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

I-2-2-2 Procedures — Aircraft Operations — Volume II
23/11/06
2.3.1 Fix tolerance areas
The fix tolerance areas are formed by the boundaries obtained from system use accuracies of the homing and
intersecting radials (or arcs as appropriate) with respect to the nominal fix position. As the system use accuracy is
expressed in angles, the size of the fix tolerance area is dependent on the distance of the fix to navigation aids.
2.3.2 Fix tolerance
The fix tolerance determines the operational acceptability of a fix. Fix tolerance is a distance measured along the
nominal track and relative to the nominal fix position. It is defined by the intersections of the nominal track with the
earliest and latest limits of the fix tolerance area, measured along the nominal track. The tolerance is expressed as a
plus or minus value around the nominal fix. See Figures I-2-2-5 and I-2-2-6. Fix tolerance and system use accuracies
are based on a 95 per cent probability of containment (2 SD).
2.3.3 System use accuracy for VOR, NDB and LLZ
System use accuracy is based on a root sum square calculation using the following tolerances:
a) ground system tolerance;
b) airborne receiving system tolerance; and
c) flight technical tolerance.
Difference between the overall system use accuracy of the intersecting facility and the along track facility is accounted
for by the fact that flight technical tolerance is not applied to the former. See Table I-2-2-1 for system use accuracies
and Table I-2-2-2 for the tolerances on which these values are based.
2.4 FIX TOLERANCE FOR OTHER TYPES OF NAVIGATION INSTRUMENTS
Radar fix accuracies need to consider:
2.4.1 Terminal area radar
a) mapping accuracies (normally 150 m (492 ft) or 3 per cent of the distance to the antenna);
b) azimuth resolutions of the radar (reduced to some extent to account for the controller interpretation of target
centre);
c) flight technical tolerance (which recognizes communication lag as well as speed of the aircraft); and
d) controller technical tolerance (which recognizes sweep speed of the antenna and the speed of the aircraft).
The total fix tolerance is the result of a combination, on a root sum square (RSS) basis, as in Table I-2-2-3.