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

III-3-6-8 Procedures — Aircraft Operations — Volume II
6.4.8.1.2 Additional material is included to enable appropriate authorities to assess realistic benefits for claims of
improved performance and associated conditions (see 6.4.8.7, “Adjustment of OAS constants”).
6.4.8.1.3 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 6.4.8.9,
“Effect of obstacle density on OCA/H”).
6.4.8.2 Frame of reference
Positions of obstacles are related to a conventional x, y, z coordinate system with its origin at threshold. See Figure III-
3-6-9. The x-axis is parallel to the precision segment track, positive x coordinates measured before landing threshold
and negative x coordinates measured after landing threshold. The y-axis is at right angles to the x-axis. Although shown
conventionally in Figure III-3-6-9, 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).
6.4.8.3 Definition of obstacle assessment surfaces (OAS)
6.4.8.3.1 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 III-3-6-7 and III-3-6-8). The geometry of the sloping surfaces is precisely defined by four simple 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 III-3-6-6).
6.4.8.3.2 For each surface a set of constants A, B and C are obtained from PANS-OPS OAS CD-ROM for the
operational range of GARP- threshold distances and glide path angles. These constants may be modified as specified in
6.4.8.7, “Adjustment of OAS constants”.
6.4.8.3.3 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.
6.4.8.3.4 Where the Annex 14 approach and transitional obstacle limitation surfaces for code numbers 3 and 4
precision approach runways penetrate inside 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). 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 III-3-6-5).
6.4.8.4 OAS constants – specification
For Category I operations the constants A, B and C for each sloping surface are obtained from the PANS-OPS OAS
CD-ROM. The PANS-OPS OAS CD-ROM gives coefficients for glidepath angles between 2.5 and 3.5 degrees in 0.1
degree steps, and for any GARP-LTP distance between 2 000 m and 4 500 m. Extrapolation outside these limits is not
permitted. if a GARP-LTP distance outside this range is entered, the PANS-OPS CD ROM gives the coefficients for 2
000 m or 4 500 m as appropriate, which must be used. For an example of the PANS-OPS OAS CD-ROM results see
Figure III-3-6-11.
23/11/06