Figure 21 - P-8010052

INSTRUMENT NAVIGATION

igure 21

TATION PASSAGE

One very important facet of airways navigation is the determination of when you have passed

directly over a navaid. This event is called station passage. Station passage is a crucial point in

airways flights because in most cases, a course and heading change will be required in addition

to the associated procedures required in VT-10.

Station passage occurs the moment the aircraft passes directly over the navaid. Recalling the

characteristics of VORs and TACANs, when station passage occurs you will be in the cone of

confusion.

As you enter the cone of confusion, the #2 needle will deflect and become unreliable. When

you reach directly overhead the navaid, station passage will be indicated by minimum DME if

using TACAN or by needle deflection with VOR. Minimum DME, when the DME stops

decreasing and begins increasing, is the best indication of station passage. (Figure 22)

Because 1 nm approximates 6,000' in altitude, you can expect station passage/minimum DME to

occur at the DME which is equal to your AGL altitude divided by 6,000'. Station passage at

FL 310, for example, would occur at 5.2 DME (assuming the station is at or near sea level).

Thus, any heading change required upon passing overhead a station is usually initiated at

minimum DME. Due to navigational error, there are times your aircraft will not pass directly

overhead the navaid. In these cases, when your DME stops decreasing and starts to increase

again, you have come as close to the navaid you ever will and you have experienced station

passage. It is now your responsibility to turn the aircraft to the outbound heading.

3-14 RADIAL TRACKING AND COURSE CONTROL