (i.e. tightness), condition of the termination, cleanliness,
and corrosion. Any grounding points that are corroded or have
lost their protective coating should be repaired.

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EWIS Inspection Locations:

Examples
• Wings
– Exposed EWIS on leading/trailing edges during
flap/slat operation

• Engine/APUs/pylon/nacelle
– Heat/vibration/chemical contamination
– High maintenance area

• Landing gear/wheel wells

– Environmental/vibration/chemical

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EWIS inspection locations. Available data indicate that the locations

shown on the slide should receive special attention in an operator’s
EWIS inspection program.
Wings - The wing leading and trailing edges are areas that experience
difficult environments for EWIS installations. The wing leading and
trailing edge EWIS is exposed on some aircraft models whenever the
flaps or slats are extended. Other potential damage sources include
slat torque shafts and bleed air ducts.
Engine, pylon, and nacelle area - These areas experience high
vibration, heat, frequent maintenance, and are susceptible to chemical
contamination.
APU - Like the engine/nacelle area, the APU is susceptible to high
vibration, heat, frequent maintenance, and chemical contamination.
Landing gear and wheel wells - This area is exposed to severe
external environmental conditions in addition to vibration and chemical
contamination.

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EWIS Inspection Locations, cont.

• Electrical panels/line replacement units

(LRU)
– High density areas
– High maintenance activity
– Prone to broken/damaged EWIS

• Batteries
– Chemical contamination/corrosion

• Power feeders
– Feeder terminations
– Signs of heat distress

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Electrical panels and line replaceable units (LRUs) - Panel EWIS is

particularly prone to broken wires and damaged insulation when these high
density areas are disturbed during troubleshooting activities, major
modifications, and refurbishment. One repair facility has found that wire
damage was minimized by tying EWIS to wooden dowels. This reduced
wire disturbance during modification. It is also recommended to remove
entire disconnect brackets, when possible, instead of removing individual
receptacles.
Batteries - Wires and EWIS hardware in the vicinity of all aircraft batteries
should be inspected for corrosion and discoloration. Discolored wires
should be inspected for serviceability. Corroded wires and/or EWIS
hardware should be replaced.
Power feeders - Operators may find it advantageous to inspect splices
and terminations for signs of overheating and security. If any signs of
overheating are seen, the splice or termination should be replaced. This
applies to galley power feeders, in addition to the main and APU generator
power feeders. The desirability of periodically retorquing power feeder
terminations should be evaluated.

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EWIS Inspection Locations, cont.

• Under galleys and lavatories
– Susceptible to fluid contamination
– Fluid drainage provisions
• Cargo bay/underfloor area
– High maintenance activity
• Surfaces, controls, doors
– Moving and bending wire harnesses
• Near access panels
– Prone to accidental damage
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Under galleys and lavatories - Areas under the galleys,

lavatories and other liquid containers are particularly susceptible
to contamination from coffee, food, water, soft drinks and lavatory
fluids, etc. Fluid drain provisions should be periodically inspected
and repaired as necessary.
Cargo bay/under floor - Cargo can damage EWIS. Damage to
EWIS in the cargo bay under floor can occur due to maintenance
activities in the area.
Surfaces, controls, and doors - Moving or bending harnesses
should be inspected at these locations.
Access panels - Harnesses near access panels may receive
accidental damage and should have special emphasis
inspections.

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25.1703 Requirements for

preemptive maintenance
• New rule assures EWIS components selection carried out
safely, consistently, & standardized
• What does it require?
1. EWIS Components must function properly when installed
2. EWIS components must be qualified for airborne use
• Household wire or wire used on consumer electronics not
acceptable unless shown to meet all part 25 certification
requirements

3. Expected service life must be addressed

• Limitations must be part of the ICA (H25.4)
4. Must consider known characteristics in relation to each
specific application
• Includes wire’s insulation susceptibility to arc tracking
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Use of Grommets
Improper

Proper

The grommet should cover the entire edge and come together at
the top of the hole.

Potential Foreign Object Damage

Metallic Shavings

These photographs show foreign object damage (FOD) that can

cause damage to EWIS components. Metallic shaving pose a
serious threat which can damage wire insulation and cause
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subsequent arcing and fire damage. This is one of the reasons
for the clean-as-you-go maintenance philosophy.

Tie Wrap Ends

Improper: tie wrap
ends have not been
cut.

Proper

It is important to cut the tie wrap ends after securing the wires in
order to avoid possible interference with other EWIS components.

Clamp Cushion
Damaged clamp
cushion

Undamaged
clamp cushion

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Damaged clamp cushions can cause EWIS damage that can lead
to arcing.

Sleeving Installation
Improperly installed
sleeving

Properly installed
sleeving

Protective sleeving should overlap at least 30% to ensure 100%

coverage of the wire bundle.

Part 25 and 26 Required Compliance

Documentation
• Project Specific Certification Plan (PSCP)
(EWIS/EZAP aspects)
– Load analysis
– EWIS Installation Drawings and EWIS Diagrams
(25.1701,including identification of EWIS components per
25.1711)
– EWIS separation requirements (25.1707)
– Systems Safety Analysis (25.1709)
– ICA for EWIS including any airworthiness limitations
(25.1729 and 25.1703, respectively)

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