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

Part II — Section 3, Chapter 1 II-3-1-3
width of the buffer area is progressively reduced to zero, ending in an area without track guidance where the full MOC
is applied (see Figure II-3-1-8).
1.4 CONSTRUCTION OF AREAS FOR VOR AND NDB ROUTES
1.4.1 General
This section contains methods for calculating the areas used for en-route obstacle clearance purposes. The statistical
derivation of these calculations, which are based on a root sum square method of the navigation system use accuracy,
appears in Appendix B.
1.4.2 Straight areas
1.4.2.1 Area descriptions. The obstacle clearance areas consist of a primary area and two lateral buffer areas on
each side.
1.4.2.2 Width abeam the facility. Abeam the facility, the total area has a constant width of 18.5 km (10.0 NM),
which is comprised of the primary area and a buffer area. The primary area maintains a constant width of 9.3 km
(5.0 NM) on either side of the nominal track. The buffer area also maintains a constant width of 9.3 km (5.0 NM) on
either side of the primary area.
1.4.2.3 Angular limits. When the distance from the facility is greater than:
a) 92.3 km (49.8 NM) for VOR; and
b) 60 km (32 NM) for NDB,
the areas diverge, following the angular tolerance lines of their respective facilities (See Table II-3-1-1).
1.4.2.4 Width after the point of divergence. After the limiting distance stated in 1.4.2.3, “Angular limits”, the
width of the primary area is increased by the angle of splay. The buffer area is determined by the angle of splay plus an
additional fixed width on the outside of the buffer area, parallel to its edge (see Figures II-3-1-2 and II-3-1-3). This
width is:
a) 3.7 km (2.0 NM) for VOR; and
b) 4.6 km (2.5 NM) for NDB.
1.4.2.5 Longitudinal limits. The longitudinal limits of the area associated with a straight segment are determined
as follows:
a) the earliest limit of the area is a half circle, centred on the first fix and tangent to the lateral limits of the total
area; and
b) the latest limit of the area is a half circle, centred on the second fix and tangent to the lateral limits of the total
area.
1.4.2.6 Combination of VOR and NDB criteria. In case of a straight segment based on a VOR at one end and an
NDB at the other end, the area is designed as shown in Figure II-3-1-4.
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