Roy Smith wrote:

Sam Spade wrote:

Offsets are a feature in TSO 145/146 (WAAS capable) sets. The smart
money will use a slight offset to fly a Victor Airway to greatly reduce
the opposite direction mid-air potential. Minor offsets are already
approved on the North Atlantic.


The CNX-80 has offsets. I didn't realize they were required by the TSO.
The story I had heard was the CAP wanted offsets so they could fly box
search patterns, and Apollo put the feature in to close a big sale to them.

More than you ever wanted to know:

2.2.1.3.16 Parallel Offsets

The parallel offset is defined as a route parallel to, but offset from,
the original active route. The basis of the offset path is the original
flight plan leg(s) and one or more offset reference points as computed
by the navigation system. The computed offset reference point is located
so that it lies on the intersection of lines drawn parallel to the host
route at the desired offset distance and the line that bisects the track
change angle. An exception to this occurs if there is a route
iscontinuity (or end of route). In this case, the offset reference
point is located abeam of the original flight plan waypoint at the
offset distance.

The offset path and associated waypoint must be created to the same
standards as the host route. The earth model must be WGS-84 and the
offset reference point must have the same or better resolution than the
host route waypoint. The parallel offset function shall be available for
enroute TF and the geodesic portion of DF leg types at a minimum.
Note: The parallel offset function enables an aircraft to be flown on a
flight path offset from the center line of a route while maintaining all
characteristics of that flightpath, as if it were being flown centrally
on the route. Examples for the use of offsets are weather avoidance, air
traffic conflict avoidance, etc.The system shall have the capability to
fly parallel tracks at a selected offset distance.

When executing a parallel offset, the navigation mode and all erformance
requirements of the original route in the active flight plan shall be
applicable to the offset route. The system shall provide for entry of
offset distance in increments of 1 nm, left or right of course. The
system shall be capable of offsets of at least 20 nm. The fact that the
system is operating in offset mode shall be clearly indicated to the
flight crew. When in offset mode, the system shall provide reference
parameters (e.g., cross-track deviation, distance-to-go, time-to-go)
relative to the offset path and offset reference points.

An offset shall not be propagated through route discontinuities,
unreasonable path geometries, or beyond the initial approach fix.
Annunciation shall be given to the flight crew prior to the end of the
offset path, with sufficient time to return to the original path. Once a
parallel offset is activated, the offset shall remain active for all
flight plan route segments until removed automatically, until the flight
crew enters a Direct-To routing, or until flight crew (manual) cancellation.

Note: RTCA/DO-236A provides additional information on parallel offsets.