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
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.