of airplane electrical systems and associated documentation
As part of its effort
to ensure the continued airworthiness of aging airplanes (i.e.,
airplanes built to type designs that are more than 20 years old),
the U.S. Federal Aviation Administration (FAA) formed a fact-finding
committee in 1998 to evaluate the airplane systems of the aging
fleet and propose enhancements to current procedures. The Aging
Transport Systems Rulemaking Advisory Committee (ATSRAC), which
is composed of representatives from various segments of the aviation
industry, is focusing its investigation on airplane wiring. (See
Airplane Systems Investigation, Aero no. 7, July 1999.)
The committee completed its initial tasks in January 2001 and is
continuing with plans to implement its recommendations. This article
findings and recommendations.
of ATSRAC recommendations.
- FAA actions.
Airworthiness Program for Airplane Systems.
ATSRAC FINDINGS AND RECOMMENDATIONS
The initial ATSRAC investigation
of aging airplane wiring studied five factors: fleet condition,
fleet service history, maintenance criteria, standard practices
for wiring, and inspection and repair training. A team of ATSRAC
members and industry representatives was assigned to evaluate each
parameter. The teams conducted analyses, made conclusions, and recommended
follow-up actions, which the ATSRAC then reviewed, approved, and
provided to the FAA.
Although none of the
teams identified any issue related to the immediate safety of the
aging fleet, they did suggest enhancements to existing design, maintenance,
and operational procedures for the continued airworthiness of all
One team reviewed the condition of the aging fleet by conducting
a nonintrusive evaluation of the wiring on 81 in-service airplanes
and a detailed, intrusive inspection of the wiring removed from
six recently retired airline airplanes.
The team found that wiring
degradation primarily is not related to the age of the airplane
(i.e., the time since manufacture), the environment in which the
airplane operates, or the type of wiring. Rather, wiring degradation
is influenced significantly by the maintenance and modification
performed throughout the life of the airplane. The team also determined
that a general visual inspection of the wiring installed on airplanes,
which typically is conducted from a distance of a few feet, cannot
adequately assess the condition of the wiring. Specific recommendations
included an increased emphasis on the periodic removal of accumulated
contaminants and clarification of the requirements regarding the
spatial separation of wiring for critical airplane systems.
A second team reviewed all existing service information applicable
to the older airplanes under study. Of the thousands of wiring-related
service documents (e.g., service bulletins, service letters, all-operator
letters, in-service activity reports) reviewed, 29 service bulletins
contained airplane modifications important enough to justify upgrading
the service bulletins to alert status. (Boeing releases alert service
bulletins to address issues of safety over the life of the fleet.)
The FAA has released 26 airworthiness directives (AD) and proposed
one AD that mandate incorporation of the modifications (table