Avionics News December 2022 - 50

Incorrect routing of additional wiring has a pipe running through the middle of the wiring bundle.
AGING AIRCRAFT WIRING
Continued from page 49
Still not convinced?
all eventually cause wire insulation to deteriorate. It
eventually fails one way or another by having an open
circuit, or even more seriously, a short circuit.
Wiring harnesses are also subject to the buildup of
lint or metal shavings, and exposure to certain oils, fuels
or other lubricants - from a leaking pipe, or accidental
spillage during a routine maintenance activity.
Items and techniques used 30 years ago to repair
wiring did not seal the repair from the environment,
leading to likely failure sooner or later.
(Incidentally, the wiring in modern aircraft has
much better insulation properties. Proof of this is the
difference in weight between the wiring of the different
eras. Newer wires are noticeably lighter. The conductor
specification, however, is still the same.)
Under the radar
I recently asked a well-known member of the New
Zealand aviation community if people were concerned
about what lies behind their instrument panels. He felt that
most owners were oblivious.
Is it time for a change of heart by aircraft owners? I
believe owners need to encourage their engineers to inspect
the wiring system thoroughly and report any defect they
identify. I think owners, being their own maintenance
controllers, should insist on their licensed aircraft
maintenance engineers fixing any wiring defects properly or
replacing that wiring.
50 avionics news * december 2022
Here are some facts that might persuade you.
In 1996, TWA flight 800, 12 minutes into a flight between
New York and Paris, exploded, killing 230.
Although the source of the explosion was never absolutely
confirmed, the investigation concluded an electrical failure
had ignited a nearly empty center wing fuel tank in the
25-year-old aircraft.
Two years later, a Swissair MD-11 crashed off the coast of
Nova Scotia, killing all 229 on board.
The Transportation Safety Board of Canada concluded the
crash was the result of faulty wiring igniting the flammable
insulation above the cockpit.
Around this time there was also a growing suspicion at
the U.S. National Transportation Safety Board that wiring
failures were likely present on all aging aircraft across the
globe - including military aircraft, commercial airliners,
private jets and helicopters.
So in 2002, it established a working group to evaluate the
wiring harnesses on 39 aircraft, across eight different models.
The working group concluded the aircraft were in overall
good condition, but that there were instances of:
*
Inappropriate wiring repair.
*
*
*
*
Incorrect routing of modification wiring.
Inappropriate clamping and wiring support.
Structural contact, and
Lack of a " clean as you go " philosophy (illustrated by
the presence of lint, dust and debris in between wiring
bundles).
The working group identified a whopping 2,256 such
issues. On 39 aircraft.

Avionics News December 2022

Table of Contents for the Digital Edition of Avionics News December 2022