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which is 1 × 10–9 (normally divided equally between integrity and COS failures).
2.3 The risk analysis above assumes no equipment design errors.
3. Example of the use of the risk tree — MLS Category III basic operations (Figure A-2).
3.1 In this case there are only two navigation elements involved (e.g. azimuth and elevation). It is assumed that no
secondary guidance is available following a COS failure of the primary guidance, the normal procedure being to maintain
heading and climb.
ATT A-3 23/11/06
Annex 10 — Aeronautical Communications Volume I
Aircraft loss due to
non-aircraft guidance
system failure
Pa = 3 x 10
-9
Calculated
Pa = 2 x 10
-9
1 x 10-9 1 x 10-9
2.5 x 10-4
4 x 10-6
1 x 10-9
0.5 x 10-9
2 x 10-6
0.5 x 10-9
2 x 10-6 2.5 x 10-4
1
1
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 failure
Pd
Secondary
guidance failure
(see Note 1)
Ps
Primary guidance
continuity of
service failure
Pp
Integrity
failure,
azimuth
Pi1
COS
failure,
azimuth
Pp1
COS
failure,
elevation
Pp2
COS
failure
Ps1
Integrity
failure
Ps2
Integrity
failure,
elevation
Pi2
Pilot risk
reduction
Pu
Note 1: Secondary guidance not applicable
Azimuth:
4 000 hours MTBO
30 seconds exposure time
Elevation:
2 000 hours MTBO
15 seconds exposure
Figure A-2. MLS Category III landing risk tree
23/11/06 ATT A-4
Attachment A Annex 10 — Aeronautical Communications
Pi1 = Pi2 = 0.5 × 10–9
Pp1 = Pp2 = 2 × 10–6
Note.— These figures are from Attachment G, Table G-15, Level 4 and assume exposure times of 30 and 15 seconds, and
MTBOs of 4 000 and 2 000 hours for the azimuth and elevation elements respectively.
Ps = 1.0
Note.— Since there is no guided discontinued approach/missed approach procedure using secondary guidance, the
probability of an accident during the procedure is taken to be 1.
Px = 1.0
Note.— It is assumed in this example that in a Category III operation the pilot is unable to intervene in the event of an
integrity failure in the ground system. The risk reduction factor is therefore equal to 1.
Pu = 2.5 x 10-4
Note.— The pilot risk reduction factor is estimated at 1 in 4 000 based on a study of accidents to aircraft conducting
approaches to land using ground guidance systems. This is the risk reduction factor assumed due to pilot intervention
following a continuity of service failure.
Therefore:
Pi = 1 × 10–9
Pp = 4 × 10–6
Pd = 2.5 × 10–4
Pc = 4 × 10–6 × 2.5 × 10–4 = 1 × 10–9
Pb = 1 × 10–9 × 1
and:
calculated Pa = 2 × 10–9.
3.2 There is therefore a margin of 1 × 10–9 on the generic requirement.
4. Application of the risk tree to an MLS/RNAV approach in an obstacle rich environment (Figure A-3).
4.1 In this case there are three navigation elements (i.e. azimuth, elevation and DME/P) and all are assumed to meet the
integrity and COS requirements for Level 4 azimuth equipment; i.e integrity = 1 – 0.5 × 10–9 and MTBO = 4 000 hours.
Pi1 = Pi2 = Pi3 = 0.5 × 10–9
Px = 1.0
Note.— It is assumed that the pilot is unable to intervene in the event of an integrity failure in the ground system.
ATT A-5 23/11/06
Annex 10 — Aeronautical Communications Volume I
Aircraft loss due to
non-aircraft guidance
system failure
Pa = 3x10
-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
Ps
Primary guidance
continuity of
service failure
Pp
Integrity
failure,
azimuth
Pi1
COS
Pp1
failure,
azimuth
COS
Pp2
failure,
elevation
COS
Pp3
failure,
DME/P
COS
failure
Ps1
Integrity
failure
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