Partial inspections of the electrical wiring and electrical systems
in fewer than 100 high-time U.S. jetliners have uncovered more than
3,000 items worth noting.
The term "item" is used deliberately, as the exact nature
of the deficiencies has not been released. However, of these 3,000
items, some 144 were deemed potentially hazardous. Operators should
be wary of this low fraction of significant findings (4.5%). Not
all the wiring on each aircraft was inspected, the rigor of the
inspections may have varied from aircraft model to model, and forthcoming
"intrusive" inspections may yield problems not discovered
by visual examination. While the results of the visual inspections
provide some comfort, there is little reason for complacency about
the integrity of old aircraft wiring.
The inspections were conducted as part of the activity of a government-industry
task force charged with evaluating the condition of non-structural
systems in aging aircraft, which have been defined as those with
20 years or more service. A report of these non-intrusive inspections
of the electrical systems in so-called "geriatric jets"
is perhaps the first work product to result from the efforts of
the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC).
This body is undertaking a two-year effort to assess the condition
of non-structural systems in jets with 20 or more years service.
The program is an outgrowth of the 1997 report of the White House
Commission on Aviation Safety & Security (the "Gore Commission").
The Commission recommended a comprehensive review of systems to
complement the aging structural inspections that were put in place
throughout the industry after the explosive decompression of a high-time
Aloha Airlines B737 in 1988. The present ATSRAC effort is intended
to identify possible design changes or modified maintenance practices
to electrical, hydraulic, pneumatic and other systems in old jets
as they age in service.
Visual inspections completed
The
ATSRAC began its work in earnest last January. Among its top
priorities was a non-intrusive (i.e., visual) inspection of the
condition of wiring and electrical components on a representative
sampling of the fleet. As of last July, some 70 aircraft had undergone
inspection. At the ATSRAC meeting last week the final report was
presented of inspections that ultimately covered some 81 aircraft
out of a candidate fleet of 3,078 jets at or beyond the 20-year
point. The inspections were conducted under bright light while the
aircraft were undergoing heavy maintenance (i.e., at a time when
interior cabin fittings, panels, and thermal/acoustic insulation
blankets were removed, exposing wiring and bundles to inspection).
The inspection teams used a common report form to record the results
of their examinations.
The inspections represent the second of three waves of activities
affecting operators of older jets. The first wave was a service
history review that resulted in the publication of previously issued
service bulletins as alert service bulletins. For example, nearly
20 service bulletins for the DC-9 alone were upgraded to Alert status.
The visual inspections represent the second wave of activity, and
intrusive inspections, now getting under way, represent the third
wave. Actually, there are two components to this third wave: (1)
a detailed visual inspection followed by (2) non-destructive testing
(NDT) using the latest test methods.
"All is well," with some exceptions
From the visual inspections, ATSRAC officials said they found nothing
that would warrant an airworthiness concern, defined as "requiring
immediate fleet action." However, a number of significant areas
"warranting improvement" were uncovered. These 144 items
of "potential hazard" have been referred to the manufacturers
for analysis and possible action. Although the January 11 Final
Report of the non-intrusive electrical systems inspections was the
first definitive work-product to emerge, coming at the halfway point
of the ATSRAC's two-year effort, the 16-page document did not outline
the specific nature of either the less or the more severe findings.
One of the complicating factors here is that most of the inspections
were completed before last September, when the ATSRAC defined its
terms, such as "significant item".
As definitions were late in arriving, the final report of the inspections
necessarily couched the results in general terms. For example, the
report did not outline the nature of the 3,000 items, such as the
number or percent involving chafed wire, inadequately supported
wire bundles, inadequate grounding, etc. Nor did the report contain
a tabulation of the 144 serious items.
As an example of the kind of assertion offered without supporting
data, the report declared, "Time in service and the systems
that they service seemed to have no appreciable bearing on the condition
of the wiring installation.... The working groups did not note any
direct correlation between the condition of the wire and the actual
time in service."
Yet fleetwide inspections of wiring in fuel tank conduits of 737's,
ordered by the Federal Aviation Administration (FAA) in 1998, revealed
virtually a direct correlation between chafing and age. Among 737's
with about 40,000 flight hours, less than 5% were found with wire
chafed more than halfway through the thickness of the insulation.
But, among 737's with 70,000 hours, more than 30% of the aircraft
had wires chafed more than halfway through, and 15% of the aircraft
had wire chafed to the point where bare conductor was showing -
a chilling finding that meant one out of every six aircraft was
flying with exposed conductor.
The ATSRAC report also asserted the discrepancies that were found
"did not appear to be wire-type dependent." However, there
was no place to record wire type on the form that was used to record
the inspection results. If wire type was not recorded, it may be
a stretch to conclude that wire type does not matter. Indeed, the
inspectors may not have been sensitized to an examination of wiring
by insulation type. Their template was the Air Transport Association's
(ATA) "Specification 117: Wiring Maintenance Practices/Guidelines."
This document outlines practices to avoid, such as using piping
carrying flammable liquids or oxygen as a support from which to
hang wire bundles. However, Spec 117 is silent on the subject of
wire type. For instance, it does not caution against mixing wire
types in the same bundle, which can lead to accelerated chafing
when insulation types of different "hardness" wear against
one another. Nor does Spec 117 enjoin against the use of certain
wire in particular applications. As an example of such guidance,
the FAA's 1991 Advisory Circular 25-16, "Electrical Fault and
Fire Prevention and Protection," urged, "Whenever practical,
aromatic polyimide (Kapton) insulation wires should not be used
for high current carrying cables."
One industry representative at the ATSRAC meeting, frustrated by
the report's lack of specificity, quipped acerbically, "We
have an executive summary of a report that does not exist."
One of the ATSRAC members explained that the report's generalized
language resulted from the need to de-identify the findings from
specific aircraft tail-numbers and from specific carriers.
Findings may be understated
It may be useful to outline how the inspections were structured.
The inspection teams each worked a specific aircraft model. This
organization may explain the variance in findings. For example,
nothing significant was found on the 737 inspections, but 62 potentially
hazardous items were found during the inspections of 14 high-time
DC-10's. That is an average rate of more than 4 potentially hazardous
items per airplane. Given the variance in approach taken by the
individual inspection teams, the results across aircraft types may
not be at all comparable. It is possible, if the DC-10 inspectors
indeed were tougher, that significantly more than 3,000 items would
have been found had this team examined all 81 airplanes.
In addition, not all the wiring was inspected. The inspections focused
on the harsh environment areas (wing leading edges, wheel wells,
air conditioning bays, cargo holds, and so forth). In these areas,
the particular emphasis was on power feeder wires (those with 15
amps or more current). Wires in protected areas, such as lighting
circuits in the cabin ceiling, generally were not inspected.
Nor were fuel tanks inspected, where the discovery of chafed wires
in 737 fuel tank conduits prompted the temporary grounding of many
high-time 737 "Classics" (-100 through -500 models) on
Mother's Day in 1998 until the wiring inspections were completed.
The fuel tank wiring and electrical component inspections are not
part of the ATSRAC program, but rather are the focus of a separate
program of intensive fuel tank inspections proposed by the FAA.
In this respect, the industry was spared the burden of having to
inspect fuel systems twice.
Taken altogether, a rough estimate suggests that the non-intrusive
inspections covered about 25%-30% of the wiring on the target aircraft.
The findings from the non-intrusive inspections, however tentative,
suggest that more serious discoveries await. If 3,000 items - 144
of them serious - were found by flashlight on fewer than 100 airplanes,
the planned intrusive inspections could reveal much more. Here's
why, according to a June, 1999 paper prepared by Lectromechanical
Design Co., a Virginia-based firm that specializes in aircraft wire
inspections:
"The comparative data are quite telling and suggest an inadequacy
in the most widely used wire inspection technique. Visual inspection
has difficulty detecting hidden breaks which are behind harnesses,
contained in the middle of harnesses, microscopic in size, under
clamps and ties, covered in Nomex, etc. In fact, most visual inspections
do not untie bundles, separate wires, loosen clamps or inspect wire
in tight locations."
This company has found in its inspections of military aircraft that
some 60% or more of the breaches in wire insulation are not detected
by visual inspection. Rear Admiral Donald Eaton, USN (Ret.), who
holds the logistics chair at the Naval Postgraduate School, strongly
supports a preventive program of wire husbandry for naval aircraft.
During his presentation at the Aging Aircraft Conference in Albuquerque,
N.M. last September, Eaton estimated that visual inspection may
uncover only about 25% of insulation breaches. In other words, the
intrusive inspections featured in the next round of the ATSRAC's
activity could potentially reveal significant, different or hidden
faults that were not found in the visual inspections, which traditionally
are not designed to find them. The intrusive inspections, by the
way, will address the issue of in-service wear by wire type.
