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

II-1-1-8 Procedures — Aircraft Operations — Volume II
1.4.7.3.3 The obstacles in Table II-1-1-3 may only be exempted if the following two criteria are met:
a) the localizer course sector has the standard width of 210 m (see 1.1.3, “Standard conditions”); and
b) the Category I decision height is not less than 60 m (200 ft) or the Category II decision height is not less than
30 m (100 ft).
1.4.7.3.4 An object that penetrates any of the basic ILS surfaces and becomes the controlling obstacle, but must
be maintained because of its function with regards to air navigation requirements, may be ignored under certain
circumstances in calculating the OCA/H, with the following provision. It must be established by the appropriate
authority that the portion which penetrates the surface is of minimum mass and frangibly mounted and would not
adversely affect the safety of aircraft operations.
23/11/06
1.4.8 Obstacle clearance of the precision segment using obstacle assessment surface (OAS) criteria
1.4.8.1 General
1.4.8.1.1 This section describes the OAS surfaces, the constants which are used to define these surfaces, and the
conditions under which adjustments may or must be made. The OAS dimensions are related to:
a) the ILS geometry (localizer-threshold distance, glide path angle, ILS RDH, localizer sector width);
b) the category of ILS operation; and
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 ILS operations at the
particular airfield.
1.4.8.1.2 Additional material is included to enable appropriate authorities to assess realistic benefits for claims of
improved performance and associated conditions. See 1.4.8.7, “Adjustment of OAS constants”.
1.4.8.1.3 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 1.4.8.9,
“Effect of obstacle density on OCA/H”).
1.4.8.2 Frame of reference
Positions of obstacles are related to a conventional x, y, z coordinate system with its origin at threshold. See
Figure II-1-1-10. 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-1-1-10, 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”).
1.4.8.3 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. The PANS-OPS OAS CD-ROM gives coefficients for glide path angles between 2.5 and 3.5 degrees
in 0.1 degree steps, and for any localizer-threshold distance between 2 000 m and 4 500 m. Extrapolation outside these