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

Part III — Section 1, Chapter 4 III-1-4-3
α = VOR system use accuracy (degrees)
DTT = DME system use accuracy
θ = arctan (D2/D1) (degrees) (if D1 = 0, θ = 90°)
VT = D1–Dcos(θ + α)
DT = DTT cos θ
AVT = D2–Dsin(θ – α)
ADT = DTT sin θ
Note.— ATT does not contain an FTT component.
4.5.1.2 Results of calculations of XTT and ATT for FAF, SDF, TP, IF and IAF are presented in Tables III-1-4-1,
and III-1-4-2. Where ground facility performance is demonstrated to be consistently better than in 4.2.3, “System use
accuracies” the total system tolerance may be reduced by using the formulae.
4.5.2 Area semi-width
Area semi-width (1/2 A/W) at a waypoint is the greater of the following:
(1.5 × XTT + BV) or the appropriate fixed value as shown in Table III-1-4-5
where: 1.5 XTT corresponds to 3 sigma
BV is the buffer value.
Table III-1-4-5 shows the criteria for determining area semi-width at the various fixes and at the beginning of each
segment. See also Tables III-1-4-3, and III-1-4-4 as well as 4.5.1, “XTT and ATT”
4.5.3 ATT and XTT track dependency
ATT and XTT are track dependent. Thus when a turn is specified at a fix, the ATT and XTT are different before and
after the turn due to the individual fix geometry.
23/11/06