needle 15º below the 90º benchmark to return the aircraft back to the arc.
Approaching the arc, lead the turn onto the arc and resume Arcing Procedures.
If DME is greater than desired, you are outside the arc. To correct, turn to
place the head of the TACAN needle 15º above the 90º benchmark.
Once established on the arc in a nowind situation, remain on the arc by altering
aircraft heading and/or bank angle to maintain the head of the TACAN needle on or
near the 90º benchmark. Strive to stay within .2 DME.
When altering aircraft heading, consider the position of the head of
the TACAN needle relative to the 90º benchmark. In a nowind
Head of the needle on the 90º benchmark
Head of the needle below the 90º benchmark
Head of the needle above the 90º benchmark
When arcing, crosswinds may cause the aircraft to drift off of the arc.
If drifting off the arc, Turn to place the head of the needle above or below
the 90º benchmark, as required. A correction of 10 to 30 degrees will normally
suffice. Once reestablished on the arc, continue Arcing Procedures, but
maintain a drift correction, just as you do when tracking a radial. If the drift
correction is insufficient, repeat the procedure and apply a more appropriate
The rate of deviation from, or correction to, an arc will vary with
the size of the arc, distance from the NAVAID, TAS, wind
direction, and velocity.
EXAMPLE: Figure 635 illustrates a typical arcing situation. The aircraft is inbound
to the TACAN station at 150 knots groundspeed, flying a course of 90º (inbound on the
270º radial). You are instructed to arc north at 10 NM.
At Point "a" the aircraft is at the leadpoint. Start the turn to intercept the arc
At Point "b" you have just intercepted the arc. The head of the TACAN needle
is on the 90º benchmark and the DME is showing 10.
RADIO INSTRUMENT FLIGHT PROCEDURES 6-59