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

Appendix B to Chapter 1
STATISTICAL CALCULATIONS FOR PRIMARY
AND SECONDARY AREAS AND THEIR ANGLES OF SPLAY
1. GENERAL
The obstacle clearance area is divided into a central primary area and two buffer areas on either side. The primary area
represents 95 per cent probability of containment (2 SD), as calculated on a root sum square basis from the system use
accuracy. The buffer/secondary area represents 99.7 per cent probability of containment (3 SD), calculated in the same
fashion.
2. NAVIGATION SYSTEM USE ACCURACY
2.1 The system accuracies used in the development of obstacle clearance criteria are based on minimum system
performance factors. The various accuracy values, when considered as statistically independent, are combined on a root
sum square (RSS) basis to produce limits corresponding to approximately 95 per cent probability of containment (2
SD) and limits corresponding to approximately 99.7 per cent probability of containment (3 SD).
2.2 The following system use accuracy values apply to VOR:
a) ± 3.5° ground system tolerance;
b) ± 2.7° receiver tolerance;
c) ± 3.5° flight technical tolerance; and
d) ± 1.0° monitoring tolerance.
2.3 The following system use accuracy values apply to NDB:
a) ± 3° ground equipment;
b) ± 5.4° airborne equipment; and
c) ± 5° flight technical tolerance.
2.4 Fix or facility tolerances
2.4.1 VOR intersecting tolerance. The VOR angular intersecting tolerance, calculated without the flight technical
tolerance, results in 7.9 per cent (4.5°).
2.4.2 NDB intersecting tolerance. The NDB angular intersecting tolerance, calculated without the flight technical
tolerance, results in 10.9 per cent (6.2°).
II-3-1-App B-1 23/11/06