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phases were investigated and reported if present. Weibull distributions, along with other
distributions, were the candidate models used in the study, and the best-fit model was chosen.
Regarding using failure rates and other quantitative numbers, the SAE ARP also states, “When
(initially) developing a new aircraft, the average flight time is usually determined from the
customer requirements for the aircraft. This is an assumed value. When modifying an existing
aircraft, the actual average flight time, based on in-service data, could be used” [12]. This also
agrees with the current study. When validating a fleet that has already been in service, customer
requirements and assumed values at initial assessment should be updated using in-service data.
6.2 FUTURE SAFETY ASSESSMENT AT CERTIFICATION.
As the result of this study, the initial safety assessment at certification should be enhanced to
address dependency and spare part issues. These are the two most significant generic issues
identified thus far. A safety factor can be built into the process of FTA, FMEA, etc., for
example, by using a 50 percent dependency factor or a 50 percent increase in removal rates as
initial design requirements. Actual in-service performance can then be validated one time in the
life of the fleet (see discussions in section 6.3). The safety factor can then be adjusted up or
down, depending on performance or other similar designs and modifications.
The CCA described above is exactly designed for dependency issues and should be
implemented. It includes a zonal safety analysis, particular risks analysis, and common mode
analysis. It is mostly qualitative; however, results of this study complement the CCA by
providing quantitative numbers, which help set priorities. Readers are referred to SAE ARP
4761 [12] for details of the procedures.
6.3 VALIDATION ONCE IN LIFE OF FLEET.
For an evaluation of aging, experience from this study suggests a one-time, in-service validation
of the original safety assessment at certification, with revisits when there is a new design or
upgrade or a new vendor. Validation times can vary depending on the component; however, the
timing is quite challenging. An ideal time is when there is an adequate number of failure modes
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or precursors of interest. Most commercial transport aircraft fleets should have enough failures
for aging evaluations. As discussed earlier, the approach is intended to detect early progression
or precursors to prevent aging-related problems. If the validation timing is off, problems can
occur before the evaluation occurs. An investigation is then needed to understand the underlying
mechanisms, severity, and need for preventative measures. Emphasis should be placed on areas
of potential high consequence. Less emphasis should be placed on highly reliable components.
Note, the frequency and timing of the validation may be different if the objective is different
(e.g., for warranty evaluation or other economic reasons).
6.4 DATA QUALITY AND LIMITATIONS OF STUDY.
Despite the efforts of the study, it is important to discuss the limitations. In one respect, the
approach used may have included unconfirmed occurrences and was, therefore, likely to be
conservative. Conversely, records may have been missing, due to the use of multiple repair
shops, company buyouts, and reorganizations, etc., and this may have resulted in underreporting.
The study was also limited by the records provided.
Operators could have been key players in the study and should be included in future studies.
Operators had removed and replaced certain components, including servocontrol cables and
numerous single rods, and this removal data was not available for the study. Removal
information on the cables, for example, could have allowed analysis into the risk of servocontrol
input failure due to the cable unit.
6.5 PREVENTATIVE USE AND/OR REGULATORY COMPLIANCE.
This study likely included precursor events and unreported occurrences. These were problems of
the same nature as those found in accidents, but most likely less severe or just in progression (see
the Heinrich pyramid discussion in section 2). Examining all incidents, events, and occurrences
has the advantage of being preventative, because the larger the number of occurrences, the higher
the chances of one being severe. Identifying these high-occurrence areas helps set priorities for
preventative measures.
Equally important, this approach has the potential to be used for regulatory compliance.
However, it requires that accidents be distinguished from incidents, events, and unreported
occurrences. It also would require a process of data collection and analysis that does not exist
today and should involve interviews with the operators and flight crews. Finally, a complete
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