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

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

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

formula.

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