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

Part II — Section 1, Chapter 3 II-1-3-9
c) other factors, including aircraft geometry, missed approach climb gradient.
Thus, a table of OCA/H values for each aircraft category may be calculated for Cat I and II MLS operations at the
particular airfield.
3.4.8.1.2 Additional material is included to enable appropriate authorities to assess realistic benefits for claims of
improved performance and associated conditions. See 3.4.8.7, “Adjustment of OAS constants”.
3.4.8.1.3 Note that the OAS are not intended to replace Annex 14 surfaces as planning surfaces for unrestricted
obstacle growth. The obstacle density between the basic ILS surfaces and the OAS must be accounted for (see 3.4.8.9,
“Effect of obstacle density on OCA/H”).
3.4.8.2 Frame of reference
Positions of obstacles are related to a conventional x, y, z coordinate system with its origin at threshold. See
Figure II-1-3-11. The x axis is parallel to the precision segment track: positive x is distance before threshold and
negative x is distance after threshold. The y axis is at right angles to the x axis. Although shown conventionally in
Figure II-1-3-11, in all calculations associated with OAS geometry, the y coordinate is always counted as positive. The
z axis is vertical, heights above threshold being positive. All dimensions connected with the OAS are specified in
metres only. The dimensions should include any adjustments necessary to cater for tolerances in survey data (see Part I,
Section 2, Chapter 1, 1.8, “Charting accuracy”).
3.4.8.3 Definition of obstacle assessment surfaces (OAS)
3.4.8.3.1 The OAS consist of six sloping plane surfaces (denoted by letters W, X, Y and Z) arranged
symmetrically about the precision segment track, together with the horizontal plane which contains the threshold (see
Figures II-1-3-9 and II-1-3-10). The geometry of the sloping surfaces is defined by four linear equations of the form
z = Ax + By + C. In these equations x and y are position coordinates and z is the height of the surface at that position
(see Figure II-1-3-8).
3.4.8.3.2 For each surface a set of constants (A, B and C) are obtained from the PANS-OPS OAS CD-ROM for
operational range of localizer threshold distances and glide path angles. Separate sets of constants are provided for
Category I and II. These constants may be modified by the programme as specified (see 3.4.8.7, “Adjustment of OAS
constants”
3.4.8.3.3 The Category I OAS are limited by the length of the precision segment and, except for the W and X
surfaces, by a maximum height of 300 m. The Category II OAS are limited by a maximum height of 150 m.
3.4.8.3.4 Where the Annex 14 approach and transitional obstacle limitation surfaces for code number 3 and 4
precision approach runways penetrate the OAS, the Annex 14 surfaces become the OAS (i.e. the surfaces for code
numbers 3 and 4 are used for obstacle assessment on runways with code numbers 1 and 2).
3.4.8.3.5 The Annex 14 inner approach, inner transitional and balked landing obstacle limitation surfaces protect
Category III operations, provided the Category II OCA/H is at or below the top of those surfaces, which may be
extended up to 60 m if necessary (see Figure II-1-3-7).
3.4.8.4 OAS constants — specification
For Category I and II operations the constants A, B and C for each sloping surface are obtained from the PANS-OPS
OAS CD-ROM with the constraint that the Category II flight director constants shall be used for MLS Category II
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