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unacceptable and therefore the required integrity level can only be predicted by rigorous design analysis of the equipment.
However, a degree of confidence in the analysis can be achieved by demonstration of independence between the
transmitter and monitor functions. The predicted performances of the transmitter and monitor can then be evaluated
independently, resulting in more feasible evaluation periods.
11.2.6 The MTBF and continuity of service of equipment is governed by basic construction characteristics and by
the operating environment. The basic construction characteristics include the failure rate of the components of the
equipment and the physical relationship of the components. Failure rate (1/MTBF) and continuity of service are not
always directly related because not all equipment failures will necessarily result in an outage, e.g. an event such as a
failure of a transmitter resulting in the immediate transfer to a standby transmitter. The manufacturer is expected to
provide the details of the design to allow the MTBF and the continuity of service to be calculated. Equipment design has
to employ the most suitable engineering techniques, materials, and components, and rigorous inspection should be applied
during manufacture. It is essential to ensure that equipment is operated within the environmental conditions specified by
the manufacturer.
11.2.7 The design continuity of service is expected to exceed that given in 12.4 by as large a margin as is feasible. The
reasons for that are as follows:
a) the MTBF experienced in an operational environment is often worse than that determined by the design calculations
due to the impact of operational factors;
b) the continuity of service objectives given in 12.4 are minimum values to be achieved in an operational environment.
Any improvement in performance above these values enhances the overall safety of the landing operation;
c) a margin between the continuity of service objective and that achieved is required in order to reduce the chance of
falsely rejecting the suitability of an equipment for a particular level of service due to statistical uncertainty.
Note.— The Level 3 and 4 continuity of service values include a factor that accounts for the pilot’s capability to avoid a
fatal accident in the event of a loss of guidance. It is particularly desirable to reduce this factor to the maximum extent
practical by achieving the best possible continuity of service for Level 3 and 4 equipment.
11.2.8 Experience has shown that there is often a difference between the calculated continuity of service and that
experienced in an operational environment both because the performance of the equipment may be different from the
calculated value and because of the impact of operational factors, i.e. airport environment, inclement weather conditions,
power availability, quality and frequency of maintenance, etc. For these reasons, it is recommended that the equipment
MTBF and continuity of service be confirmed by evaluation in an operational environment. Continuity of service may be
evaluated by means of mean time between outages, where an outage is defined as any unanticipated cessation of signal-inspace.
It is calculated by dividing total facility up-time by the number of operational failures. For integrity and continuity of
service Levels 2, 3 or 4, the evaluation period is to be sufficient to determine achievement of the required level with a high
degree of confidence. To determine whether the performance record of an individual equipment justifies its assignment to
Levels 2, 3 or 4 requires judicious consideration of such factors as:
a) the performance record and experience of system use established over a suitable period of time;
b) the average achieved MTBO established for this type of equipment; and
c) the trend of the failure rates.
23/11/06 ATT G-32
Attachment G Annex 10 — Aeronautical Communications
11.2.9 The minimum acceptable confidence level for acceptance/rejection is 60 per cent. Depending on the service
level of the MLS, this may result in different evaluation periods. To assess the influence of the airport environment, a
minimal evaluation period of one year is typically required for a new type of installation at that particular airport. It may be
possible to reduce this period in cases where the operating environment is well controlled and similar to other proven
installations. Subsequent installation of the same type of equipment under similar operational and environmental conditions
may follow different evaluation periods. Typically, these minimal periods for subsequent installations are for Level 2, 1 600
hours, for Level 3, 3 200 hours and for Level 4, at least 6 400 hours. Where several identical systems are being operated
under similar conditions, it may be possible to base the assessment on the cumulative operating hours of all the systems. This
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