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modified since they were in service, followed by, how they had been routinely checked and
maintained. If data allowed and analysis was needed, a full analysis included descriptive
statistics on removal reasons, graphical presentations, and risk computations of the removals
over time. The analyses compared removal times and inspection intervals. In addition, they
examined single and multiple failure modes to check for failure-safe design within each unit and
subsystem. Operator differences were also evaluated. Safety concerns, not necessarily
confirmed, were investigated. The risks were propagated in a fault-tree model in a few cases.
Performance of the original and refurbished parts was also compared. Refurbished parts were of
interest because of their prevalent use as the fleet gets older.
4.2 HAZARD ASSESSMENT TABLES.
Unit and System Hazard Assessment tables were prepared with the results of the risk analyses.
The Unit Hazard Assessment table included the following criteria from the original safety
assessment at certification: individual failure modes, detection methods by crew, and
maintenance intervals. The Unit Hazard Assessment table also included results of this study to
enhance the initial safety assessment: estimated failure probability projected to the next 5 years
and adequacy of the maintenance and recommendations. An example of the artificial feel units
(AFU) is shown in table 2. Potential areas of elevated probabilities, if any, were highlighted and
discussed.
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Table 2. The AFU Hazard Assessment Table
Failure Mode Detection by Crew
Hazard
Category
Scheduled
Maintenance
(if not detectable)
Failure
Probability
Adequate
Maintenance
Intervals
Inner spring inoperative May be
undetectable
Major 14,000 flying hours
Inner spring disconnection Pedal untimely
movement
Major
Inner spring disengagement
failure
Pedal high force None
Autopilot override jamming Jammed pedals None
Artificial feel jamming Jammed pedals Minor
Trim drive disconnection Loss of trim
function
Minor a
Trim drive jamming Rudder trim system Minor a
Detectable
<1.2 X 10-7 Adequate
a Major with engine failure
The System Hazard Assessment table (table 3) included the following criteria summarized
from the original safety assessment at certification: units or subsystem, failure condition,
consequence, hazard classification, certification probability, and mitigating actions. The System
Hazard Assessment table also included results from this study: failure probability per flight hour
and remarks. Failure probabilities were compared with regulatory requirements for each hazard
classification (i.e., minor, major, hazardous, and catastrophic). The results showed that the
failure probabilities fell reasonably within the regulatory requirements.
Table 3. System Hazard Assessment Table (Partial)
Part or
Subsystem
Failure
Condition Consequence
Hazard
Classification
Certification
Probability per
Flying Hour
Mitigating
Action(s)
In-Service
Probability
per Flying
Hour Remarks
Disconnected No/limited
mechanical
control
Minimum Extremely
improbable
Use copilot
pedals
Rudder
pedal–pilot
Jammed No mechanical
control
Minimum Extremely
improbable
Use copilot
pedals
Disconnected No/limited
mechanical
control
Minimum Extremely
improbable
Use pilot
pedals
Rudder
pedal–
copilot
Jammed No mechanical
control
Minimum Extremely
improbable
Use pilot
pedals
Flight
control
cable
Jammed No mechanical
control from
cockpit
Major Extremely
improbable
Autopilot
and/or Trim
(insufficient
for some
landing
conditions?)
…
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As discussed, because accidents and incidents were rare, potential progressions or events, even
though less severe and unconfirmed, were observed and noted in the remarks column to provide
insight and help set preventative safety priorities.
5. GENERAL DISCUSSION AND ISSUES.
This section presents generic issues identified during the study.
5.1 PERFORMANCE DEPENDENT ON DESIGN COMPLEXITY.
Of the components reviewed, simple mechanical units were highly reliable. For each one
evaluated, only a handful had been removed since they went into service.
Electromechanical, electrohydraulic, and mechanical/hydraulic components were more complex
and intricate and, therefore, were less reliable. Components logically controlled by electronic
　
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