AIRCRAFT ELECTRICAL WIRE TYPES

associated with

AIRCRAFT ELECTRICAL FIRES

 

An aviation safety article

by

Alex PATERSON

http://www.vision.net.au/~apaterson/aviation/wire_types.htm

Last Updated: 22 July 2012


MASTER INDEX of articles written, posted online, or recommended by Alex Paterson


INTRODUCTION

This article provides a list of electrical wire types most commonly used in jet transport aircraft. The articles lists both the positive and negative characteristics of each wire type and the aircraft that these wires have been installed in. It needs to be understood that the article is by necessity incomplete because aircraft manufacturers and airlines have historically given scant regard to the potential dangers posed by different wire types when installing them in their aircraft and therefore have not kept accurate records of what types of wire have been installed in the same. The complacency within the aviation industry towards the dangers posed by electrical wire is endemic and is best summed up by the comment of United States Federal Aviation Authority (FAA) deputy head, Tom McSweeny, who is reported to have said before a Congressional Committee in 19-- that "wire is wire". (more on Mr McSweeny's alleged comment below)

NOTE: This is a draft web page only, setup for discussion between contributors. It undoubtedly contains omissions, and possibly some mistakes.  

Readers are invited to agree with, disagree with, seek clarification about or put their point of view about any of the issues discussed in the article.

Alex Paterson (May 2007)


AIRCRAFT WIRE TABLE

The following table relates to general purpose aircraft electrical wire. It is important to understand that all transport jet and turboprop aircraft have a mixture of the following different wire types installed in them. The wire types listed in the table relate to the predominant type of wire used in each aircraft. It would appear that even aircraft manufacturers themselves are not completely sure as to what wire is installed in individual aircraft as their attitude towards electrical wire in the past has been that "wire is wire".

Table Colour code:

DANGEROUS WIRE

SAFETY UNKNOWN

PROBABLY SAFE

SAFE WIRE

NOTE: Wire is listed in the table by date of introduction into aircraft, with the oldest wire typed listed at the top.

WIRE TYPE

DESCRIPTION

AIRCRAFT INSTALLED IN

(some)

PVC/Nylon

(Polyvinyl-Chloride)

Introduced 1950s

Specification No: 5086

Fails Far 25
  • Weight 6.8 lbs. per 1,000 ft
    (Heaviest and thickest)
  • Rated temperature low: 105C
  • Flammable - burns readily creating copious amounts of thick, toxic smoke rendering it virtually impossible for pilots to see their flight instruments or breathe. (e.g. Valujet 592)
  • Insulation when burning turns to hydrochloric acid when exposed to water.
  • Outgasses onto electrical & electronic contacts
  • Soft - Susceptible to chafing
  • Susceptible to aging in that it dries out and becomes brittle.
  • Banned by US Air Force.
  • US Air Force had 800 autopilot anomalies due to defective PVC in a 6 month study in --?
  • Still used as general purpose replacement wire.
  • Implicated in Valujet Flight 592 DC9 which crashed into the Florida Everglades on 11 May 1996
Dangerous Wire

Installed in

  • Early DC-9s up until 1979
    (e.g. Valujet 592)
  • Early B727s up until 1976
  • Early B737s up until 1976
Still used as general purpose
replacement wire by sections
of the aviation industry.

Kynar

Introduced in 1964

Specification number:

  • 81044/9
Fails Far 25
  • Thickness: 15 microns
  • Weight 5.5 lbs per 1,000 ft.
  • Rated Temperature: 150C
    (fails temperature spec)
  • Poor fluid resistance
  • No longer used

Installed in

  • DC9s from 1970 until 1976

Kapton

(complex aromatic polyamide)

Manufactured by Dupont Chemical Co.

Introduced 1966

Specification Numbers:

  • 81381/11
  • BMS 13-51
    (Boeing)
Fails Far 25
  • Thickness: 8.4 microns (Very thin)
  • Weight: 4.6 lbs per 1,000 ft (Very light weight)
  • Rated temperature: 200C
  • 'Explodes' and burns fiercely at flash-over during an arc tracking event due to the production of free hydrogen, severely damaging adjacent wires and igniting surrounding structure. (i.e. behaves like detonator fuse.) 1
  • High ignition temperature to start burning (usually associated with an electrical short circuit of 5000C), but when it does finally ignite it burns very fiercely (explodes) creating virtually no smoke.
  • Fumes are clear and fairly benign.
  • Susceptible to wet and dry Arc Tracking.
  • Susceptible to aging in that it dries out forming hairline cracks which can lead to micro current leakage (i.e. electrical 'ticking' faults ) which in turn can eventually culminate in an explosive arc tracking event. (i.e. short circuit) 1
  • Stiffness (straight line memory) makes it prone to vibration chafing, (rubbing) and stressed by bending.
  • Abrasive to other wires. (due to its hardness)
  • Hygroscopic (i.e. absorbs water ) rendering it susceptible to wet arc tracking.
  • Installation difficulties (difficult to strip and mark)
  • Banned by
    * US Air Force
    * US Navy
    * Canadian military
    * Boeing in 1992
    * Bombardier?
VERY DANGEROUS WIRE

Installed in

  • Airbus A310 (all)
  • Airbus A320 (currently) 2
  • Airbus A330 (currently)
  • Airbus A340 (currently)
  • B727 (after 1979, EB)
  • B737 (after 1979 to 1990)
  • B747-400 (some from 1989 - 1991)
  • B757 (up until 1990)
  • B767 (up until 1991)
  • BAe 146 (unconfirmed reports)
  • DC-10
  • MD-8x (all)
  • MD-11 (up until early 1992)
  • A300 -600 (with Teflon top-coat)
  • L-1011 Tristar
  • Concorde SST
  • B-707 (but not according to EB)
  • Dassault Mercure
  • CL 600 Series (but not RJ/CL604 or Global Express (Challenger)
  • Shorts SD-330
  • Gulfstream G-II, G-III
  • HS125-700
  • Bell 212, 214
  • Sikorsky S-61, S-70B, S-76
  • Westland 606
  • Plus 31 military types such as P-3, C130, F-14, F-18, Hawkeye, etc
Still used by AIRBUS
in A319, A320, A330, A340
until about 2005
(
see footnote 2 below)

Teflon

(Polytetrafluoroethylene - PTFE)

Introduced in 1969

Specification Numbers:

  • 22759/11
Fails Far 25
  • Thickness: 10 microns
  • Weight 5.43 lbs/1,000 ft.
  • Rated temperature: 200C
  • Longitudinal splitting problem due to manufacturing process.
  • Susceptible to cold-flow (creeping of conductor).
  • Type of insulation found as ignition source on Apollo 13
  • Type of insulation found split in TWA 800-fuel tank wires [Fuel Quantity Indicating System] (FQIS)
  • Banned by major manufacturers in 1983

Installed in

  • B747
  • BAe146

Poly-X

(alkane-imide)

an Aliphatic polyimide

Manufactured by Raychem

Introduced in 1970

Specification Numbers:

  • 81044/16-29

Fails Far 25
  • The first exotic blend of insulation (due to oil embargo)
  • Thickness: 10 microns
  • Weight: 4.7 lbs. per 1,000 ft (Light weight)
  • Rated temperature: 150C
  • Susceptible to solvents
  • Susceptible to radial cracking. Projected service life 60,000 hrs/but circumferential cracks found after just 2000 hrs by US Navy.
  • Susceptible to premature aging. Banned by US Navy in 1978 due to premature aging of insulation after 4000 hrs
  • Brittle. Due to brittleness, 1" bare spots not uncommon.
  • Susceptible to chafing.
  • Fails FAR 25 (airworthiness testing standards)
  • Caused 323 USN F-14s to be re-wired
  • Banned by US Navy.
  • Implicated by Edward Block (and others) in the downing of TWA Flt 800 in 1996.  4
  • No longer used in civilian aircraft.

Dangerous Wire

Installed in

  • Early 747s (e.g. TWA 800)
  • Early DC-10s

Stilan

Introduced 1972

Specification Numbers:

  • 81044/20
Fails Far 25
  • Thickness: 10 microns
  • Weight 4.7 lbs. per 1,000 ft (Light weight)
  • Rated Temperature: 150C
  • Insulation breaks down in hydraulic and de-icing fluid
  • Microscopic crazing problem seen under microscope
  • Cracks under stress
  • Found to arc over
  • Susceptible to spurious signal generation (EMI hazard)
  • Absorbs water (i.e. hygroscopic)
  • No longer used

Installed in

  • B-747s built in mid-to-late 1970s
  • DC-10s built in mid-to-late 1970s

Tefzel

(ETFE)

Introduced 1972

 

Specification numbers

  • F-5

Fails Far 25
  • Rated temperature 150C
  • Soft at rated temperature
  • Used as general installation wire but should never be mixed in bundle with other wire types due to its softness.

Installed in

Arcturus

Tefzel was found in Swiss Air flight SR111's Inflight Entertainment System (IFEN) which was suspected as being the cause of the inflight fire and subsequent crash of the aircraft off Nova Scotia in November 1998.

Cross Linked Tefzel

(XL-ETFE)

Manufactured by Judd Wire and Raychem.

Known by some sections of the aviation industry as "Spec 55" wire. Apparently the name "Spec 55" has been trademarked by Raychem.

Introduced 1977

 Specification numbers

  • MIL-W-22759/34
  • Spec 55
  • Spec 55A
  • BMS 13-48 (Boeing)
Fails Far 25
  • Thickness: 10 microns
  • Weight: 5.0 lbs/1000' (light weight)
  • Rated temperature: 150C
  • Wet arc tracks
  • Flammable producing copious amount of Dense toxic smoke (96%+ density) when it burns rendering it virtually impossible for flight crew to see their flight instruments.
  • NASA states will fail flammability requirements in 30% oxygen.
  • Toxicity - the worst of all wires, banned for manned aerospace use by major manufacturer. (Grumman Corporation banned it in 1982 and NASA followed suit in 1983 due to its toxicity)
  • Soft at rated temperature
  • Loses mechanical strength properties at rated temperature
  • Fails FAR 25 (airworthiness standards test)
  • Projected life 50,000 hrs
  • Notch propagation problems
Dangerous Wire

Installed in

  • B737 (currently)
  • B747 (currently)
  • B757 (currently)
  • B767 (currently)
  • B777 (currently)
  • BAe146
  • Airbus A320
  • Airbus A330
  • Airbus A340
Still used by BOEING in
B737, B747, B757, B767, B777
and Airbus

TKT Boeing

(Teflon/Kapton/Teflon)

Introduced 1992

Boeing Specification No:

  • MIL-W-22759
  • BMS 13-60 (Boeing)

Tufflite brand manufactured by Tensolite
http://www.tensolite.com

Passes FAR 25
  • Weight: 5.0 lbs. per 1,000 ft (Light weight)
  • Arc-track resistant
  • Abrasion resistant
  • Superb insulation protection
  • High heat tolerance
  • Resists smoking when burning (less than 2% density)
  • Displays all the positive aspects of Kapton (i.e. lightweight, resistance to burning, no fumes when burning etc) without any of Kapton's negatives.
  • No known problems
SAFE WIRE

Installed in

  • B737s built after 1992
  • B757s built after 1992
  • Reported by some LAMEs to be partly installed in some B747-400 aircraft manufactured between 1989 - 1999.

NOTE: Airbus Industries now use their own version of TKT (See below)

KKF BAe

Two layers of Kapton within a FEP laquer topcoat.

Installed within the pressure cabin of BAe 146 aircraft.

Note: Source of info:
BAe Statement dated 7 July 1999

FAR 25 attributes unknown

Undoubtedly safer than Kapton if only because it reduces Kaptons propensity to dry out and form cracks.

Resistance to Arc Tracking unknown.

Installed in

  • BAe 146

Source: BAe Statement 7 July 1999

KT BAe

Single layer of Kapton overlaid by single wrap of PTFE (i.e. Teflon)

Installed outside the pressure cabin of BAe 146 aircraft.

Note: Source of info:
BAe Statement dated 7 July 1999

FAR 25 attributes unknown

Undoubtedly safer than Kapton if only because it reduces Kapton's propensity to dry out and form cracks. However, similar to Airbus' KTT (see below) which according to the America's foremost independent aircraft wire expert, Edward Block, "this type of wire is just Kapton with a cosmetic coating of Teflon which is used for marking purposes only and does little to reduce Kapton's propensity to explosively arc track".

Safety Unknown

Installed in

  • BAe 146

Source: BAe Statement 7 July 1999

KTT Airbus

Kapton with two very thin outer layers of Teflon.

Called by Airbus
Polimide/PTFE/PTFE

Airbus Specification No:

  • ASNE0261CF

FAR 25 attributes unknown

Undoubtedly safer than Kapton, but Airbus refuses to disclose performance attributes or specifications of this wire to independent researchers.

According to specifications available, this wire is made up of 25m Kapton, sandwiched between two layers of 2.5m FEP.

Safety Unknown

Used by Airbus to replace Kapton as a general purpose wire.

Installed in Airbus FBW aircraft up until mid 2006 when it was replaced by Airbus' TKT specification EN2267-008 listed below. See footnote 3 below.

TKT Airbus

Called by Airbus
PTFE/Polimide/PTFE

Note: PTFE/Polimide/PTFE is just another name for TKT as
Teflon is a PTFE and
Kapton is a Polimide

Airbus Specification No:

  • EN2267-008

Probably meets FAR 25

Airbus refuses to fully disclose the performance attributes or specifications of this wire to independent researchers.

  • No known specifications released by Airbus although suspected to be very similar to Boeing's TKT wire listed above.
  • Allegedly has a much thicker outer layer of PTFE (i.e. Teflon) than the KTT wire used earlier by Airbus. (see below)
Probably Safe

Reportedly now installed in Airbus Aircraft as from mid 2006 as a general purpose wire.

Sources:


NOTES

FAR 25 comprises clauses mandating aircraft design safety rules. However, there are no specific clauses within FAR 25 pertaining to the flammability, toxicity or smoke visibility criteria of electrical wire insulation. That said, FAR Section 601 mandates a general statement that;


FAR 25-601: "The airplane may not have design features or details that experience has shown to be hazardous or unreliable. The suitability of each questionable design detail and part must be established by tests.
"

Source: http://www.flightsimaviation.com/data/FARS/part_25-601.html

As argued in this document the suitability of aircraft electrical wire insulation materials are "questionable", yet they have never been the subject of a comprehensive formal testing program and as such are in breach of FAR 25-601.

Only Boeing's TKT wire has no known problems and meets FAR 25 requirements. Airbus' version of TKT probably meets FAR 25 requirements.

No specific standards spelt out by aircraft regulatory authorities such as US FAA or European JAR regarding aircraft electrical wire. Specifically no standards defined or any requirement to test wire for:

Modern jet transport aircraft are required by law (FAA 25 & JAR 25) to ensure all safety of flight items and aircraft systems have adequate backup systems installed in the event of a failure of the main system, (and that includes aircraft electrical systems), yet no thought was given to the failure of the aircraft wiring system itself.

Wire is deemed by most in the aviation industry (i.e. aircraft manufacturers, pilots, airline management and regulatory authorities) as an "install and forget" item. This attitude is best summed up by the comment of United States Federal Aviation Authority (FAA) deputy head, Tom McSweeny, who is reported to have said before a Congressional Committee that "Wire is wire". This attitude ignores the fact that:

According to Ed Block: "Only TKT wire insulation (BMS 13-60) meets FAR 25 Standards."



RECOMMENDATIONS

The aviation industry as a whole needs to acknowledge that the shortcomings associated with different electrical wire types are a serious issue and potentially very dangerous as evidenced by the information presented in this paper. The notion held by many with in the aviation industry that "wire is just wire" is irresponsible.

Practical steps that the industry should take to begin to address the situation include:


GLOSSARY


FOOTNOTES

1. Kapton Arc Tracking and Flashover: According to Dr Armin Bruning of the Lectromechanical Design Company of Dulles, Virginia USA - a company which has been contracted by the US Navy amongst others to investigate Kapton arc tracking - the reason Kapton explodes during an arc tracking event and 'flashover' is because "the arc will cause a temperature of 5000 degrees Celsius ... and in this condition carbon is vaporized and free hydrogen is liberated."
Source: email from Armin M. Bruning Lectromechanical Design Co to Alex Paterson dated 5 May 2001

It would appear that encapsulating Kapton between layers of Teflon [i.e. Teflon - Kapton - Teflon (TKT)] prevents the Kapton layer from drying out and cracking, as well suppressing the production of hydrogen during a short circuit, rendering the Teflon coated Kapton (i.e. TKT) relatively benign from an arc tracking point of view.

___________________________

2. Airbus Industries began using a hybrid version of Kapton comprising Kapton coated with Teflon (TK) made by Dupont in some parts of its Fly by Wire aircraft in the late 1990s. However, according to the America's foremost independent aircraft wire expert, Edward Block, this type of wire is "just Kapton with a cosmetic coating of Teflon which is used for marking purposes only and does little to reduce Kapton's propensity to explosively arc track".
However, see note 3 below.

___________________________

3. As of mid 2006 Airbus Industries have started to install their own version of Boeing's TKT in their new Airbus aircraft. This wire is designated Airbus Specification No: EN2267-008. This wire would almost certainly be much safer than bare Kapton, but Airbus Industries refuses to disclose the performance attributes of this wire so it is difficult to determine for certain how safe this wire actually is. See main wire table above for more details about this wire.

___________________________

4. Source: Assertion made by Edward Block in 'Aerospace Testing International' magazine dated June 2009 on page 39.


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ABOUT ALEX PATERSON

Alex PATERSON is an Australian airline pilot. He writes articles and advises on issues pertaining to aviation, politics, sociology, the environment, sustainable farming, history, computers, natural health therapies and spirituality.

He can be contacted at:


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