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maintained by operators and were not pursued.
As tasked, this study focused on the wear and tear and aging of the components and related
safety issues. It did not focus on acute onsets of failures that were sudden with little or no
progression or aging. Human interactions were part of this study if they affected the wear and
tear and had a progression. Sudden human errors and weather impacts that caused high
consequences were not part of this study. These issues and consequences should be covered in a
separate reporting and lessons learned process.
This evaluation stressed the safety philosophy of the Heinrich pyramid (figure 2) [2], especially
the preventative aspect, which posits that for every accident or event, there is a higher number of
incidents and an even higher number of unreported occurrences. These incidents, or unreported
occurrences, are problems of the same nature. They may be less severe or in the same
progression. Through proper assessment and maintenance, they can be reduced to a minimum,
thereby reducing the number of accidents or events. Accidents and incidents are rare. In many
cases, the threats are unknown. Therefore, it is critical to identify precursors and to examine
failure progression so that preventative measures can be taken to preclude future accidents.
3
Figure 1. The A320 Family Rudder Control System and Critical Components
Major, hazardous, or
catastrophic events
Incidents
Unreported
occurrences
Assessment and
maintenance
Figure 2. Heinrich Pyramid
3. METHODOLOGY.
3.1 STUDY FRAMEWORK.
The overall framework that guided this study is shown in figure 3. Key players included the
FAA, Airbus, and their OEMs (see left side of figure 3) who provided necessary documents for a
four-step process and a final evaluation. This four-step process included reviewing
requirements, background information, initial assessments, and in-service data and is described
in sections 3.2, 3.3, 3.4, and 3.5. The final evaluation produced hazard assessment tables and
included life cycle risk analysis. Examples of hazard assessment tables are presented in section
4. Graphical presentations of the in-service data and risk analyses are described in sections 3.6
and 3.7.
4
5
Figure 3. Study Process Framework
CFRs,
ACs,
MPDs,
ADs,
MSG-3s,
CMMs,
AMMs,
FTAs,
FMESs,
HAs, SSAs,
SBs,
Technical
letters,
Other
documents
Repair
records,
Fleet data,
Airclaims
database
Vendors,
Airbus,
FAA
Hazard Assessment Tables
Conclusions
Life Data/Risk Analysis
(1) Optimal maintenance
(2) Sound redundancy
(3) Low failure probabilities
4. In-Service Data (1988-2003)
3. Safety Review of Initial Assessment at Certification
2. Background
1. Requirements
Review FARs/JARs
Regulatory Requirements and Maintenance Planning/
Inspection Intervals
Review System/Part Designs/Requirements, Drawings/
Redundant Paths, and Inspection/Functional Tests
Review FTAs, FMESs, HAs, SSAs, MSG-3s
Request and Prepare Data, Review Findings, and Code
Primary Function Failures
3.2 REVIEW OF REGULATORY REQUIREMENTS AND MAINTENANCE PLANNING
DOCUMENTS.
3.2.1 Code of Federal Regulations and Joint Aviation Regulation Regulatory Requirements.
Risk thresholds were defined as thresholds beyond which preventive measures should be taken.
Depending on the consequence of the failure type, the CFR [1] and Joint Aviation Regulation
(JAR) [3] have mandated that the probability of failure be kept below 10-3, 10-5, 10-7, and 10-9 per
flying hour for each of the hazard categories, respectively, minor, major, hazardous, and
catastrophic. The hazard categories were classified according to the consequence of failure and
used in this study as the risk thresholds (see table 1).
Table 1. Regulatory Risk Thresholds
CFR Qualitative
Probability Terms Probable Improbable
Extreme
Improbable
JAR qualitative probability
terms
Frequent Reasonably
probable
Remote Extremely remote Extremely
improbable
JAR effect category Minor Major Hazardous Catastrophic
CFR and JAR qualitative
probability ranges
10-3 10-5 10-4 10-7 10-9
3.2.2 Maintenance Practices.
Copies of the Maintenance Review Board reports and Maintenance Planning Documents (MPD)
were obtained and reviewed. The recommended maintenance requirements and intervals were
also reviewed.
3.3 REVIEW OF DESIGN REQUIREMENTS, SYSTEM/ASSEMBLY/UNIT DRAWINGS,
AIRCRAFT MAINTENANCE MANUALS, AND COMPONENT MAINTENANCE
MANUALS.
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