Intercepting a Radial from an Arc - P-12150092

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Flight Procedures
Instrument Flight
Adjustments to the position of the head of the bearing pointer relative to the 90 degree benchmark will
have to be made to compensate for the position of the wind relative to the aircraft on the aircraft as it
moves around the arc.
Intercepting a Radial from an Arc
When intercepting a radial from an arc, you must determine which way you have to turn to intercept and
fly the radial in the correct direction (Figure 24). Since you will most often be performing an arc as part of
an approach or departure procedure, you can obtain this information from the appropriate approach plate
or SID. To turn from an arc to a radial, your main consideration is to determine the proper lead in radials.
Radials diverge as you get further from a station and are 1 nm apart at 60 DME. Take this divergence into
account when calculating your lead point for the turn. To calculate the lead point for intercepting a radial
from an arc, first you must calculate or estimate the ground speed. Then apply the following formula:
Divide the arc DME into 60 then multiply the quotient by 1 percent of the ground speed.
For example, if you are on a 15 DME arc at 250 knots ground speed, your lead point will be 10 radials
(60 divided by 15 equals 4, 1 percent of 250 is 2.5, and 2.5 multiplied by 4 equals 10).
When making your intercept turn, you can also use the movement of the bearing pointer and CDI as a
guide to determine when to initiate the turn. When you are flying close to the station, the CDI will move
too quickly to follow at 30 degrees AOB. Therefore, the turn must be initiated at the calculated lead point.
To intercept a radial from an
arc, first set the desired
course in the course select
window. Next, determine
your lead and then turn
using a 1/2 SRT when you
reach the lead point.
Finally, vary your AOB in
the turn with the movement
of the CDI so that it is
centered when the turn is
complete. Do not exceed
30 degrees AOB.
18 Radial Lead
The above formula assumes
300 KGS
a 90 degree turn. However,
1/2 SRT
inbound radial intercepts will
60 X 1% GS = Lead Radial
require a greater lead than
outbound because the
aircraft turns through more
than 90 degrees.
Page 77
(12-00) Original


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