Now, I'll tell you something even more disturbing. I was aware of the
Kapton problem c. 1983. At that time, I was working closely with the UK
Ministry of Defence Aquila in Bromley, Kent on ionic contamination. My
interlocutor was Alan Stopes who retired (c. 1987?) to go sheep farming
in Wales, after the DEF STAN on contamination was published in 1986.
During my numerous visits to Aquila, Alan discussed with me a number of
other problems that crossed his desk, such as corrosion of the
epoxy-encapsulated winding wire at the tag joint of a solenoid valve
used as part of the main rotor control of a helicopter which had
tragically crashed as a result. One day, I think in 1983, Alan invited
me to watch a film. It showed a freshly made bundle, about 5 cm across,
of polyimide insulated wiring, typical of what you would find in an
aircraft fuselage, with wires of different diameters. This bundle was
bent once through 90° over a radius of what I estimate to be ~5 cm on
the inside, allowing the individual wires to slide, so that the only
strain was that caused by the actual bending. The bundle was put on a
test rig behind armoured glass and the wires were powered up to voltages
and currents such as were typical in aircraft (I think, mostly 28 V DC,
if I remember correctly, with currents up to 40 A). No problem. Then a
burette containing tap water was opened so that there was a slow drip
falling on the bend. After a few minutes, it happened. It exploded or,
at least, it deflagrated violently, to the extent that the armoured
glass was splattered with bits of wiring.
The problem was discovered in the early 1980s when a Royal Air Force
troop carrier (I don't know what make/type) was preparing for take-off
when a deflagration occurred in the baggage hold. Happily, there were no
injuries and the hold had a major halon release which stopped the
aircraft from catching fire. Naturally, there was major damage so an
inquiry was held. It transpired that the cable that caused the problem
was bent round some pipework carrying "bluepoo" to or from one of the
toilets and a little leak of the stuff happened to drip onto where the
cable circumnavigated the pipe. What I did learn is that "bluepoo" is an
ethylene glycol solution with a strictly controlled pH of 7, that the
blue colour is a powerful dye (to show maintenance guys where there are
leaks) and it contains a deodorant of doubtful effectiveness, if you
happen to be the 100th user of the toilet on a flight.
What I say now is hearsay, but Alan told me that they had been in
contact with the polyimide manufacturer who categorically denied that
the insulation could be damaged with radiussed bends.
As a result of this investigation, the MoD specified that all new RAF
aircraft should not have polyimide-insulated wiring and this
specification was certainly sent to all companies supplying aircraft or
components thereof in the mid-1980s. It is to be supposed that
McDonnell-Douglas was likely to be one of them, yet, there we are,
several years later, in 1991, a brand spanking new MD-11 was delivered
to Swissair with polyimide wiring, with the horrible consequences that
we know. I certainly flew in that MD-11, several times. I KNEW that
polyimide wiring was unsafe and had naively assumed that it was no
longer used. YET, at
http://www.pogo.org/p/contracts/ca-021102-wiring.html I see that, in
2002, it is still being used. WHY?
Not really connected with this, but an amusing story about "bluepoo",
read
http://www.flatrock.org.nz/topics/flying/what_happens_to_dry_ice.htm :-D
Brian
Hfjord wrote:
>
>
> Here is the report. Read chapter 2.1 the aircraft was destined to
> accident. Cause: Polyimide wiring!
>
> The start of the accident was in the microscale, but the result was
> horrible.
>
> It�s spoky reading.
>
> When I got the preliminary report many years ago, I sent it to our staff
> immediately, and after that we have very little Polyimide wiring. But
> new generations come, and this accident will be forgotten.
>
> http://www.iasa-intl.com/pdf/SEVEN_YEARS.pdf
>
> Inge
>
>
>
>
>
>
>
>
>
>
> -----Ursprungligt meddelande-----
> Från: Hfjord [mailto:[log in to unmask]]
> Skickat: den 9 maj 2007 20:33
> Till: 'TechNet E-Mail Forum'; [log in to unmask]
> Ämne: SV: [TN] Strange one?
>
> Ioan,
>
> I have to check carefully to be sure, but some years ago, we had a fire
> in a equipment at high altitude. Luckily, it was just experimental
> stuff. The reason was that the flex material undergoes certain change,
> and cold+mechanical friction+voltage caused leakage and finally arcing.
> An airplane went down by that cause years ago. I got the report, but
> don't know if I have it still. Think it's not confidential any more.
> You've certainly heard of that already. I don't remember the exact
> failure sequence, so I repeat that I need check. Would be disastrous to
> say something wrong here. Anyone who got information on that accident?
>
> Such failures must be extremely rare.
>
> Inge
>
> -----Ursprungligt meddelande-----
> Från: TechNet [mailto:[log in to unmask]] För Chuck Brummer
> Skickat: den 9 maj 2007 16:49
> Till: [log in to unmask]
> Ämne: Re: [TN] Strange one?
>
> Iain,
>
> I look forward to hearing. I t is hard to believe that there we not a
> high current arc on the burnt looking sample. The white or near white
> could be something else like moisture. I have seen failures of the
> oxide
> but they are not usually that even. If there are larger blisters and
> you
> cut them open then catch the smell of ammonia that would be from
> moisture.
>
> Are you X-sectioning?
>
> Chuck Brummer
> 3M Manufacturing Engineer
> 8357 Canoga Ave.
> Canoga Park, CA 91304-2605
> (818) 734-4930
>
>
>
>
> "Braddock, Iain" <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 05/09/2007 02:42 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> "Braddock, Iain" <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> Re: [TN] Strange one?
>
>
>
>
>
>
> Chuck,
>
> That was my comment to the Lab however they were adamant that wasn't the
> case. The area under the blasted part on pic 1 was delaminated but due
> to the surface of the Cu being etched away. They managed to replicate
> the etched area on the Cu under the Polyimide and the fact it became
> conductive by placing a spot of Alkaline on the board.
>
> I will be checking another board with similar test fault though that
> will not be until next week.I'll let you now the outcome.
>
> Regards,
> Iain.
>
> ________________________________
>
> From: [log in to unmask] [mailto:[log in to unmask]]
> Sent: 08 May 2007 17:54
> To: TechNet E-Mail Forum; Braddock, Iain
> Subject: Re: [TN] Strange one?
>
>
> *** WARNING ***
>
> This mail has originated outside your organization,
> either from an external partner or the Global Internet.
> Keep this in mind if you answer this message.
>
>
> Iian,
>
> Stop fixing small shorts with your hi pot tester. Wow, Big flash and
> the short is gone leaving you on pic 1 a delaminated area and on pic 2
> the burnt remains.
>
> Bet if you look real close you will find a sliver of metal or a copper
> fiber.
>
> Chuck Brummer
> 3M Manufacturing Engineer
> 8357 Canoga Ave.
> Canoga Park, CA 91304-2605
> (818) 734-4930
>
>
>
>
> "Braddock, Iain" <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
>
> 05/08/2007 09:23 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> "Braddock, Iain" <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
> Subject
> [TN] Strange one?
>
>
>
>
>
>
> Hi all,
>
> Stevie G has been kind enough to post a couple of pics of a flexi
> anomaly I've come across.
>
> Been having problems with some flexi-rigid boards failing at Ins/Cons
> test due to insulation breakdown - tested at 200v.
>
> Had the lab do some exploratory work and the guys have come up with
> these pics. along with the fact that the Polyimide was conductive
> around the areas highlighted on the pics. but only localised. The Lab
> view seems to be that something was left on the board during fabrication
> and caused the problem; they have their suspicions but would like to
> know if anyone else has encountered a similar problem.
>
>
> http://stevezeva.homestead.com/files/Flex_Problem_1.JPG
> <http://stevezeva.homestead.com/files/Flex_Problem_1.JPG>
>
> http://stevezeva.homestead.com/files/Flex_Problem_2.JPG
> <http://stevezeva.homestead.com/files/Flex_Problem_2.JPG>
>
>
> Regards,
> Iain.
>
>
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