Basic Data of Route Segment

Start Point & End Point

Route Segment ICAO defines a route segment as

A portion of a route to be flown usually without an intermediate stop, as defined by two consecutive significant points.

In AIXM 5 the start and the end properties of the RouteSegment class are used to define the "From point" i.e. the first point of the route segment and "To point" i.e. the second point of a route segment.

The EnrouteSegmentPoint class used for that purpose is a specialisation of the SegmentPoint. It provides the reportingATC attribute which is used for the indication of the ATS / MET reporting requirement 'COMPULSORY' or 'ON_REQUEST' for the corresponding point.

The actual start or end point of a RouteSegment is defined via the pointChoice property that allows to select a SignificantPoint. Not all available choices of significant points (e.g. RunwayCentrelinePoint, etc.) will make sense to be used for a route segment point (see also page SignificantPoint [SPN] ).

Mainly DesignatedPoint and Navaid will be the start or end point of a RouteSegment. In a few countries also theairport reference point of an AirportHeliport may be used for that purpose.

For defining that the point used for the route segment is an RNAV waypoint (see also topic Designated Point (Overview)) the EnrouteSegmentPoint.waypoint attribute will be encoded with 'YES'.

In addtion AIXM 5 provides as an option to encode the extend of the route segment as geometrical element, using the Curve class. This data may be used for charting purposes. The curve will be encoded according to the corresponding GML specification (see topic  Geometry (Overview)). It will contain the coordinates of the start and end point of the route segment. It has to be ensured that the data (i.e. coordinates)  provided in the Curve element(s) match with those provided as start and end of the RouteSegment.

Implicit direction

As every route segment is defined 'from' a start point 'to' an end point, it has an implicit direction. This direction should be consistent with the direction of the route. The start point of a route should also be the 'from' point of the first route segment and the end point of the route should be the 'to' point of the last segment.

The implicit direction of a route segment is relevant when specifying the route availability (see also page Direction of Cruise Levels).

Rules are included in the model to enforce consistency between the general direction of the route and the implicit direction of every segment (see below).

Track & Lenght

The RouteSegment class provides several attributes to define the initial trueTrack, initial magneticTrack, the initial reverseTrueTrack and the initial reverseMagneticTrack of it.

For "ENR 3.3 Area navigation routes", PANS-AIM explicitly requires that the magnetic bearing of the route segment is provided. For all other routes defined in ENR 3, PANS-AIM requires a track or VOR radial applying the following note

Bearings, tracks and radials are normally magnetic. In areas of high latitude, where it is determined by the appropriate authority that reference to Magnetic North is impractical, another suitable reference, i.e. True North or Grid North, may be used.

Tracks and reverse tracks if any, have to be encoded taking into account the implicit direction of the route segment, i.e. the defined start and end point, (see also above). In this regard the tracks are considered to be from the start point of the segment whereas the reverse tracks are considred to be to from the end point of the route segment.

The length attribute allows to define the length of the path of the RouteSegment. The length actually depends on the pathType, which may be a great circle ('GRC'), a rhumbline ('RHL') or a geodesic line ('GDS'). 

According to PANS-AIM an the length of a route segment shall be provided as geodesic distance.

Lateral Limits

For "ENR 3.1 Lower ATS routes" and "ENR 3.2 Upper ATS routes", PANS-AIM requires the provison of  lateral limits.

The RouteSegment class provides the attributes widthLeft and widthRight to define the lateral limits. These are the distances from the centreline of the route segment to the left and right edge respectivley. For example, if the route segment has a defined width of 10 NM for each of the attributes will carry the value '5' uom equal-to 'NM'.

Vertical Limits

PANS-AIM requires that all routes/route segments have at least a lower limit and an upper limit.

For some types of routes, viz. ENR 3.1 (Lower ATS Routes) and ENR 3.4 (Helicopter Routes), also minimum obstacle clearance altitudes and minimum flight altitudes, respectively shall be provided.

According to ICAO Annex 11:

The minimum flight altitudes determined shall provide a minimum clearance above the controlling obstacle located within the areas concerned.

Whereas, the States should publish, “The criteria used to determine minimum flight altitudes”, in AIP section GEN 3.3.5.

Depending on the purpose and classification of the route different vertical limits may be required by PANS-AIM:

ENR 3.1 (Lower ATS Routes):

upper and lower limits or minimum en-route altitudes.

Since en-route flights at or above the lowest usable flight level are flown at flight levels, it is suggested that these limits be expressed in flight levels (FL) whenever such flight levels are not likely to fall below the lowest safe altitude; otherwise, they should be expressed in altitude.

ENR 3.2 (Upper ATS Routes):

upper and lower limits (usually in flight levels "FL”).

ENR 3.3 (RNAV Routes):

upper and lower limits (usually in flight levels "FL”).

ENR 3.4 (Helicopter Routes):

upper and lower limits; minimum flight altitudes

ENR 3.5 (Other Routes):

No requirements stated regarding vertical limits.

The following table gives an overview which vertical limits are required by PANS-AIM for the AIP data set and the corresponding AIXM attributes of the RouteSegment class (Note 1) :

Regarding the general encoding guidelines and rules for vertical limits see topic Vertical Limits [VER].

Coding Rules for Basic Data for Route Segment 

Coding Examples

More examples TBD in the scope of the DONLON XML.