FAULT AND FIRE
||AC No: 25.16
||Initiated by: ANM-100
1. PURPOSE. This advisory circular (AC) provides 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. These means are not mandatory. An applicant may use any
other means found to be acceptable by the Federal Aviation Administration
for compliance with the Federal Aviation Regulations (FAR).
2. RELATED FAR SECTIONS. The related sections of the FAR are as
Where applicable, corresponding sections of part 4b of the Civil Air
Regulations (CAR) of 1962 follow each cited Part 25 section of the FAR.
25-23 or later amdt.)/4b.320(b)
3. RELATED ADVISORY CIRCULARS. The guidance in this AC supplements
the following existing guidance on safe electrical design and installation
- AC 43.13-1A, Change 3, Acceptable Methods, Techniques and Practices,
Aircraft Inspection and Repair.
- AC 43.13-2A, Change 1, Acceptable Methods, Techniques and Practices,
- AC 25.9, Smoke Detection, Penetration, Evacuation Tests, and Related
Flight Manual Emergency Procedures.
- AC 25.10, Guidance for Installation of Miscellaneous, Non-required
a. Electrical Component: For the purpose of this AC an
electrical component is defined as an electrical power source,
or a component receiving electricity from any source. Sources
of electricity are not limited to power sources or distribution
buses. They also include signal sources, such as the output of
an autopilot servo amplifier, or data sources that transfer information
electrically from one component to another.
b. Circuit Protection Device (CPD): A device used to protect
electrical/electronic circuit components from an over-voltage
or over-current condition, by automatically interrupting the current
flow. The most common types of CPDs used in aircraft are
the circuit breaker and the fuse.
c. Aromatic Polyimide Insulation: A wire insulating material
formed as the result of a polycondensation reaction between an
aromatic dianhydride and an aromatic diamine.
d. Arc Tracking: A phenomenon in which a conductive carbon
path is formed across an insulating surface. This carbon path
provides a short circuit path through which current can flow,
normally as a result of electrical arcing. Also referred to as
"Carbon Arc Tracking", "Wet Arc Tracking"
or "Dry Arc Tracking".
e. Insulation Flashover: A result of Arc Tracking, an
instantaneous burn-through of the insulated wire with the possibility
of continuing the burn into surrounding wires. This failure mode,
which is a result of the high temperature degradation of the insulation
experienced during arcing, can propagate through a complete wire
bundle, severing the entire grouping.
- WIRING FAULT AND WIRE INSULATION FLAMMABILITY INFORMATION.
a. Background. Amendment 25-32,
effective May 1, 1972, added a new §1359(d) which applies the
flammability requirements of Appendix F of Part 25 to wire insulation
used in aircraft and also
revised Appendix F to make the burn test requirements more stringent. These requirements are effective on airplanes for which application
for a type certificate is (or was) made on or after May 1, 1972;
on the Boeing 747, Douglas DC-10, and Lockheed L-1011 airplanes
by special conditions; and on certain other airplanes. (Reference
the Type Certificate Data Sheet of each airplane type for its
type certification basis.) Before
these regulatory actions, there were no wire insulation flammability
requirements in either Part 25 or Part 4b.
Certain types of insulation, including polyvinyl chloride
(PVC) insulation, do not comply with the §25.1359(d) flammability
b. Discussion. Airplanes subject to the §25.1359(d) flammability
requirements are referred to as "later" airplanes, and
all other airplanes are referred to as "earlier" airplanes.
Similarly, wire having insulation which complies with these flammability
requirements is referred to as "later" wire, and all
other wire is referred to as "earlier" wire.
- General Guidance. The guidance in this AC supplements
existing guidance provided in AC 43.13-1A and AC 43.13-2A and
should be applied to new airplanes, as well as to modifications
of previously-certificated airplanes in all locations where electrical
systems, components, or wires are affected. It may also be useful
in developing corrective Airworthiness Directive (AD) actions
taken in response to hazards discovered in service. It should
be noted that this guidance is not intended to take the place
of instructions or precautions provided by wire, wire insulation
or equipment manufacturers. Also, if any requirement of Part 25
that is included in the airplanes type certification basis
supersedes any particular guidance in this AC, compliance with
that requirement of Part 25 should be ensured. This guidance reflects
past certification practices and is considered to provide acceptable
means of compliance with, or equivalent safety to, §25.1359(d)
- Earlier wire may be used in all locations in earlier airplanes.
- Earlier wire may be used inside the fuselage of later airplanes
only if its use does not create any significant potential
for hazard. Under the equivalent safety provision of §21.21(b)(1),
the potential for hazard is considered insignificant if the
guidance provided in any of the following three paragraphs
- Earlier wire may be used inside equipment designed, on an
overall basis, to be as fire resistant as practicable if it
- Enclosed in a case made of metal or other material that
complies with the flammability requirements of Amendment
25-32 that will either contain an internal fire so that
it cannot propagate to other locations or is sufficiently
airtight that internal ignition sources cannot cause a
- Located and installed where a fire cannot damage safety-related
parts, propagate to flammable parts, or cause personal
injury. Maximum physical or spatial separation is especially
important above the equipment or downstream of any consistent,
- Earlier wire may be used inside the cases of video or audio
tape players, television receivers, telephones, or other passenger
convenience or entertainment equipment purchased on the general
commercial market where similar, economically feasible equipment
having wire which complies with §25.1359(d) does not exist.
In such instances, the equipment should be located where smoke
or fire would readily be noticed, and a readily identifiable
switch, located away from the equipment, should be provided
to enable its safe and rapid disconnection.
(iii) Where reasonable and appropriate, very small
amounts of earlier, special purpose wire, such as telephone
interconnection wires, may also be used outside equipment
- General Wire Installation Guidance.
- All instructions and precautions provided by wire and wire insulation
manufacturers should be followed and observed.
- Machines used to stamp identification on wire insulation should
be adjusted so as not to penetrate through the insulation. Quality
assurance procedures to verify that penetration has not occurred
should be established. An acceptable method would be to employ
an approved high voltage test. Alternatively, a non-impact process
to place identification on wire insulation may be used.
- Care should be taken to ensure that the clamps and ties around
wire bundles do not present a rough surface that may damage the
wire insulation. These clamps and ties should be tight enough
to hold the wires in place but not so tight that insulation damage
would occur during fabrication or installation, or later in service.
To prevent insulation chafing, wires and bundles should be installed
by routing and clamping to ensure sufficient spacing from structure
or other parts after any single failure. Alternatively other means
may be used to prevent insulation chafing after any single failure.
Some examples of such single failure would include the failure
of any single clamp or clamp fastener. To prevent insulation damage
during installation or maintenance, avoid routing wires or bundles
in the vicinity of parts having sharp edges, corners or protrusions.
Alternatively, sharp edges, corners or protrusions should be covered
with smooth protective material or other devices. Bend radii should
be large enough to ensure that insulation cracking does not occur
during the fabrication or installation of wires or bundles, or
later in service due to excessive mechanical stress. The clamps
should be oriented such that abrasion of the wire or the clamp
insulation does not occur. 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. Quality assurance procedures should
be established to verify that insulation damage has not occurred
during the process of fabricating and installing wires and bundles.
- Abrasion of wire insulation caused by differences in hardness
can be hazardous. Therefore, wires having significantly different
insulation hardness, or abrasion characteristics, should be routed
in separate bundles. This is particularly important in areas of
high vibration. Abrasion of either the insulation or the insulation-facing
material of the clamps, conduits, or other devices used to secure
or support wires or bundles can also be hazardous. Therefore,
wire and bundle installations should be designed so that the insulation-facing
material has a hardness compatible with that of the
insulation. Compatible materials are often known by the wire or wire
insulation manufacturer or the airplane manufacturer or modifier. Compatible
materials may also have been established by prior satisfactory service
experience or tests. However, if such information is unavailable, insulation
and insulation-facing materials should be tested to ensure that differences
in hardness would not result in abrasion in service.
- To prevent insulation damage, wires or
wire bundles should not be routed in locations where liquid spillage
or leakage may be anticipated in service. It should be assumed that
water or other liquids from any source are electrolytic. This assumption
applies whether the liquids are pure or have other chemical(s) dissolved
in or mixed with them. Service experience shows that locating wires
or bundles under lavatories and galleys has sometimes resulted in insulation
damage. Locating them in the wheel-wells and some areas of the wings
has also resulted in damage. Locating them in some areas of the empennage
could also result in damage. Care should also be taken that condensation,
rain, snow, hail, ice or slush will not result in insulation damage
or deterioration of wires or bundles exposed to these environments.
This may be established by satisfactory service experience, tests, or
comparison of the insulation chemistry or design with those of other
types of insulation that are known to be safe when exposed to these
environments. In particular, the use of aromatic polyimide insulation
material in these areas should be carefully evaluated.
- Installations that are inherently difficult sometimes lead to post-maintenance
reinstallations that do not conform to any approved type design. Service
experience shows that non-conforming reinstallations, especially if
done hurriedly, can significantly increase the potential for electrical
faults, smoke or fires. Therefore, to the greatest practicable extent,
a type design should be provided for wire and wire bundle installations
that allows for easy reinstallation after the completion of maintenance.
- Wires and wire bundles should be routed or otherwise protected to
minimize the potential for maintenance personnel to step, walk or climb
on them, or use them for handholds. The wire bundles should be routed
along heavier structural members whenever possible. Sharp metal edges
must be protected by grommets to prevent chafing. Wires should not be
routed between aircraft skin and fuel lines. Avoid running wires along
the bottom of the fuselage, over the landing gear, in areas of the leading
edge of the wing where fuel spillage is anticipated, or adjacent to
flammable fuel lines or tanks.
installations where wires or wire bundles are expected to flex,
such as landing gear harnesses, aromatic polyimide insulated
wires should be avoided. If this wire type is used in flexible
conduit, then the conduit installation should be properly designed
for this purpose.
- CIRCUIT PROTECTION DEVICE (CPD) INFORMATION.
a. Background. Historically,
the FAA criterion for circuit protective device (e.g. circuit
breaker or fuse) selection can simply be expressed as: "to
protect the aircraft wiring but not the equipment". This
limited criterion is based on designing electrical components
to be as fire-resistant as practicable and either enclosing them
in metal cases that will contain an internal fire or are sufficiently
airtight that internal ignition sources cannot cause a fire, or
isolating them from flammable materials and safety-related parts.
In the vast majority of instances metal cases have been used.
The few exceptions which are known to have resulted in or contributed
to fires have been corrected by airworthiness directives (AD)
action. However protecting electrical system installations by
using CPD' to protect wiring and through component design
to protect the rest of the system is not adequate.
Circuit protection devices (circuit breakers and fuses) are considered
to be slow-acting devices and may not offer sufficient disconnect
protection from events such as arc-tracking or insulation flash-over.
For example, service experience shows that:
- Selection of CPD ratings; protection of three-phase loads;
design of connectors, wire bundles, routing and clamping;
and component temperatures have not always been safe.
- Faulty maintenance sometimes occurs in regard to routing,
clamping and cleanliness of locations surrounding vertain
- Aging, weathering, vibration and the normal wear and tear
of maintenance sometimes cause chafing, abrasion, or deterioration
of insulation, which can cause cracks or cuts that can expose
- The effects of electrical faults can include component overheating;
toxic fumes; smoke; fire; damage to wires, wire bundles or
parts; melting of holes in sheet metal parts by faulted, high-current
feeder cables; melting and burning of titanium bleed air ducts
by a chafed high-current feeder cable; electromagnetic interference
(EMI) with equipment; and the simultaneous and unrestorable
loss of both engine-driven generators in a two-engine airplane.
While these effects occur infrequently, guidance is considered
necessary to minimize them when they do occur and to minimize
their potential causes. These effects can be caused by the
following conditions or failure modes for which CPDs
often do not provide timely, if any, automatic protection.
- Intermittent, low impedance, short circuits, such as
chafing or arcing of wires on grounded metal parts, or
chafing between conductors of different phases.
- Higher impedance short circuits which, for example,
can be caused by locating wire in a moist or wet environment
if the wire insulation is cracked or cut.
- Lower or higher impedance short circuits inside electrical components
themselves; or in component power, signal or data transfer interconnection
- Failure of power to one or two phases of components intended for three-phase
- Incorrect assumptions of worst-case load conditions in regard to safe
- Inadequate design in regard to safe connector or wire bundle temperatures.
- High current cable faults.
- Arcing on wire insulation or the resulting insulation damage. Service
experience documents only a relatively small number of incidents of
arcing damage for all types of insulation. However service experience
of aromatic polyimide insulation, as presently constructed, documents
a failure mode called "insulation flashover" where conduction
at insulation breakdown areas has damaged or destroyed the wire or wire
bundle in which it occurs. Also, other adverse effects have sometimes
occurred as a result of this failure mode. Arcing on wire insulation,
or "arc tracking" can result from electrolytic contamination
of wire having insulation cracks or cuts that expose the conductor.
It can also result from chafing damage that reduces the dielectric strength
of dry insulation. Each successive attempt to restore an automatically
disconnected CPD can result in progressively worse effects from arc
- Circuit Protective Device Selection and Related Guidance.
- Linear corrections should be made of any measured temperatures to
the maximum sea-level outside ambient temperature approved for operations
in the Limitations Section of the FAA-approved airplane Flight Manual
(AFM) or AFM revision or supplement. Alternatively, they may be corrected
in accordance with an analysis that establishes a rational method
- Quality Assurance (QA) procedures should be established to ensure
that circuit breaker time versus current automatic trip characteristics
conform to their specifications, whenever a specific type or lot is
suspect. Such procedures should be followed before installing circuit
- To protect against unsafe wire temperatures, consideration should
be given to the wire size and specification used at the maximum deliverable
continuous current of the CPD. The time-versus-current automatic
disconnection specification for each CPD should also be considered,
taking into account its ambient temperature and that of the protected
wire. If doubt exists regarding safe wire insulation temperatures, measurements
should be taken and corrected as described in paragraph 6b(1).
- To minimize deliverable power to potential fault loads, the minimum
commercially available CPD rating should be used. This is the rating
that will power the normal (intended) loads without spurious automatic
disconnections due to temporary conditions such as transients, surges
or momentary overloads.
NOTE: It is recommended that thermal circuit breakers not be specified
for a continuous load in excess of 85 percent of the circuit breaker
rating because it may cause deterioration of the trip point.
- To protect against propagation of an internal electrical component
fire to surrounding locations, electrical components should be designed
using non-flammable or self-extinguishing materials to minimize the
potential for fire. Electrical components should be enclosed in metal
or other fire-resistant cases (reference paragraph 5c(2)(i)(A) that
will either contain an internal fire or are sufficiently airtight that
internal ignition sources cannot cause a fire. However if such a case
is impracticable, the component should be located and installed where
a fire cannot damage safety-related parts, propagate to flammable parts,
or cause personal injury (reference paragraph 5c(2)(I)(B). Maximum physical
or spatial separation is especially important above the component, or
downstream of any consistent known airflow.
- Precautions should be taken to ensure that three-phase loads are not
a smoke or fire hazard and do not result in unsafe component temperatures
when powered by fewer than three phases. Note that three-phase integral
(ganged) circuit breakers seldom detect the loss of phases when an overcurrent
condition does not exist. If component over-heating could occur, separate
thermal protection should be provided in the equipment case. This could
occur, for example, as a result of a bus phase outage, a load control
component failure, or an open wire. If doubt exists regarding safe component
temperatures, measurements should be taken and corrected as described
in paragraph 6b(1). The recommended maximum safe external temperatures
are considered to be 450ºF for components in dry, clean and isolated
locations; 400ºF for components located in the vicinity of safety-related
or flammable parts (including liquids or gases) or where flammable waste
or other foreign material (such as used paper towels) might inadvertently
accumulate without readily being noticed and 140ºF for components in
- Some internally faulted components may not cause automatic disconnection
of their CPDs before causing excessive temperatures that would
result in a serious personnel, smoke, toxic fumes or fire hazard.
Some examples would include certain transformers and motors. In such
cases, adequate backup protection should be provided. If over-temperature
protection systems are used, the disconnection of the components protected
by these systems should be considered when determining compliance with
the applicable regulations. This is especially important for components
in more than one channel of a redundant system or components in various
systems that perform operationally similar functions. Consideration
should also be given to foreseeable operational and local environmental
conditions, and the failure of any necessary cooling air supply.
- Protection should be provided against unsafe temperatures in connectors
and wire bundles. Consideration should be given to probable combinations
and durations of the maximum normal (intended) loads, combined with
the simultaneous maximum single continuous fault load for that individual
wire which would cause the maximum increase in connector or bundle temperature
(reference paragraph 6b(3)). The maximum safe temperature should be
determined from the specifications of the connector or the wires with
the lowest temperature rating. The maximum safe temperature should not
be exceeded, except in unavoidable applications with high ambient temperatures
(e.g. the power-plant). If doubt exists regarding safe connector or
bundle temperatures, measurements should be taken and corrected as described
in paragraph 6b(1).
- MISCELLANEOUS DESIGN, OPERATIONAL, AND MAINTENANCE-RELATED GUIDANCE.
- Care should be taken to ensure that EMI caused by intermittent faults
does not adversely affect systems or equipment. Such intermittent
faults can be caused, for example, by chafing of conductors on grounded
metal parts, chafing between conductors of different phases, or arcing
on wire insulation. Digital equipment, including digital computer-based
equipment, is usually more susceptible to such EMI than other equipment.
- It is accepted practice to demonstrate by tests that the generator
protection system responds properly to faults in the electrical generation
and distribution system, or in any utilization system. Demonstrations
of arcing on wire insulation should be allowed to progress to the
point of "insulation flashover". The tests may be supported
by any relevant analysis. If laboratory tests are conducted instead
of airplane tests, compliance should be shown with §25.1363.
- Penetration of the effects of electrical faults or failure modes
into tanks, tubes, or components containing fuel, other flammable
fluid, oxygen, or concentrated oxidizing or reducing agents (such
as chemical oxygen generators) can be extremely hazardous. Therefore,
it is important to ensure that any foreseeable penetration will not
occur. Some examples of foreseeable faults or failure modes that could
result in penetration would include short circuits of conductors,
arcing on wires or wire bundles, or "insulation flashover".
Consideration should be given to the maximum power which could be
produced by such faults or failure modes. And the physical or spatial
separation provided between their possible
locations and the areas of potential hazard. Additionally, physical
or spatial separation should be provided between high-current cables
and the areas of potential hazard. However, if adequate separation is
impracticable, protection should be provided against the effects of
foreseeable faults or failure modes by providing alternative physical
protection such as an adequate barrier or conduit or by other acceptable
means. For example, adequate separation is impracticable for wires or
bundles that are necessarily located inside of or close to fuel tanks,
and may be impracticable for some wires or bundles located in nacelles
or pylons. Whenever practical, aromatic polyimide
insulation wires should not be used for high current carrying cables.
- Electrical components should be assessed for potential fire or smoke
(which includes harmful or hazardous concentrations of gases or vapors)
assuming a failure has occurred. Typically, circuit protective devices
and metal enclosures are all that are used to control the failure conditions,
but not in every case (example overheat protective devices).
Each installation must be evaluated on its own merit. Electrical equipment
bays should contain smoke and overheat detectors and methods should
be devised to contain a fire and prevent fire from penetrating into
the cabin or the cockpit. Smoke detection tests, smoke penetration tests
and smoke evacuation tests should be conducted in flight to demonstrate
that the methods used to detect, control and evacuate the smoke are
functioning as designed. The tests should be conducted in accordance
with the test procedures outlined in AC 25-9.
- Electrical components located in fuel vapor zones should be qualified
as explosion proof in accordance with section 9 of RTCA Document DO-160B,
"Environmental Conditions and Test Procedures for Airborne Equipment",
dated July 20, 1984 or later approved revision. Fuel vapor zones are
defined by the airplane manufacturer.
- Burning of a metal part can be extremely hazardous; therefore it is
important to ensure that any foreseeable burning will not occur. Some
parts made of certain metals, such as magnesium or titanium, can sometimes
ignite and burn before they melt and drip away. Ignition can be caused
by electrical faults or failure modes. Whether such a party will burn
before it melts depends on various factors. Some examples of such factors
would include mass; electrical and thermal conductivity; design, construction
and installation; rate of heat dissipation into surrounding locations;
smaller dimensions (e.g. metal thickness); presence of sharp or thin
edges, corners or protrusions (e.g. cooling fins); air temperature;
airflow and oxygen content. If the use of such a part otherwise complies
with §25.601 and 25.603, consideration should be given to the maximum
power which could be produced by foreseeable faults or failure modes
and the physical or spatial separation provided between their possible
locations and the part. Some examples of such faults or failure modes
would include short circuits of conductors, arcing on wires or wire
bundles or "insulation flashover". Routing high-current cables
in the vicinity of such parts should be avoided; however if adequate
separation is impracticable,
protection should be provided against the effects of foreseeable faults
or failure modes by the use of alternative physical protection, such
as an adequate barrier or conduit, or by other acceptable means. For
example, adequate separation may be impracticable for some wires or
bundles located in nacelles or pylons.
- Information should be provided in FAA-approved AFMs or AFM revisions
or supplements that the crew should make only one attempt to
restore an automatically disconnected power source or reset or replace
an automatically disconnected CPD that affects flight operations or
- Some electrical faults or failure modes can result in the automatic
disconnection of a power source, bus, or high-current load for which
power cannot be restored (or will not remain restored) without maintenance
action. Such a disconnection could result in a serious latent failure
of a flight control system component if the fault or failure mode occurs
in its vicinity. For this reason, it is important that maintenance personnel
determine by close inspection of related and non-related components
in the vicinity of the fault, and before the next flight, that such
a latent failure has not occurred. Therefore this maintenance information
should be provided to owners and operators of the airplane early enough
for well-planned, timely incorporation into FAA-approved maintenance
Leroy A. Keith
Manager, Transport Airplane Directorate
Aircraft Certification Service, ANM-100
US Government Printing Centre WM-4790101