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permanent changes to the operating conditions.
2.8.2.7 Continuity of service performance may also be demonstrated by means of MTBO (mean time between outages)
where an outage is defined as any unanticipated cessation of signal-in-space. It is calculated by dividing the total facility uptime
by the number of operational failures. MTBF and MTBO are not always equivalent, as 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 minimum MTBO values expected for the continuity of service in 2.14 have been derived from several years
of operational experience of many systems. To determine whether the performance record of an individual ILS system
justifies its assignment to Levels 2, 3 or 4 requires a judicious consideration of such factors as:
a) the performance record and experience of system use established over a suitable period of time (see 2.8.2.6);
b) the average achieved MTBO established for this type of ILS; and
c) the trend of the failure rates.
An assigned designation should not be subject to frequent change. A suitable method to assess the behaviour of a particular
installation is to keep the records and calculate the average MTBO over the last five to eight failures of the equipment. A
typical record of this method is given in Figures C-12A and C-12B.
2.8.2.8 During the equipment evaluation, and subsequent to its introduction into operational service, records should be
maintained of all equipment failures or outages to confirm retention of the desired continuity of service.
2.8.2.9 The following configuration is an example of a redundant equipment arrangement that is likely to meet the
objectives for integrity and continuity of service Levels 3 or 4. The localizer facility consists of two continuously operating
transmitters, one connected to the antenna and the standby connected to a dummy load. With these transmitters is associated
a monitor system performing the following functions:
ATT C-33 23/11/06
Annex 10 — Aeronautical Communications Volume I
23/11/06 ATT C-34
Figure C-12A. Example of a localizer outage record
Figure C-12B. Example of a glide path outage record
10 000
2 000
1 000
100
Outage Single No Effect
MTBF
MTBO
GP XX
30-06-92 29-12-92 30-06-93 30-12-93 30-06-94 30-12-94 1-07-95 30-12-95 30-06-96 30-12-96 1-07-97
8
6420
jul-92
oct-92
jan-93
jan-94
jan-95
jan-96
jan-97
apr-93
apr-94
apr-95
apr-96
jul-93
jul-94
jul-95
jul-96
oct-93
oct-94
oct-95
oct-96
10 000
4 000
2 000
1 000
100
Outage Single No Effect
MTBF
MTBO
LLZ XX
30-06-92 29-12-92 30-06-93 30-12-93 30-06-94 30-12-94 1-07-95 30-12-95 30-06-96 30-12-96 1-07-97
8
6420
jul-92
oct-92
jan-93
jan-94
jan-95
jan-96
jan-97
apr-93
apr-94
apr-95
apr-96
jul-93
jul-94
jul-95
jul-96
oct-93
oct-94
oct-95
oct-96
Attachment C Annex 10 — Aeronautical Communications
a) monitoring of operation within the specified limits of the main transmitter and antenna system by means of majority
voting among redundant monitors;
b) monitoring the standby equipment.
2.8.2.9.1 Whenever the monitor system rejects one of the equipments the facility continuity of service level will be
reduced because the probability of cessation of signal consequent on failure of other equipment will be increased. This
change of performance must be automatically indicated at remote locations.
2.8.2.9.2 An identical monitoring arrangement to the localizer is used for the glide path facility.
2.8.2.9.3 To reduce mutual interference between the main and standby transmitters any stray radiation from the latter is
at least 50 dB below the carrier level of the main transmitter measured at the antenna system.
2.8.2.9.4 In the above example, the equipment would include provision to facilitate monitoring system checks at
intervals specified by the manufacturer, consequent to the design analysis, to ensure attainment of the required integrity level.
Such checks, which can be manual or automatic, provide the means to verify correct operation of the monitoring system
including the control circuitry and changeover switching system. The advantage of adopting an automatic monitor integrity
test is that no interruption to the operational service provided by the localizer or glide path is necessary. It is important when
using this technique to ensure that the total duration of the check cycle is short enough not to exceed the total period specified
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