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items so identified are derived from empirical data.
1.2 The method gives a visual representation of sequences and combinations of events leading to the top failure event.
The method can also be used to determine the probability of the top event occurring, provided that the probabilities of the
individual events are known or can be estimated. In the case of simple fault trees probabilities can be directly calculated, but
care must be taken if the primary failure events are not independent, i.e. if failure events are common to more than one path.
1.3 In this guidance material the acceptable probability of the top level event occurring is determined by the risk
allocation and the fault tree is used to further partition the risk into integrity and continuity of service risks. Therefore, the
term “risk tree” is used rather than “fault tree”.
2. A generic risk tree for aircraft landing operations is given in Figure A-1. The top event for this tree is taken to be the
loss of the aircraft due to a failure of the non-aircraft guidance system. The causes of this event are either an integrity failure
of the primary non-aircraft guidance equipment or a continuity of service (COS) failure of the non-aircraft guidance system
(i.e. both the primary system and any secondary system used to support a discontinued approach/missed approach). The
primary non-aircraft guidance system is considered to have a number of elements, 1 to N, for example azimuth, elevation and
DME/P in the case of MLS. The secondary guidance system may be an alternative non-aircraft system, or in some cases an
aircraft navigation system such as an inertial reference system.
2.1 The following probabilities can be defined:
Pa = Probability of aircraft loss due to a failure of the non-aircraft guidance system.
Pb = Probability of aircraft loss due to primary guidance integrity failure.
Pc = Probability of aircraft loss due to COS failure.
ANNEX 10 — VOLUME I ATT A-1 23/11/06
Annex 10 — Aeronautical Communications Volume I
Aircraft loss due to
non-aircraft guidance
system failure
Pa = 3 x 10
-9
Aircraft loss due to
primary guidance
integrity failure
Pb
Aircraft loss due to
continuity of service
failure
Pc
Primary guidance
integrity failure
Pi
Pilot risk
reduction
Px
Discontinued
procedure practice
Pd
Secondary
guidance failure
(see Note 1)
Ps
Primary guidance
continuity of
service failure
Pp
Integrity
failure nav
element 1
Pi1
COS
failure nav
element 1
Pp1
COS
failure nav
element 2
Pp2
COS
failure nav
element N
PpN
COS
failure
Ps1
Integrity
failure
Ps2
Integrity
failure nav
element 2
Pi2
Integrity
failure nav
element N
PiN
Pilot risk
reduction
Pu
Ppn =
Exposure time
MTBON Note 1: Secondary guidance may be aircraft or non-aircraft guidance system.
Figure A-1. Generic risk tree
23/11/06 ATT A-2
Attachment A Annex 10 — Aeronautical Communications
Px = Probability that the pilot is unable to detect and intervene successfully following a primary guidance integrity
failure. This risk reduction factor is only relevant in those cases where an integrity failure of the guidance
system may be detected by the pilot, e.g. at decision height in a Category I ILS approach.
Pp = Probability of primary guidance COS failure.
Pd = Probability of aircraft loss during a discontinued approach/missed approach procedure.
Pi = Probability of primary guidance integrity failure.
PiN = Probability of integrity failure in Nav element N.
PpN = Probability of COS failure in Nav element N.
Ps = Probability of aircraft loss during a discontinued approach/missed approach with secondary guidance.
Ps1 = Probability of secondary guidance COS failure.
Ps2 = Probability of secondary guidance integrity failure.
Pu = Probability that the pilot is unable to intervene successfully following primary guidance COS failure with no
secondary guidance available.
Where:
Pa = Pb + Pc
Pb = Pi × Px
Pi = Pi1 + Pi2 + ... PiN
Pc = Pp × Pd
Pp = Pp1 + Pp2 + ... PpN
Pd = Ps × Pu
Ps = Ps1 + Ps2
2.2 The acceptable probability of the top event, Pa, can be determined by partitioning the global risk factor for the
approach and landing operation to the various classes of accident. Using this method an acceptable value for Pa of 3 × 10–9
has been determined. This is consistent with the smallest probability that can be assigned to each ground navigation element,
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