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periodically with a calibrated receiver to ensure that the plus or minus 2-degree tolerance is satisfied, irrespective of the
orientation of the VOR receiving antenna.
Note.— The tolerance of plus or minus 2 degrees relates to the consistency of the information at the selected point and
includes a small tolerance for the accuracy of the calibrated VOR receiver used in checking the point. The 2-degree figure
does not relate to any figure for acceptance or rejection of an aircraft VOR installation, this being a matter for determination
by Administrations and users in the light of the operation to be performed.
2.2.5 Check-points which can satisfy the foregoing requirements should be selected in consultation with the operators
concerned. Provision of check-points in holding bays, at runway ends and in maintenance and loading areas, is usually
desirable.
ATT E-3 23/11/06
Annex 10 — Aeronautical Communications Volume I
2.3 Marking of VOR check-points
Each VOR check-point must be distinctively marked. This marking must include the VOR bearing which a pilot would
observe on the aircraft instrument if the VOR installation were operating correctly.
2.4 Use of VOR check-points
The accuracy with which a pilot must position the aircraft with respect to a check-point will depend on the distance from the
VOR station. In cases where the VOR is relatively close to a check-point, particular care must be taken to place the aircraft’s
VOR receiving antenna directly over the check-point.
___________________
23/11/06 ATT E-4
ATTACHMENT F. GUIDANCE MATERIAL CONCERNING
RELIABILITY AND AVAILABILITY OF RADIOCOMMUNICATIONS
AND NAVIGATION AIDS
1. Introduction and fundamental concepts
This Attachment is intended to provide guidance material which Member States may find helpful in providing the degree of
facility reliability and availability consistent with their operational requirement.
The material in this Attachment is intended for guidance and clarification purposes, and is not to be considered as part of
the Standards and Recommended Practices contained in this Annex.
1.1 Definitions
Facility availability. The ratio of actual operating time to specified operating time.
Facility failure. Any unanticipated occurrence which gives rise to an operationally significant period during which a facility
does not provide service within the specified tolerances.
Facility reliability. The probability that the ground installation operates within the specified tolerances.
Note.— This definition refers to the probability that the facility will operate for a specified period of time.
Mean time between failures (MTBF). The actual operating time of a facility divided by the total number of failures of the
facility during that period of time.
Note.— The operating time is in general chosen so as to include at least five, and preferably more, facility failures in
order to give a reasonable measure of confidence in the figure derived.
Signal reliability. The probability that a signal-in-space of specified characteristics is available to the aircraft.
Note.— This definition refers to the probability that the signal is present for a specified period of time.
1.2 Facility reliability
1.2.1 Reliability is achieved by a combination of factors. These factors are variable and may be individually adjusted
for an integrated approach that is optimum for, and consistent with, the needs and conditions of a particular environment. For
example, one may compensate to some extent for low reliability by providing increased maintenance staffing and/or
equipment redundancy. Similarly, low levels of skill among maintenance personnel may be offset by providing equipment of
high reliability.
ANNEX 10 — VOLUME I ATT F-1 23/11/06
Annex 10 — Aeronautical Communications Volume I
1.2.2 The following formula expresses facility reliability as a percentage:
R = 100 e–t/m
where:
R = reliability (probability that the facility will be operative within the specified tolerances for a time t, also referred
to as probability of survival, Ps);
e = base of natural logarithms;
t = time period of interest;
m = mean time between facility failures.
It may be seen that reliability increases as mean time between failures (MTBF) increases. For a high degree of reliability, and
for operationally significant values of t, we must have a large MTBF; thus, MTBF is another more convenient way of
expressing reliability.
1.2.3 Experimental evidence indicates that the above formula is true for the majority of electronic equipments where
the failures follow a Poisson distribution. It will not be applicable during the early life of an equipment when there is a
　
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