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relatively large number of premature failures of individual components; neither will it be true when the equipment is nearing
the end of its useful life.
1.2.4 At many facility types utilizing conventional equipment, MTBF values of 1 000 hours or more have been
consistently achieved. To indicate the significance of a 1 000-hour MTBF, the corresponding 24-hour reliability is
approximately 97.5 per cent (i.e. the likelihood of facility failure during a 24-hour period is about 2.5 per cent).
1.2.5 Figure F-1 shows the probability of facility survival, Ps, after a time period, t, for various values of MTBF.
Note.— It is significant that the probability of surviving a period of time equal to the MTBF is only 0.37 (37 per cent);
thus, it is not assumed that the MTBF is a failure-free period.
1.2.6 It may be seen that adjustment of MTBF will produce the desired degree of reliability. Factors which affect
MTBF and hence facility reliability are:
a) inherent equipment reliability;
b) degree and type of redundancy;
c) reliability of the serving utilities such as power and telephone or control lines;
d) degree and quality of maintenance;
e) environmental factors such as temperature and humidity.
23/11/06 ATT F-2
Attachment F Annex 10 — Aeronautical Communications
1.3 Facility availability
1.3.1 Availability, as a percentage, may be expressed in terms of the ratio of actual operating time divided by specified
operating time taken over a long period. Symbolically,
Actual time operating (100)
A =
Specified operating time
Duration of equipment operation – hours (t)
Probability of survival (PS) (per cent)
1 000
950
900
850
800
750
700
650
600
550
500
450
400
100 90 80 70 60 50 40 30
m = 5 000 hrs m = 2 000 hrs
m = 1 000 hrs
m = 500 hrs
m = 333 hrs
m = 250 hrs
m = 100 hrs
m = 167 hrs
350
300
250
200
150
100
50
0
P = Probability of survival
t = Duration of operation
m = MTBF
S
Figure F-1. Plot of Ps = 100 e–t/m
ATT F-3 23/11/06
Annex 10 — Aeronautical Communications Volume I
For example, if a facility was operating normally for a total of 700 hours during a 720-hour month, the availability for that
month would be 97.2 per cent.
1.3.2 Factors important in providing a high degree of facility availability are:
a) facility reliability;
b) quick response of maintenance personnel to failures;
c) adequate training of maintenance personnel;
d) equipment designs providing good component accessibility and maintainability;
e) efficient logistic support;
f) provision of adequate test equipment;
g) standby equipment and/or utilities.
2. Practical aspects of reliability and availability
2.1 Measurement of reliability and availability
2.1.1 Reliability. The value that is obtained for MTBF in practice must of necessity be an estimate since the
measurement will have to be made over a finite period of time. Measurement of MTBF over finite periods of time will enable
Administrations to determine variations in the reliability of their facilities.
2.1.2 Availability. This is also important in that it provides an indication of the degree to which a facility (or group of
facilities) is available to the users. Availability is directly related to the efficiency achieved in restoring facilities to normal
service.
2.1.3 The basic quantities and manner of their measurement are indicated in Figure F-2. This figure is not intended to
represent a typical situation which would normally involve a larger number of inoperative periods during the specified
operating time. It should also be recognized that to obtain the most meaningful values for reliability and availability the
specified operating time over which measurements are made should be as long as practicable.
23/11/06 ATT F-4
Attachment F Annex 10 — Aeronautical Communications
Operating
Non-operating
Specified operating time
Actual operating time = a + a + a + a + … + a
a = operating period
Non-operating time = s + … + s + f + f + … + f
s = scheduled shutdown period
f = failure period
Specified operating time = Sum of actual operating time and non-operating time
1 2 3 4 n
1 n 1 2 n
a1
s1
a2
f1
a3
f2
a4
f3
a5
f4 f5
a6 a7
Figure F-2. Evaluation of facility availability and reliability
2.1.4 Using the quantities illustrated in Figure F-2, which includes one scheduled shutdown period and five failure
periods, one may calculate mean time between failures (MTBF) and availability (A) as follows:
　
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