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5)  Each month a three-month running average for each system is calculated. First, a three-month average is obtained by compiling and analysing data for three consecutive months — the total pilot reports for three months are divided by the number of aircraft hours flown during the same three-month period. To maintain a running average, each month the first month's data is deleted and the data for the current month added. Any system which either exceeds the alert or which has a trend indicating the target will not be met is considered to be in need of special attention.
1.8.10 Establishing alert values statistically
1.8.10.1 Many programmes establish alert values by reviewing past performance and then, by using “good judgement”, establish the numerical value for the alert. Although this generally works well, the value can become controversial since the “good judgement” of one person may well be different from that of another person. In an effort to avoid controversy, some operators prefer the statistical or mathematical approach. This is a broad term that covers a number of methods of gathering numbers of instances and evaluating the result; all methods, however, require a sufficient quantity of accurate data to be available for analysis.
1.8.10.2 In order to establish system alert values, an evaluation is made of the operational performance of each system to be controlled by the programme. The yardsticks covering failure performance are clearly defined in the programme. Using these definitions, the failure data for each system are extracted from pilot-reported malfunctions for at least a 12-month period. The “mean” and the “standard deviation” are then computed from those data and each system's alert value is established equal to the mean plus three standard deviations.
1.8.10.3 The current performance level of each system is computed on a monthly basis as a three-month cumulative performance rate. This rate is computed by multiplying the number of in-flight malfunctions for a three-month period by 1 000 and dividing by the total aircraft flight hours for the same period. Maintaining a cumulative rate requires that the first month's data be deleted and the data for the current month be added to the sum of the previous two months. When a trend of deteriorating system performance is detected, or if a system is over the alert value, an active investigation is conducted to assess the causes of the change in system performance and to develop an active corrective programme, if required, to bring the system performance under control.
1.8.11  Condition-monitored maintenance programmes
1.8.11.1  Other techniques are used which monitor the functional condition of systems or components without disturbing them in their installed environment. These programmes are based on the establishment of acceptable performance as base line data. Internal and external leakage, functional testing, and unit teardown analysis are the factors used to determine the base line. The results of this test become a part of the aircraft’s permanent record. The point to be established is that the tests accurately and conservatively identify discrepancies before operational reliability is degraded.
1.8.11.2  This type of programme lends itself readily to components. It has also proven very successful in monitoring the functional condition of aircraft systems such as hydraulics, air conditioning and pneumatics (the system primarily utilizing this type of programme is hydraulics). The various tests perform the function of system or subsystem interrogation to determine the presence or absence of component degradation. Internal leakage rates serve as the criteria to evaluate wear and rigging effect on component performance while pressures are used to determine certain component functional responses.
1.8.11.3  During the test, individual parts, components and subsystems are evaluated by selective positioning of the various system controls and isolation points. From the comparison of the response produced by sequential steps to the established tolerance, the general location or the specific location of the faulty unit can be determined.
　
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