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1.8.8 Performance standard
1.8.8.1 Each reliability programme should include a performance standard expressed in mathematical terms. This standard becomes the point of measure of maximum tolerable unreliability. Thus, satisfactory reliability trend measurements are those which fall at or preferably below the performance standard. Conversely, a reliability trend measurement exceeding the performance standard is unsatisfactory and calls for some type of follow-up and corrective action.
1.8.8.2 A performance standard may be expressed in terms of system or component failures per thousand hours of aircraft operation, number of landings, operating cycles, departure delays, or of other findings obtained under operational conditions. In some instances, an upper and lower figure may be used. This is known as a reliability band or range and provides the standard by which equipment behaviour may be interpreted or explained.
1.8.8.3 When the performance standard is exceeded, the programme should provide for an active investigation which leads to suitable corrective action.
1.8.8.4 A description of the types of action appropriate to the circumstances revealed by the trend and the level of reliability experience should be included in the programme. This is the central core of maintenance control by reliability measurement. It is the element that relates operating experience to maintenance control requirements. Statistical techniques used in arriving at reliability measurements presented in support of maintenance control actions should be described. Appropriate corrective actions might be:
a)  verify that engineering analysis is appropriate on the basis of collective data in order to determine the need to change the maintenance programme;
b)  actual maintenance programme changes involving inspection frequency and content, functional checks, or overhaul limits and times;
c)  aircraft system or component modification, or repair; or
d)  other actions peculiar to the condition that prevails.
1.8.8.5 The results of corrective action programmes should become evident within a reasonable time from the date of implementation of corrective action. An assessment of the time permitted should be commensurate with the severity or safety impact of the problem. Each corrective action programme should have an identified completion date.
1.8.8.6  Due to the constantly changing state-of-the-art, no performance standard should be considered fixed — it is subject to change as reliability changes. The standard should be responsive and sensitive to the level of reliability experienced. It should be “stable” without being “fixed”. If, over a period of time, the performance of a system or component improves to a point where even abnormal variations would not produce an alert, then the performance standard has lost its value and should be adjusted downward. Conversely, should it become evident that the standard is consistently exceeded in spite of taking the best known corrective measures to produce the desired reliability, then the performance standard should be re-evaluated and a more realistic standard established. Each programme should contain procedures to effect, when required, such changes to the prescribed performance standards.
1.8.9  Establishing initial standards
1.8.9.1 In order to establish the initial standards for structural components, powerplants and systems, the past operating experience with the same (or, in the case of new aircraft, similar) equipment should be reviewed in sufficient depth to obtain a cross-section of the subject system’s performance. Normally, a period of six months to one year should be sufficient. For a system common to a large fleet of aircraft, a representative sample may be used, while small fleet systems may require 100 per cent review. Operators introducing a new aircraft into service may establish their alert by using this available data. After the operator completes about one year's operating experience, however, the alert value should be adjusted based upon his experience.
　
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