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

Part II — Section 1, Chapter 1 II-1-1-3
1.1.5 Methods of calculating OCA/H
1.1.5.1 General. Three methods of calculating OCA/H are presented, which involve progressive increases in the
degree of sophistication in the treatment of obstacles. Standard conditions (as specified in 1.1.3) are assumed to exist
unless adjustments for non-standard conditions have been made.
1.1.5.2 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 1.4.7.2, “Definition of basic ILS surfaces”.
From this point forward, these are termed “basic ILS surfaces”. Where the standard conditions exist as specified in
1.1.3 and where the basic ILS surfaces are free of penetrations (see 1.4.7.1), the OCA/H for Cat I and Cat II is defined
by aircraft category margins, and there are no restrictions on Cat III operations. If the basic ILS surfaces are penetrated,
then the OCA/H is calculated as described in 1.4.7.3, “Determination of OCA/H with ILS”.
1.1.5.3 Second method. The second method involves a set of obstacle assessment surfaces (OAS) above the basic
ILS surfaces (see 1.4.8.4, “Definition of obstacle assessment surfaces (OAS). If the OAS are not penetrated —and
provided the obstacle density below the OAS is operationally acceptable (see 1.4.8.9, “Effect of obstacle density on
OCA/H”) — the OCA/H for Cat I and Cat II is still defined by the aircraft category margins, and Cat III operations
remain unrestricted. However, if the OAS are penetrated, then an 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.
1.1.5.4 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 judgment in the
choice of an OCA/H value.
Note.— The CRM does not take into account the characteristics of helicopters. The CRM can be used but the
method should be conservative.
1.1.6 References
The following appendices 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 (Attachment to Part II,
paragraph 2);
b) turning missed approach after precision approach (Appendix A);
c) independent parallel approaches to closely spaces parallel runways (Appendix D);
d) determining ILS glide path descents/MLS elevation heights and distances (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