Instrument Flight
Flight Procedures
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 kts
ground speed, your lead point will be 10 radials (60
divided by 15 equals 4, 1 percent of 250 is 2.5, and
C
2.5 multiplied by 4 equals 10).
When making your intercept turn, you can also use
the movement of the bearing pointer and course line
as a guide to determine when to initiate the turn. Use
the HSI display scale and relative position of the
Planimetric course line to initiate and execute the
radial intercept. When you are flying close to the
station, you must initiate the intercept turn before the
CDI course line begins to move. Therefore, the turn
must be initiated at the calculated lead point. Figure
24 depicts the HSI display indications during the
intercept procedure.
To intercept a radial from an arc, first set the desired
course on the HSI display. Next, determine your
B
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 course line so that it is centered
when the turn is complete. Do not exceed 30 degrees
AOB.
For radial intercepts from arcs less than 10 DME, a
correction factor must be applied to the arc-to-radial
formula to account for the turn to the radial being
more or less than 90 degrees. In the case of a turn
inbound, the turn will actually be more than
90 degrees, and the correction factor will be added to
the standard 90-degree arc-to-radial formula. In the
case of an outbound turn, the turn will actually be less
than 90 degrees, and the correction factor will be
subtracted from the standard 90-degree arc-to-radial
formula.
A
Figure 24: ARC-TO-RADIAL INTERCEPT
PROCEDURE—2
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(4-01) Original
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