Flight 111 Tragedy
Wiring and Other Related
July 27, 1998
Predicting wire insulation breakdowns.
When the FAA
grounded 737s recently for wiring inspections, Administrator Jane
Garvey offered this perspective in the May 14th edition
of USA Today newspaper: "Respected safety analysts say that
failure of wiring - absent wear on the wire - is a random
event (emphasis added)."
She went on to explain, "The variety or brand of wire in Boeing
737 fuel tanks is irrelevant to recent FAA actions because our
concerns are about insulation wearing down and exposing the metal
wire inside. When the wiring system was designed with insulation and
a Teflon coating, it was thought it would remain intact for the
plane's life span. Last week, it was shown wing vibration is wearing
down the insulation."
However, the molecular structure of wire insulation degrades over
time with exposure to heat, moisture, etc., and this breakdown, some
experts point out, is not random. Indeed, it can be predicted with
sufficient confidence to guide selective replacement of wiring in
Wiring is not trivial.
Here's another perspective that
came to our attention just recently. It is from remarks made by Air
Marshal C.G. Terry, Chief Engineer of the Royal Air Force, at an
RAF-hosted wire and cable symposium in the UK last September:
"...my main message this morning:
Firstly, wiring is not trivial. We have policies and standards...
We must acknowledge that wire is not without risks and we must
understand the risks...Even small amounts of damage can result in
the loss of an aircraft - we have painful experiences in this
Make no mistake, as Chief Engineer (RAF), I view the integrity of
an aircraft's wiring as of equal importance to that of the airframe
components, engines and flight control systems."
Wire erosion over time. Shown
here, the effect of current leaking to the surface of the
insulation and creeping along the surface, burning the
insulation (about 1/2 inch of carbonized insulation in this
case). In extreme instances, the creepage can run for many
feet, burning up the wire insulation.
The U.S. Navy's experience with wiring failures.
In a one-year period there were 143,000+ aircraft wiring
failures. The repairs represented nearly 9% of all aircraft
maintenance man-hours. The data are a bit dated (circa late 1980s)
but relevant for two reasons: the number of airplanes in the U.S.
Navy (3,000+) is roughly the same as the number of airliners in the
U.S. fleet, and the average age is increasing in both the military
and the civilian aircraft fleets.
A picture of aging.
A structural crack in the
electrical insulation of a conductor can lead to electrical noise
caused by current leakage, or worse. This photograph, taken of
wiring on an in-service Navy aircraft, illustrates the effect of a
current leaking to the surface of the insulation, and moving along
that surface to a point where it touched structure (i.e., going to
ground). The phenomenon is known as wet arc tracking (dry arcing is
metal-to-metal contact). Over time, the degradation is seen as more
noise. The photograph illustrates the kind of wear that can occur in
an accelerated environment, in this case on a Navy aircraft exposed
to salt-water humidity. Almost certainly it is occurring in the
commercial fleet, where accumulated hours and cycles can be
significantly greater than for military aircraft flying 20 hours or
so a month (a typical jetliner will accumulate that many hours in
2-3 days of revenue service).
Dr. Armin Bruning, an expert on wire
insulation, explains the difference between voltage and current,
offered here for its succinct clarity: "Voltage is pressure. Current
is flow." Hence:
|Voltage = Current |
Audit of success.
After a lapse of some years, the
National Transportation Safety Board has just published the 2d
edition of its report, "We Are All Safer; NTSB-Inspired
Improvements in Transportation Safety." The 73-page report
highlights the Board's success in improving aviation, railroad,
maritime, highway and pipeline safety (the aviation section is the
largest part). "This publication records some of the major lessons
learned and the changes that have been made to prevent future
accidents," Chairman Jim Hall explained in his introduction.
The improvements run the gamut from fire to ice. On fire: the
Board pushed for the installation of automatic-discharge fire
extinguishers in lavatory waste-paper containers in all airliners.
On ice: 9 icing-related airline accidents have occurred since 1982.
Among the worst, the 1994 crash of an ATR-72 in Roselawn, Indiana,
prompting changes which the Board claims have made flying in icing
conditions "considerably safer."
A reading between the lines of this report shows how it often has
taken years for the Board to see its recommendations implemented.
Copies are available through the NTSB's Public Inquiries Branch.
>> NTSB Public Inquiries, tel. 202/314-6551 <<
New product coming on the block.
The SnakeEye is not a
flashlight, but a new hand-held remote video inspection tool.
According to the manufacturer, the system delivers clear, full-color
images under low-light conditions. It features an interchangeable
camera head that can be attached to a rigid wand, a ring finger
adapter or variable-length cables. For aircraft, the system can be
used to inspect engines, fuel tanks, under instrument panels, and
interstices of the fuselage. It weighs 2.5 lbs and operates on
regular power (or 2 hrs on battery). A nice plus: the camera can be
coupled to a video recorder. A borescope attachment is optional.
Cost for the basic system: $1,750. William McCafferty, an official
with the manufacturer, Aqua Communications Inc. of Cambridge, Mass.,
says the system is available for September delivery.
>> McCafferty, tel. 617/354-6353, ext. 3029;e-mail:
firstname.lastname@example.org; web site: http://www.aquacomm.com <<
Jet fuels' answer man.
A new web site has been created
offering information to users about fuel quality and handling tips,
technical support, and an area called "Ask The Expert," where users
can ask fuel-related questions. The site was created by Hammonds
Technical Services, Inc., of Houston, Texas. The company specializes
in jet fuel additives. Featuring a built-in e-mail capability ("We
understand that people need a quick response," said Hammond's Betsy
Donnelly), the site is at http://www.hammondscos.com.
>>Donnelly, tel. 913/236-7757 <<
An impressive agenda.
The annual safety conference
hosted by the Air Line Pilots Association (ALPA) last year was
thankfully short on empty rhetoric and long on discussions packed
with substance. The agenda for this year's conference looks even
better, if we may suggest, with sessions such as "V1 and the Go/No
Go Decision" and "Accident Survival - Crash Fire Rescue." The latter
topic, by the way, was mentioned at last week's NTSB hearing as a
continuing deficiency, where the track record is not good,
seems to be little improvement in the capability to rapidly fight
This year's conference, with the theme "Schedule with Safety -
Raising the Standards," is scheduled for August 18-20 at the Hyatt
Regency Washington hotel on Capitol Hill, 400 New Jersey Ave., NW,
Washington, D.C. For more information, contact ALPA's Engineering
and Air Safety Department at its toll-free, 24-hour safety reporting
line, tel. 1-800-424-2470.
Flight 111 Tragedy
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