Introduction - P-8010082

INSTRUMENT NAVIGATION

INTRODUCTION

TACAN arcs are used most frequently during SID's (Standard Instrument Departures) and

instrument approaches. Arcing is simply flying around a navaid while maintaining a constant

distance from it.

DETERMINING LEAD TO ENTER AN ARC

To establish yourself on an arc, either tracking inbound or outbound on a radial, a lead must

be determined. When using 30 degrees of bank, an approximate lead point for the arc may be

determined from the aircraft ground speed.

The lead point should be % of your ground speed. The resultant number will be the

distance by which you will lead your turns into an arc. For example:

GS 180 Lead Turn by 0.9 nm

GS 250 Lead Turn by 1.2 1.3 nm

GS 380 Lead Turn by 1.9 nm

These lead points are necessary because aircraft turn radius is a function of speed. If an

aircraft traveling at 180 kts ground speed inbound on a radial should turn off that radial

(using a standard rate turn of 3 per sec) at 10 nm, when it rolls out wings level, it will not be at

10 nm but approximately 9.1 nm. That's why at the airspeed given above, you should lead the

turn by .9 nm (i.e. turn at 10.9 nm.) so that you will end up on the 10 nm arc as desired.

When in a climb or descent, true airspeed and ground speed will be changing due to winds at

different altitudes, differing pressure altitudes, and temperatures. Lead points for arcing should

be based on your aircraft speed over the ground. Practically speaking, however, there is very

little time available to you during the departure and approach phases of flight to figure out your

ground speed. The next best speed to use in calculating lead points would be true airspeed which

would equal ground speed under no-wind conditions. Because of the time restriction again on

departures and approaches and the fact that TAS is changing in a climb or descent it would be

difficult to estimate an exact TAS to use in determining arc lead points. While actually flying

departures and approaches use your Indicated Airspeed to determine your lead point.

If you have a headwind or tailwind prior to transitioning to an arc, you must consider altering

your lead point or varying your angle of bank. Altering your lead point is the preferred method

entering the clouds suddenly at a severe angle of bank may lead to disorientation.

Example: You are proceeding outbound on the 090 radial and desire to establish yourself on

the 15 nm arc (Indicated Airspeed = 180).

You correctly determine that a lead of 0.9 nm is necessary, and that you would turn at 14.1

DME. (Figure 1)

4-2 TACAN ARCING