Over the Station Intercepts

CHAPTER THREE

RADIO INSTRUMENTS

A clearance for direct routing, as defined, would be "straight line flight between two navigational

aids, fixes, points, or any combination thereof". This is in fact, a matter of federal law: FAR

91.181 states: Unless otherwise authorized by ATC, no person may operate an aircraft within

controlled airspace under IFR except as follows: (a) On a Federal airway, along the centerline

of that airway. (b) On any other route, along the direct course between the navigational aids or

fixes defining that route. For example, an aircraft cleared "from NPA direct to SJI direct NMM"

would be expected to fly the direct course from SJI to NMM (348). The aircraft is NOT cleared

to turn at SJI, and from whatever random point in space, which might result from the turn, tune

in NMM TACAN, and proceed to NMM directly from THAT point. The correct procedure at

the TACAN would be an "Over the Station Intercept", followed by compliance with "Change

Over Point (COP)" procedures at the appropriate distance between the NAVAIDs. Thus, the

navigation equipment would always be tuned to the NAVAID, which would assure maximum

accuracy.

OVER THE STATION INTERCEPTS

Actual TACAN "station passage" occurs when the aircraft is directly over the station, but due to

the cone of confusion over a TACAN (which can temporarily render the TACAN needle

useless), this point is often difficult to accurately identify. Station passage is considered to occur

when the DME stops decreasing (-- NATOPS INSTRUMENT FLIGHT MANUAL.) If the aircraft

passes directly over the station, this will occur at minimum DME:

Minimum DME = altitude above the TACAN / 6000.

Over the Station Intercepts:

The turning radius of an aircraft (relative to a surface NAVAID) depends on a number of factors,

all of which are continually changing, such as wind, groundspeed, G-load, and angle of bank.

Calculations to determine a lead point which would result in precisely rolling out on a new

course after crossing a NAVAID would require consideration of variations in all those factors,

and the angle between the old course and the new course as well. When turning over a

NAVAID at jet cruising altitudes, such hair-splitting computations rarely result in a more

accurate outbound course interception than simply turning to a heading the same as the new

course when the DME stops decreasing which should be at, or very near minimum DME). The

following procedure is recommended:

1. When the DME stops decreasing, turn to a heading, which is the same as the new course (if

the TACAN needle on the BDHI is stabilized during the turn, continuing the turn to a heading

which will intercept the outbound course would be preferable).

2. As soon as the TACAN needle on the BDHI indicates the aircraft location outbound, turn

to intercept the desired course outbound.

3-22

RADIO INSTRUMENTS