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1.8.9.2  Due to different operating conditions and system design, it is necessary to use different measuring devices (either singly or combined) to obtain satisfactory performance criteria. As stated before, there are various methods used to evaluate and control performance — aircraft diversions, mechanical interruptions in flight, delays and flight cancellations, component unscheduled removal rates, etc.
1.8.9.3  The following are typical examples of methods that can be used to establish and maintain alert values. It should be understood that the methods of evaluation given below are only illustrative and that other suitable methods of evaluation could be used:
a)  Pilot reports per 1,000 aircraft departures:
1)  Several operators have selected pilot reports as related to the number of departures as the primary measure of aircraft systems performance reliability. The reference base for the computation of alert values is a cumulative rate of the previous calendar year's experience. This provides a large statistical base and takes into consideration the extremes in seasonal effects. The baseline for each system is initially calculated by compiling the number of pilot reports logged for the previous twelve-month period times 1 000 divided by the number of aircraft departures for the same twelve-month period. The purpose of multiplying the pilot reports by 1 000 is to arrive at a figure that expresses the rate per 1 000 departures.
2)  In order for this to be a cumulative or rolling  rate for the immediately previous twelve month period, it should be recalculated each month. The data for the first month of the existing twelve month data set is dropped, and the data compiled for the last month is added; i.e. if the initial calculation was from March 1998 to February 1999, the next month’s calculation would cover the period from April 1998 to March 1999.
3)  When the base line is computed for a particular system, an alert value is established at a point above the base line equal to, say, five pilot reports per 1 000 aircraft departures. The alert values assigned to each system represent the maximum rate of pilot-reported malfunctions considered to deviate sufficiently from the base line to require investigation.
b)  Pilot reports per 1 000 aircraft hours:
1)  For the purpose of measuring reliability, pilot reports per 1 000 aircraft flight hours may be selected as the indicator of aircraft systems performance. Performance standards in terms of pilot reports per 1 000 hours are established for each of the aircraft systems. Several programmes in current use utilize two performance numbers, an “alert” number and a “target” number. A review and evaluation of a minimum of six to twelve months’ history of pilot reports are done to establish the initial alert and target numbers. Established alert and target numbers are valid for a six-month period, at the end of which all alert and target numbers are reviewed and adjusted as necessary.
2)  The alert number is defined as the three-month moving (running) average which is considered to indicate unsatisfactory performance.
3)  Historically, alert numbers show seasonal variations. To provide a more realistic alert number, the year is divided into six-month periods. One period encompasses the winter months, the other, the summer months. When reviewing a particular six-month period to ascertain if the alert number is still practical, it is important that the comparison is made between similar periods.
4)  The target number is defined as the operator’s goal and predicted level of performance at the end of a six-month period. Target numbers are set to specify the operator’s desires and expectations for future system performance. The target number is established in the same manner as the alert number, the difference being that the alert number is the upper limit of the range and, when exceeded, indicates unsatisfactory performance. The target or the lower limit is set as a goal which represents a level that the operator believes is attainable.
　
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