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design analysis. Such an analysis can be used to calculate the level of integrity of the system in any one landing. The
following formula applies to certain types of ILS and provides an example of the determination of system integrity, I, from a
calculation of the probability of transmission of undetected erroneous radiation, P.
(1) I = 1 – P
P = 1 2
1 2
1 2 1 2
when
TT T T
M M
<
α α
where
I = integrity
P = the probability of a concurrent failure in transmitter and monitor systems resulting in erroneous
undetected radiation
M1 = transmitter MTBF
M2 = MTBF of the monitoring and associated control system
1
1
α
= ratio of the rate of failure in the transmitter resulting in the radiation of an erroneous signal to the rate of
all transmitter failures
2
1
α
= ratio of the rate of failure in the monitoring and associated control system resulting in inability to detect
an erroneous signal to the rate of all monitoring and associated control system failures
T1 = period of time (in hours) between transmitter checks
T2 = period of time (in hours) between checks on the monitoring and associated control system
When T1 ≥ T2 the monitor system check may also be considered a transmitter check. In this case, therefore T1 = T2 and the
formula would be:
(2) P =
2
2
1 2 1 2
T
α α M M
2.8.2.5 With regard to integrity, since the probability of occurrence of an unsafe failure within the monitoring or
control equipment is extremely remote, to establish the required integrity level with a high degree of confidence would
necessitate an evaluation period many times that needed to establish the equipment MTBF. Such a protracted period is
unacceptable and therefore the required integrity level can only be predicted by rigorous design analysis of the equipment.
23/11/06 ATT C-32
Attachment C Annex 10 — Aeronautical Communications
2.8.2.6 The MTBF and continuity of service of equipment is governed by basic construction and operating environment.
Equipment design should 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. The manufacturer is required to provide the details of the design to enable the MTBF and
continuity of service to be calculated. It is expected that the equipment MTBF is confirmed by evaluation in an operational
environment to take account of the impact of operational factors, i.e. airport environment, inclement weather conditions, power
availability, quality and frequency of maintenance. For integrity and continuity of service Levels 2, 3 or 4 the evaluation period
should be sufficient to determine achievement of the required level with a high degree of confidence. The following
considerations apply:
a) the minimum acceptable confidence level is 60 per cent. Depending on the service level of the ILS, 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 will result in a reduced evaluation period; and
b) during the evaluation period, it should be decided for each outage if it is caused by a design failure or if it is caused
by a failure of a component due to its normal failure rate. Design failures are, for instance, operating components
beyond their specification (overheating, overcurrent, overvoltage, etc. conditions). These design failures should be
dealt with such that the operating condition is brought back to the normal operating condition of the component or
that the component is replaced with a part suitable for the operating conditions. If the design failure is treated in this
way, the evaluation may continue and this outage is not counted, assuming that there is a high probability that this
design failure will not occur again. The same applies to outages due to any causes which can be mitigated by
　
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