Figure 9 - P-8010087

INSTRUMENT NAVIGATION

MAINTAINING ARCS

In theory, it is a simple matter to maintain an arc. The aircraft will fly in an exact circle

around the station if you maintain a relative bearing of 090 or 270 to the station at all times. In

practice, this would require a constant angle of bank throughout the whole arc.

Another method for maintaining an arc is to fly a series of short legs, keeping the head of the

#2 needle on or near the wingtip position (i.e., 9 or 3 o'clock) while maintaining the

desired range. This is sometimes known as the "Chord" method, for the aircraft will be tracing a

series of chords to the arc as it progresses along the arc.

Always remember the relationship between the head of the needle and DME. If the head of

the needle is above the "wingline" (90 fiducial marker on BDHI/RMI) the DME will decrease.

If the head of the needle is below the wingline DME will increase. These are no wind rules.

Figure 9

The flight path of an aircraft employing the Chord Method might look like the path depicted

by the dotted line.

(Figure 9)

The steps to follow when usinq the chord method are:

1. Start with the head of the #2 needle on the wing tip and the aircraft at the desired range.

2. Maintain heading and allow the head of the needle to move 10 degrees behind the

wingtip position. (This will cause the ran ge to increas e slightl y.)

3. Turn toward the station to place the head of the needle 10 degrees ahead of the wing tip.

4. Maintain this heading until the head of the needle again falls behind the wing tip.

TACAN POINT-TO-POINT 4-7