Aircraft Operations. Volume II - Construction of Visual and Instrument ...

Part II — Attachment II-Att-3
defined as those located beyond 900 m after threshold. By that range all aircraft were considered to be climbing, and
the margin above obstacles accounted for the fact that an increase in OCA/H also increased the distance available to
climb prior to reaching a given obstacle.
1.10 The partitioning of approach/missed approach obstacles by range was the simplest method to produce the
desired operational penalty differential and was safe in all cases. However, the resulting OCA/H could be such that the
‘on glide path’ OCA/H point was so far before the obstacle that it should be more correctly treated as a missed
approach obstacle. Provision was therefore made for a more complex partitioning by defining approach/missed
approach obstacles relative to a plane surface originating 900 m after threshold, and sloping upwards into the approach
area parallel to the plane of the glide path.
1.11 The higher of the heights necessary for clearance of approach or missed approach obstacles was then taken
as the obstacle clearance altitude/height to be applied in calculating operating minima as specified in Annex 6.
1.12 The use of obstacle assessment surfaces in calculating OCA/H involved applying the same margin above all
obstacles without regard to the location of obstacles relative to the flight path. To account for this, and to provide a
means of assessing obstacle density, a ‘‘collision risk model’’ was developed. This was a computer programme
containing data describing the spread of aircraft about their intended path, both in the approach and instrument missed
approach. The programme used these distributions to evaluate the risk or collision probability associated with
individual obstacles. To allow for the fact that only a proportion of the approaches results in a missed approach, the
computed risk of each obstacle in the missed approach region was factored by a missed approach rate. Taking account
of the variability in missed approach rate experienced over different periods of time and at different locations, one per
cent was deemed to be representative of the general order of missed approach rates likely to be experienced and was
used in the CRM. Risks associated with individual obstacles were then accumulated to produce a total risk for the
complete set of obstacles of interest. This final value, representing a probability of collision per approach, could then be
compared with a predetermined target level of safety. In this way the effects of operational adjustments (i.e. reduction
in obstacle density, increase in OCA/H, change of GP angle) could be assessed on an objective basis.
2. AIRBORNE AND GROUND EQUIPMENT PERFORMANCE VALUES
ASSOCIATED WITH CATEGORIES I AND II OBSTACLE ASSESSMENT
SURFACES USED IN THE MATHEMATICAL MODEL
2.1 Airborne and ground equipment values
used in the mathematical model
Details of the equipment (ground and air) values associated with the Categories I and II obstacle assessment surfaces
are contained in Tables II-Att-1 and II-Att-2. This is background information only and cannot be used directly as a
means of assessing changes in equipment performance. It represents the actual performance of the systems observed. It
is included as a permanent record of the values used to match the model with the observed aircraft positions and to
provide a complete reference for any future revision. Guidance material relating to equipment performance
characteristics is contained in Annex 10, Volume I, Part I, Attachment C.
2.2 Beam holding
The approach surfaces were based on observed displacement data rather than on indicated deviations. However, when
the mathematical model was matched to predict the actual approach path envelopes it was found that a good fit could
be obtained by assuming that pilots attempt to limit indicated deviations at 75 μA on both localizer and glide path. For
the Cat I surfaces this was factored by the value 1.4.
23/11/06