Exposed Electrical Wires in Boeing 737 Fuel Tanks!

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AVwebFlash Volume 12, Number 29a -- July 17, 2006
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EXPLODING FUEL TANKS -- FIXES NOT ENOUGH
(http://www.avweb.com/12_29a/leadnews..._192684-1.html)
Meanwhile, the NTSB says that earlier fixes aimed at preventing sparks
from igniting vapors in aircraft fuel tanks don't work. In a news
release last week, the board said the wing tank of a Transmile Boeing
727 exploded even though it had been properly fitted with electrical
shields designed to prevent the electrical arcing that most likely
ignited the vapors. An airworthiness directive required the wiring
harness in question to be inspected, repaired and then wrapped in
plastic before being returned to the conduit in the wing tank. "This
accident illustrates that ignition sources continue to exist and fuel
tank explosions continue to occur in both wing and center wing fuel
tanks despite the corrective efforts of government regulators and
industry," the board concluded.
http://www.avweb.com/12_29a/leadnews..._192684-1.html



On Thu, 05 Oct 2000 17:56:47 GMT, Larry Dighera
wrote:


Exposed Electrical Wires in Boeing 737 Fuel Tanks!
---------------------------------------------------

The article quoted below refers to exposed electrical conductors
within the fuel tanks of Boeing 737 aircraft. This is an alarming
situation.

BOEING CO

The FAA has proposed asking operators of some BOEING CO. 737
jets to inspect a portion of the fuel quantity measuring system
for a potentially dangerous electrical fault. The FAA said in a
notice placed in the Federal Register on Tuesday that it had
received two reports of wires chafed down to the conductor in the
fuel measuring system in right main fuel tanks of 737s. Although
the fuel measuring wires operate at only five volts, the problem
could become an in-tank ignition source if a short occurred with
an adjacent wire of higher voltage.

(Reuters 04:27 PM ET 10/04/2000)

Mo

http://q1.schwab.com/news/lookup?a=6...15a&s=rb001004

The use of open wiring within aircraft fuel tanks is absurd and
alarming. How could the FAA certify such a practice when it is
clearly in violation of established authority on safe wiring methods?


Nationally Accepted Wiring Methods
----------------------------------

The National Electrical Code, which governs approved wiring methods
throughout this nation, in Article 500 classifies, in order of
decreasing severity, three categories of hazardous locations. A Class
1 hazardous location, the most hazardous, is defined as:

500-4. Class I Locations. Class I locations are those in which
flammable gasses or vapors are or may be present in the air in
quantities sufficient to produce explosive or ignitible mixtures.
Class I location shall include the following:

(a) Class I, Division I. Locations (1) in which hazardous
concentrations of flammable gasses or vapors exist continuously,
intermittently, or periodically under normal operating
conditions, (2) in which hazardous concentrations of such gasses
or vapors may exist frequently because of repair or maintenance
operations or because of leakage, or (3) in which breakdown or
faulty operation of equipment or processes which might release
hazardous concentrations of flammable gasses or vapors, might
also cause simultaneous failure of electrical equipment

This classification usually includes location where volatile
flammable liquids or liquefied flammable gases are transferred
from one container to another; interiors of spray booths and
areas in the vicinity of spraying and painting operations where
volatile flammable solvents are used; locations containing open
tanks of vats of volatile flammable liquids; drying rooms ...
and all other locations where hazardous concentrations of
flammable vapors or gasses are likely to occur in the course of
normal operations.

From this description, it would appear that the interior of a fuel
tank containing kerosene would be classified as a Class I hazardous
location.

In Article 501 The National Electrical Code sets forth the rules for
the instillation of electrical wiring within Class I hazardous
locations. The National Electrical Code specifically mandates the use
of Threaded Rigid Metal Conduit (or MI cable...) for Class I hazardous
locations. (For those unfamiliar with the term 'Rigid Metal Conduit',
it is very similar to ordinary, heavy-walled, steel water pipe.)
Article 501-4 goes on to state, all fittings and joints shall be
threaded and be explosion-proof. Threaded joints shall be made up
with at least 5 threads fully engaged. Article 501-5 goes on to
describe the necessity of sealing the conduit to prevent the passage
of gasses, vapors or flames from one portion of the electrical
installation to another through the conduit.

Article 501-14(a) describes approved wiring methods for Signal, Alarm,
Remote-Control, and Communication Systems within Class I hazardous
locations. It specifies that all apparatus and equipment,
irrespective of voltage, shall be approved for Class I locations, and
shall conform to Sections 501-4(a) and 501-5(a) and (c).

So, it would seem that if a storage tank containing kerosine were
wired with a 5-volt electrical fluid-level sensor, its wiring would
have to be housed in heavy-wall, threaded steel conduit (or lead
shielded Mineral Insulated cable) regardless of the voltage if it were
to conform to the nationwide standards set forth in the National
Electrical Code. The National Electrical Code clearly prohibits
running un-enclosed conductors in a fuel tank.


Is the FAA Culpable?
--------------------

While it is obvious that airplanes are not a buildings (although some
are big enough to be: http://www.maxpoweraero.com/homes/pageone.htm),
they a temporary home to hundreds of people for several hours. The
wiring methods for hazardous locations prescribed for buildings by the
National Electrical Code may not be directly applicable to aircraft,
because they fail take into account their weight. However, shouldn't
the safety they attempt to impart be adapted for use in aircraft
wiring? Is not the safety of airline passengers as important as those
persons who inhabit buildings on the ground?

I'll say it again: The use of open-wiring (unprotected from damage by
metallic raceway or shielding) within aircraft fuel tanks is absurd
and alarming. How could the FAA certify such a practice when it is
clearly in violation of established authority on safe wiring methods?

Larry Dighera




There is no expedient to which a man will not resort
to avoid the real labor of thinking.
Sir Joshua Reynolds