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tasked to evaluate two mechanical systems as sample studies: the Boeing 757 Elevator System
and the Airbus A320 Rudder Control System. This report focuses on the evaluation of the aging
Airbus A320 Rudder Control System.
The ultimate objectives of the program are (1) to identify and bring generic issues of
nonstructural mechanical systems to industry attention, (2) to evaluate whether the existing
design and maintenance philosophy at initial certification remains valid as the system ages, and
(3) to evaluate the health of commercial aircraft as they approach their design service goal. This
study was the first to demonstrate this process by examining maintenance intervals, fail-safe
designs, and failure probabilities.
Collaborative efforts came from the FAA, Airbus, and their vendors. They provided required
documents and in-service data to allow SNL reviews of (1) regulatory requirements, (2) system
descriptions, (3) initial safety assessment at certification, and (4) in-service data. SNL also
conducted a risk analysis. A full life-cycle approach used in-service data from 1988 to 2003.
The analysis included graphical presentations and model fitting (e.g., a bathtub Weibull model)
of the data. The study focused on major and procurable parts of the rudder control system for
which repair histories were obtainable.
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1. INTRODUCTION.
As part of the Aging Transport Systems Rulemaking Advisory Committee’s efforts that began
after the TWA Flight 800 accident in 1996, it was determined that generic problems of aging
aircraft fleets should be identified, brought to the attention of industry, and researched. Generic
issues included aging, wiring, corrosion, and the design concept of dual-load paths for
continuous airworthiness.
The Federal Aviation Administration (FAA) Aging Aircraft Program expanded its research to
include nonstructural components, wiring, and mechanical systems to evaluate the health of
commercial aircraft as they aged and approached their design service goal. The program was
intended to evaluate whether the existing design and maintenance philosophy at certification
remained valid as mechanical systems aged. The study summarized in this report focuses on the
rudder system of the Airbus A320 family (A318, A319, A320, and A321) and included a review
of the system description and a safety/reliability analysis. Of particular interest were
recommendations for aging, infrequently maintained, or uninspectable parts, and parts that may
have latent failures.
As of June 2003, a total of 558 A318s, A319s, A320s, and A321s had been in service in the U.S.
as commercial carriers. The oldest of the four models, the A320s, entered service in April 1988.
The A321 entered service in April 1994, the A319 in April 1996, and the A318 in April 1999.
This study followed the safety principles of the FAA Title 14 Code of Federal Regulations
(CFR) Part 25 design standards (see Advisory Circular (AC) 25.1309-1A) [1]. The FAA
requires that any single failure during a flight, and any combination of failures not shown to be
extremely improbable, should not prevent continued safe flight and landing. The FAA
consequently describes two design principles: (1) having redundancy or backup systems to
enable continued functioning after any failure(s), and (2) having independence of systems,
components, and elements so that any failure does not cause the failure of another system,
component, or element essential to continue safe flight and landing.
Following these design principles of desired redundancy and independence, this evaluation was
carried out in two steps: (1) product life at the component level (single path) was estimated and
(2) the probabilities of certain failures (single and backup paths), potentially related to safety,
were estimated. Based on the results, optimal maintenance intervals were suggested or
recommended. A life-data analysis approach was used to analyze operational data. The
operational data were requested primarily for major and procurable components from the aircraft
manufacturer (Airbus) and component original equipment manufacturers (OEM). Due to time
constraints, other operational data from the maintenance, repair, and overhaul shops and the
carriers were not pursued.
1
2
2. SCOPE AND SAFETY PHILOSOPHY.
This study reviewed the rudder control system, excluding the rudder surface. It focused on the
major and procurable parts of the rudder control system when the repair history was obtainable
from Airbus and the OEMs. The reviews gave an understanding of the interfacing systems.
Figure 1 depicts the rudder control system design and identifies the specific components that
were evaluated. Repair histories of other subassemblies and component parts were only
　
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