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

Part III — Section 3, Chapter 6 III-3-6-3
6.1.5 Methods of calculating OCA/H
6.1.5.1 General. Three methods of calculating OCA/H are presented, which in turn involve progressive increases
in the degree of sophistication in the treatment of obstacles. Standard conditions (as specified in 6.1.3) are assumed to
exist unless adjustments for non-standard conditions have been made.
6.1.5.2 First method. The first method involves a set of surfaces derived from the Annex 14 precision approach
obstacle limitation surfaces and a missed approach surface described in 6.4.7.2, “Definition of basic ILS surfaces” and
from this point forward termed “Basic ILS surfaces”. Where the standard conditions exist as specified in 6.1.3 and
where the basic ILS surfaces are free of penetrations (see 6.4.7.1, “General”) the OCA/H for Cat I is defined by aircraft
category margins. If the basic ILS surfaces are penetrated, then the OCA/H is calculated as described in 6.4.7.3,
“Determination of OCA/H with basic ILS surfaces”.
6.1.5.3 Second method. The second method involves a set of obstacle assessment surfaces (OAS) above the basic
ILS surfaces (see 6.4.8.3, “Definition of OAS”). If the OAS are not penetrated, and provided the obstacle density below
the OAS is operationally acceptable (see 6.4.8.9, “Effect of obstacle density on OCA/H”), the OCA/H for Cat I is still
defined by the aircraft category margins. However, if the OAS are penetrated, then the aircraft category related margin
is added to the height of the highest approach obstacle, or to the adjusted height of the largest missed approach
penetration, whichever is greater. This value becomes the OCA/H.
6.1.5.4 Third method. The third method, using a collision risk model (CRM), is employed either as an alternative
to the use of the OAS criteria (second method) or when the obstacle density below the OAS is considered to be
excessive. The CRM accepts all objects as an input and assesses, for any specific OCA/H value, both the risk due to
individual obstacles and the accumulated risk due to all the obstacles. It is intended to assist operational judgement in
the choice of an OCA/H value.
Note 1.— While specific GBAS distributions for the existing CRM are being developed, use should be made of the
current ILS CRM.
Note 2.— The CRM does not take into account the characteristics of helicopters. The CRM can be used but the
method should be conservative.
6.1.6 References
The following relate to and amplify the material contained in this chapter:
a) background information relating to the derivation of the OAS material (Attachment to Part II, paragraph 1) and
to airborne and ground equipment performance assumed in the derivation of the OAS (paragraph 2);
b) turning missed approach after precision approach (Part II, Section 1, Chapter 1, Appendix A);
c) independent parallel approaches to closely spaced parallel runways (Part II, Section 1, Chapter 1, Appendix D);
d) determining ILS glide path descents/MLS elevation heights and distances (Part II, Section 1, Chapter 1,
Appendix C); and
e) PANS-OPS OAS CD-ROM.
Examples of OCA/H calculations can be found in the Instrument Flight Procedures Construction Manual (Doc 9368).
23/11/06