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A Risk Analysis for a Nervous Passenger

Firstly, read the 1991 FAA Advisory Circular on Aircraft Wiring. On first inspection it sounds like a straight-forward, bland, technical document that is outlining good practices for aircraft wiring. However there are a few terms used in it that may make you uneasy (hazardous, fire, overheat, smoke, short circuit, burn-through, earlier and later safety standards, melting and burning of metal, latent faults, EMI effects on digital computers, explosion-proof, circuit breakers not a protection against flash-over etc). You are aware that the Swissair MD11 crash in Sep 98 had a lot to do with aircraft wiring and it gives you pause for thought about the background that gave rise to this circular. Well, read the commentary below and then re-read the circular and see if you feel any better about the subject of aircraft wiring. You may then wonder why it’s only a non-mandatory "Advisory Circular". If it had been more than that, would TWA800, VJ592, sr111, Silkair, UA811 etc etc have happened? What are they really trying to say about aromatic polyimide insulation (the wiring on sr111)? Perhaps they should have said this:

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Comments on the 1991 FAA Advisory Circular on Aircraft Wiring

[Extracts from document AC25-16 in purple / comments in black]

  1. "Later" versus "earlier" wire (see para 5b on page 2). It would be interesting to hear Ralph Nader’s opinion about a wire that is not safe enough to be used in a later generation aircraft – but quite ‘safe’ for continuous "aged" and "aging" use in older model passenger aircraft that are still in service. Quite a ridiculous and duplicitous outlook. If this situation existed with an automobile, the manufacturer would be forced to do a recall.
  2. This 1991 document advises that the newly applied wire insulation flammability rules apply to B747, DC-10, L-1011 and later aircraft (including therefore the MD-11). Surely this means that wire which doesn’t meet FAR 25 specs (i.e. Kapton) cannot be used in these aircraft? It then goes on to say:
  3. 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,

  4. 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. So they have always known that mixed wire in a bundle is a no-no. Why was this never mandated in any way?
  5. In particular, the use of aromatic polyimide insulation material in these areas should be carefully evaluated. Whenever practical, aromatic polyimide insulation wires should not be used for high current carrying cables. In installations where wires or wire bundles are expected to flex, such as landing gear harnesses, aromatic polyimide insulated wires should be avoided. A lot of tip-toeing around the (unspecified) problem. See also page 7 para 6 (viii) where they are obviously talking specifically about aromatic polyimide (i.e.Kapton) hazards - but not naming it as such.
  6. If this wire type is used in flexible conduit, then the conduit installation should be properly designed for this purpose. Bit non-specific and airy-fairy ("properly designed for this purpose"?? – Oh, I see now, just like TWA800’s and the B727’s). No mention of Teflon sleeving (or the hazards of its porosity).
  7. However, protecting electrical system installations by using CPD's 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. No great revelation to us in IASA -but I think it would be to about 95% of the World’s airline and military crews (still). There is no circuit protection device available that can guard against an sr111-style calamity. For this reason alone Kapton (and its clones) should not be airborne.
  8. 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 conductor. "Sometimes"?? It’s a given that it will – but much more with some wire-types than others.
  9. Information should be provided in FAA-approved AFM’s 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 safety. Once again - No great revelation to us in IASA -but I think it would be to about 95% of the World’s airline and military crews (still).
  10. 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. Pity that the Swissair electrician who incorrectly installed (and shorted out) the replacement bus-tie sensing relay on HB-IWF on 4 Aug didn’t know about this "latent failure" business. I’d suggest that the downstream implications of an electrical system "short" during maint is still being resolved by "having another go at getting it right" followed by a "GROUND_CHECKED SERVICEABLE" write-up (instead of a jolly good look for any weakened wiring insulation dielectrics or component latent damage that might re-surface later).
  11. Certain types of insulation, including polyvinyl chloride (PVC) insulation, do not comply with the 25.1359(d) flammability requirements. And also??????
  12. No mention of the attendant hazards of condensation moisture traps. They simply say to avoid running wire bundles along the bottom of the fuselage and mention electrolytic fluids. You’re supposed to guess the rest. Also no mention of lint traps and hazards of those.
  13. No discussion of bonding, static wicks, earthing points or the ramifications of inflight lightning strikes upon electrical systems, wiring or fuel tanks.
  14. Note that three-phase integral (ganged) circuit breakers seldom detect the loss of phases when an overcurrent condition does not exist. I doubt that many crews would be aware of this or the overheat condition that can result from the loss of a phase. An example of a further hazard is that an instrument (say an artificial horizon) can remain operating or freeze but you’ll never know which it’s doing – because the phase that powers the Off Flag has dropped out.

14. Summary

  1. Electrical systems are the fundamental (sine qua non) system aboard modern aircraft (and in particular, airliners where the cost-saving elimination of the third man [Flight Engineer] has led to maximum computerised automation). Navigation, communication, flight instrumentation, systems monitoring, data recording, lighting and warning systems are all totally dependent upon the integrity of the electrical system. All other systems that aren’t powered by electrics (hydraulics, pneumatics, fuel, pressurization, aircon, oxygen etc) are either controlled, status-monitored (i.e. indication gauge) or otherwise dependent upon a secure, stable and 100% reliable electrical supply. This is particularly so in the case of frequency and voltage sensitive digital electronics. These "other systems" have been designed with, in the most case, triple redundancy and individual functional integrity. Aircraft electrical systems, on the other hand, have a quasi redundancy that revolves around redundancy in duplication (at least) of components, buses, generators, inverters, rectifiers and CPD’s. However once, as this circular admits, arcing or flashover occurs, CPD’s are not going to protect the electrical wiring system and whole wire bundles "may be severed". There is NO integrity or redundancy in aircraft electrical wiring. It is the Achille’s Heel of the modern Electric Airliner.
  2. Compounding the concerns raised by para 14a above, is the post-sr111 situation whereby airlines are simply informally telling their crews that, in the event of "smoke in the cockpit", they should simply "Land ASAP". This is a tacit admission that the standard practice of following the two lengthy checklists ("aircon smoke" then "smoke of unknown origin") - as did the crew of sr111 - can have a fatal outcome. Unfortunately "Land ASAP" is not always an option and can itself lead to further grief. The basic problem with trouble-shooting checklists for ‘smoke’ is that the power stays on the wires and the fires and system failures are being stoked by this. Eventually, the situation will deteriorate and the crews will be totally non-plussed by the seemingly disassociated, yet continuing, systems failures stemming from the destruction of wire bundles. At the same time the possibility of crews being overcome by events, workload, smoke and passenger panic are in themselves real threats. Loss of control in IMC due autopilot unavailability and flight instrument outage is highly likely. Simulator drills cannot possibly entertain realistic scenarios for ‘smoke’ – because the possibles outcomes are so very variable. The debilitating effects of noxious fumes also cannot be "simulated".
  3. Investigators will always be behind the eight-ball when it comes to tracing the origins of a catastrophic airborne fire. The aftermath is usually smithereened debris and a paucity of CVR and DFDR data. They legally have to substantiate any findings because the financial ramifications for the ensuing litigation are enormous. The death tolls are usually 100% and the family trauma is aggravated by the inconclusive outcomes. The NTSB has proposed some improvements in CVR and DFDR technology but what is logically needed is a solution that addresses the problem and not simply ameliorates the investigator’s task. Averting these electrically initiated tragedies should be the first priority. In order to do that you need to look at first principles – giving aircrews a guaranteed, workable solution for "smoke-in-the-cockpit".
  4. If, at the first significant sign/smell of smoke-in-the-cockpit, airline crews could revert to a stand-alone, integral, "get-you-home" in IMC (or at night) Virgin Bus, then the power could come off the wires and the stoking electrics would be stilled, the nascent fires and smoke stifled at birth. The present risk of escalation to a calamitous crash drops to zero. Is it possible to do this? Of course it is. What would be the cost? Would it only be introduced in new aircraft or could in-service aircraft be so modified? These are questions that spring to mind and can only be answered by design analysis. There is of course the overweening cost/benefit analysis that governs all safety initiatives. The FAA argues that the risk is small and so must be "risk managed" by good maintenance practices and crew awareness. As a passenger, you might argue that, if there is a risk, you are prepared to pay more for the ‘fix’ to be put in place. Otherwise, your Fear of Flying is a justifiable loss of peace of mind – because you know that you are susceptible to a real (not notional) risk that could be eliminated. What is the magnitude of the risk? Statistics seem to indicate that flying is the safest form of transport. Not necessarily so.
  5. The study that can be accessed at this web-site URL analyses the air-crash statistics differently and gives the lie to any claim that air-travel is the safest mode of transport:
  6. Click on :

  7. That study was recently published in the prestigious Air Safety Week magazine. It was put together by an ex Chief Engineer of Northrop aircraft (Sy Levine) who lost a dear friend in the Pittsburgh B737 crash. He has credentials in the B2 bomber program and the nuclear submarine program and was one of those responsible for the introduction of airborne inertial navigation systems. He has proposed an airborne safety system for the new millenium (called RAFT) that is presently being studied by an NTSB/FAA committee – for possible introduction after 2015. You can read about it at this site:
  8. Click on :

  9. 2015 is a long way off and the threat is here and now. How can I evaluate the risk you might ask? Well the FAA is expert at obfuscation and statistics are their primary tools in this shady trade. They can claim, in the year that sr111 happened, that no US passenger fatalities occurred in 1998 aboard a US airline. True, however some of the passengers on sr111 were Delta passengers being carried under a code-sharing arrangement. In fact, because of code-sharing and "alliances", you may not realise it, but you can end up travelling on a Third World airline. In fact Delta has only recently cancelled their code-sharing arrangements with Korean Air Lines. KAL has an absolutely abysmal air safety record. A safety audit report might give you some insight as to "why?" (accessible at this URL Click on : ). The best way for you to gauge the risk is to look at a few facts, re-read all the above, and then ruminate a little about what makes the airline industry tick. Last things first: in a deregulated, cost-cutting, low profit margins environment, profits are the bottom line. Without a healthy yield and a stable share-price, airlines cannot expand. Airlines that do not expand their operations and route structures are here today and gone tomorrow. Safety compliance is expensive and only the whip-hand of the FAA keeps them toeing the safety line. However the FAA has a double remit, to maintain safety standards whilst at the same time promoting US airlines’ and manufacturers’ prosperity (both at home and abroad). At most times both the FAA and the ATA’s powerful industry lobby in Washington can force issues or kill initiatives. A case to point is EVAS and passenger smoke-masks.(click)
  10. EVAS was invented (and is FAA approved) to give pilots a fighting chance in the event of dense continuous smoke in the cockpit. It was built so that the FA Reg that says that there must be such a system (that guarantees pilot visibility) could have specified just such a safety fall-back. However, because of cost and falling back on U.S. accident statistics, the FAA has decided not to enforce that regulation. That decision was made prior to the Valujet DC9 crash and has not been reversed. EVAS stays on the shelf except for a few corporate jets. The same applies to the passenger smoke-hoods that can give you a proven extra 10 survival minutes to evacuate the burning jet. Only a few dollars, but not a justifiable expense says the FAA. Many airline flight (and back-end) crews buy and carry their own. They know that the real killer is the lung-searing smoke, not the fire. That’s been proven in countless numbers of crashes all over the world.
  11. So it’s a circuitous argument and you have to make your own judgement call – because the FAA is hopelessly mired in its two loyalties conundrum - and ineffectual because of that. Think about the wire problem in these terms:
    1. There are many hundreds of kilometres of wire in modern passenger jets and just like a rubber band, it’s all (in arcing and flashover terms) only as strong as its weakest point. A flash-over can take out its own bundle, adjacent wiring bundles and cripple systems and start fires in the flammable thermal/acoustic blankets (beneath the cabin lining) through which it runs.
    2. Much of this wire is un-inspectable (hidden inside conduits, behind structural members, buried in the wings, obscured where it passes through bulkheads). As has been revealed in the B737 (May 98) and the B727 (May 99) fuel tanks, it’s precisely in those hidden unseen places that the latent insulation faults go undetected and worsen via the chafing that happens naturally as a result of high frequency vibration. It caught up with TWA800 and will do so again. Expert opinion is that the NTSB got it wrong with Valujet 592, the oxy generators only fuelled a wire-initiated fire. That aging DC-9 had a long history of severe electrical troubles, including on the day it crashed. It was wired with PVC, the wire that is outlawed by FAA Advisory Circular 25-16, as being an earlier wire-type (but it can still be used to repair "earlier" jets).
    3. The fix for the B727? Encase wire bundles in teflon sleeves so that the chafing against the metal conduits is reduced. Any problems with that? Well, yes, because the teflon is porous and moisture can penetrate it. The most dangerous type of arcing is wet-arc tracking that can be easily induced by exposure to electrolytic fluids. Where do the fluids come from? Toilets, galleys and the ever present moisture condensation that you will find pooling inside the fuselage.
    4. Where do pilots stand on these issues? Well most of them think it won’t ever happen to them and their pilot organizations are too busy chasing terms and conditions to rock the safety boat. They don’t have a stand, in fact all they can do is don their oxy masks and remain seated. They are tied umbilically to their seats and will be as much a victim of industry inaction on the wire issue as will their passengers and their surviving families.


To date there is no solution to the perplexing problem of faulty wiring insulation in a large percentage of the World’s airliners. The FAA is in denial and all the players are just hoping that the problem will go away with time. It’s a problem that has no affordable, digestible fix. The Aging Aircraft Committee cranked up by the Vice-Presidential Commission is licenced to look at wiring but it has been hijacked (and is chaired) by the industry Lobby known as the ATA. Don’t expect anything tangible from that smoke-screen.

AC 25-16 is a dusty old document, full of half-measures and understatement, but whose message anyways went unheeded. But it’s a good back-grounder upon which a nervous passenger can whet his fears.

IASA Australasia

From the Covering Email:

1. What they (the FAA and NTSB) didn't already know about duff wire insulation in 1991 wasn't worth knowing. Conveniently they forgot all about it in the interim as the crashes and incidents occurred, the enormity of the problem became apparent - and TWA and United allowed their displeasure to be known. Since that time it's been a case of deny, deny, deny, sit on your hands, fudge the data-bases and let the problem disappear with the strands of time.

2. Why it should have been simply a non-mandatory AC in the first place is mystifying. But then again, the ramifications of the problem are so general that the specificity of an AD or SB was not appropriate (although the authority, priority and mandating was justifiable on hazard and threat grounds). An AD normally advises operators that something very specific needs fixing on a particular aircraft type -and gives the method and time constraints for carrying out the mod. A Service Bulletin draws attention to the existence of a condition that might need attention or regular inspection (once again on a specific type). With an AC, they were simply ringing silent alarm bells. It was soon to be forgotten because an AC would simply be filed away and not be a part of aircraft maint paperwork. Maybe they need another vehicle for general alarums.

3. If you tie this AC to that very comprehensive (and recently re-surfaced) RAF Document on

their extensive Kapton failure experience, it shows just how a conspiracy of silence works...... quietly.

3. An opportunity lost. All a bit sad really.


IASA Australasia


1. AC 25-16 in Word 6/95 format

2. My commentary (in the form of a letter to a nervous passenger - I hear from a lot of those and

it's good to have something to easily throw-back) -- HTML format

email.gif May I ask a question ? Tautology, pleonasm or oxymoronic?to Tell-Tale Docs homew.gif