Figure 10 - P-8010031

INSTRUMENT NAVIGATION

GONE OF CONFUSION

The line-of-sight transmission pattern from a VOR or TACAN station creates an area

directly overhead the facility where azimuth signal reception is very weak or random. This

area is called the "cone of confusion". Although the azimuth information displayed is

considered to be unreliable, the DME is not affected and will continue to display slant range to

the station. The cone of confusion for a TACAN is much larger than that of the VOR, and at

40,000 feet, the TACAN's cone of confusion encompasses a 15 nm diameter about the TACAN

station.

Figure 10

When you analyze the diagram, it should be obvious that the diameter of the cone is a

function of altitude. Understanding this concept will help eliminate potential navigational

problems in close proximity of the NAVAID.

At 30,000 feet, we could expect less time in the cone; however, at 50,000 ft. we could

expect a long time period of unreliable TACAN or VOR azimuth information. Within the cone

of confusion, the behavior of the TACAN or VOR azimuth will be completely random. It will

oscillate from side to side and will not present accurate information. It is important to

recognize when you have entered the cone, so as not to follow the unreliable indications of the

needle.

The DME, however, will continue to behave in a predictable manner and the numbers

displayed will continue to decrease until the aircraft is directly overhead the TACAN station.

At this point you have reached "minimum DME". As noted earlier, this number will be equal

to your AGL altitude in nautical miles. Upon reaching this point your DME will not continue

to decrease. Rather, after minimum DME is observed, it will again increase in the same

predictable manner that it decreased as you were headed inbound to the station.

NAVAIDS POSITIONING 2-9