Continue on the desired radial to the DME that identifies the lead point. When
turning onto an arc from a radial (using a StandardRate Turn), the amount of lead
(in nautical miles) should be 0.5% of the aircraft's groundspeed.
RADIUS OF TURN
When an aircraft commences a turn, it turns about an imaginary
point. The distance from the aircraft to this point is referred to as
radius of turn. Radius of turn is determined by turn rate and
airspeed. In Figure 6-36 (inset), an aircraft has been flying a 90º
heading and turns to 360º (or a 90º turn onto an arc). Position "a"
represents the point where the turn was started and position "b" is
the point where 360º is reached. As you can see, some eastward
travel occurred between the start of the turn and rollout. For a 90º
turn, this distance equals the radius of turn. For a StandardRate
Turn at 150 knots, this distance is .8 NM and at 120 knots it is .6
.8 NM and .6 NM are actually approximations. The actual
value is .5% of your GROUNDSPEED. In most applications, the
amount of error introduced by using these approximations is
negligible. However, excessive headwinds or tailwinds may require
adjusting the leadpoint to turn onto the arc.
The values of .8 and .6 NM will be less accurate if you are
sloppy rolling into and out of turns or if you fail to maintain a
For example, if you are inbound to the station at 150 knots and are instructed to arc at 10
DME, you would begin the turn onto the arc at 10.8 DME. If you are outbound from the
station and desire to arc at 10 DME, your lead point would be 9.2 DME.
Determine the direction of turn at the leadpoint.
At the leadpoint, start a turn in the appropriate direction. Continue the turn until the
head of the TACAN needle is on the 90º benchmark.
Check your DME.
If DME is less than desired, you are inside the arc. To correct back to the arc,
simply maintain the heading. If you have flown excessively inside the arc, (.5
DME or more inside the arc), make a turn to place the head of the TACAN
6-58 RADIO INSTRUMENT FLIGHT PROCEDURES