"Roy Smith" wrote in message
...

OK guys, now that we've established that, let me repeat my original
question, which seems to have gotten lost in the shuffle...

Why can't a TACAN-only aircraft hold directly over a TACAN?


The short answer is because it's not permitted, see the note following
subparagraph d. below. The long answer? The technical reason behind the
restriction? Beats the hell outta me.


FAA Order 7110.65P Air Traffic Control

Chapter 4. IFR

Section 1. NAVAID Use Limitations

4-1-5. FIX USE

Request aircraft position reports only over fixes shown on charts used for
the altitude being flown, except as follows:

NOTE-
Waypoints filed in random RNAV routes automatically become compulsory
reporting points for the flight unless otherwise advised by ATC.

a. Unless the pilot requests otherwise, use only those fixes shown on high
altitude en route charts, high altitude instrument approach procedures
charts, and DP charts when clearing military turbojet single-piloted
aircraft.

b. Except for military single-piloted turbojet aircraft, unpublished fixes
may be used if the name of the NAVAID and, if appropriate, the
radial/course/azimuth and frequency/channel are given to the pilot. An
unpublished fix is defined as one approved and planned for publication which
is not yet depicted on the charts or one which is used in accord with the
following:

REFERENCE-
FAAO 7130.3, Holding Pattern Criteria.

1. Unpublished fixes are formed by the en route radial and either a DME
distance from the same NAVAID or an intersecting radial from an off-route
VOR/VORTAC/TACAN. DME shall be used in lieu of off-route radials, whenever
possible.

2. Except where known signal coverage restrictions exist, an unpublished
fix may be used for ATC purposes if its location does not exceed NAVAID
altitude and distance limitation, and when off-route radials are used, the
angle of divergence meets the criteria prescribed below.

NOTE-
Unpublished fixes should not negate the normal use of published
intersections. Frequent routine use of an unpublished fix would LEFT
establishing a fix.

REFERENCE-
FAAO 7110.65, Altitude and Distance Limitations, Para 4-1-1.

3. Do not hold aircraft at unpublished fixes below the lowest assignable
altitude dictated by terrain clearance for the appropriate holding pattern
airspace area (template) regardless of the MEA for the route being flown.

4. When the unpublished fix is located on an off-route radial and the
radial providing course guidance, it shall be used consistent with the
following divergence angles:

(a) When holding operations are involved with respect to subparas (b)
and (c) below, the angle of divergence shall be at least 45 degrees.

(b) When both NAVAIDs involved are located within 30 NM of the
unpublished fix, the minimum divergence angle is 30 degrees.

(c) When the unpublished fix is located over 30 NM from the NAVAID
generating the off-course radial, the minimum divergence angle shall
increase 1 degree per NM up to 45 NM; e.g., 45 NM would require 45 degrees.

(d) When the unpublished fix is located beyond 45 NM from the NAVAID
generating the off-course radial, the minimum divergence angle shall
increase 1/2 degree per NM; e.g., 130 NM would require 88 degrees.

c. Fixes contained in the route description of MTRs are considered filed
fixes.

d. TACAN-only aircraft (type suffix M, N, or P) possess TACAN with DME,
but no VOR or LF navigation system capability. Assign fixes based on TACAN
or VORTAC facilities only.

NOTE-
TACAN-only aircraft can never be held overhead the NAVAID, be it TACAN or
VORTAC.

e. DME fixes shall not be established within the no-course signal zone of
the NAVAID from which inbound holding course information would be derived.

REFERENCE-
FAAO 7110.65, NAVAID Fixes, Para 2-5-3.
FAAO 7110.65, Methods, Para 5-6-2.