Lectrogram - Issue 15
Author: Lectromec
Published: 05-01-2007
Lectromec
LECTROGRAM©
May
2007 Newsletter
A
Resource for your Wire Maintenance Program
The results of recent testing indicated
a rate of 5.4 breaches per 1000 feet of wire tested. Lectromec
and InterConnect Wiring, at the 10th Aging Aircraft
Conference, delivered a report describing a recently completed series
of tests to evaluate the status of wire harnesses removed from an
F-16 A-model aircraft. These tests were performed on a total of
35 Nomex® protected electrical wire harnesses that were being
replaced with new harnesses. Both insulation breach detection
(DelTest™) and chemical degradation tests (Inherent Viscosity)
were performed on the wires.
The
laboratory DelTest™ detects breaches in the wire insulation
that reach to the conductor. It is a snapshot of the condition
of the harness on the day it was removed from the airplane. The
DelTest™ process identified those wires in a harness with
insulation breaches and was done without removing the Nomex®
braid. The results of testing indicated 58 insulation breaches
in the more than 1500 wires tested, corresponding to a rate of 5.4
breaches per 1000 feet of wire tested. This rate is over three
times higher than the insulation breach rate that was found on
recently retired commercial aircraft during the FAA’s Intrusive
Inspection in 1999 and 2000.
On a representative number of
harnesses, a visual examination of the Nomex at the breach locations
was performed and only in a few cases did moderate damage to the
Nomex correspond to the breach location. Though there could
have been physical trauma during the manufacturing process or while
on the aircraft that did not leave a lasting mark, most of the
insulation breaches seen after the removal of the Nomex® braid at
the failure points are quite similar to those found in aged polyimide
insulated wiring systems.
From
this visual examination of 28 breaches, there were five cases of
multiple wire breaches that were found within an inch of one
another. Photos included here show the proximity of these
breaches. While it is possible for wire with breaches to
operate with no problems, the likelihood of system problems, and
possibly flashover, is significantly increased. In these five
cases, the wires carried 28VDC. Additionally, in some of the
harnesses, 115VAC wires were also present. If the breaches
appeared in these wires, the potential for a damaging arc would have
increased.
In addition to the identification of
the insulation breaches in the wires, the chemical degradation of the
insulating material was examined. Inherent Viscosity testing of
the wire insulation is a procedure that can evaluate the
environmental and physical strains to which the wire was subjected.
The chemical analysis of the wires showed that more than two-thirds
of the wires sampled had indications of aging with half showing
significant degradation of the insulation.
These tests were performed on wire from
a single aircraft and it is uncertain how well the results can be
extrapolated to the fleet. However, these tests could provide
insights into planning for future harness modification and
replacement schedules.
Further details of the testing and the
subsequent risk assessment performed on the F-16 EWIS can be found
online at www.Lectromec.org/AAC2007.
Copyright
©2007 Lectromechanical Design Company