Start & 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.
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.
Open question
AIXM 5.1.1 does not provdie the possibility to define if the path between on the route segment is a VOR radial. Shall 'OTHER:VOR_RADIAL' for RouteSegment.pathType be used for that purpose.
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 different vertical limits to be specified depending on the purpose and classification of the route.
The following table gives an overview which vertical limits are required by PANS-AIM taking into account the defined properties and route sections and the corresponding AIXM attributes of the RouteSegment class (NOTE 1) :
| AIXM 5.1.1 RouteSegment attributes | PANS-AIM | |||||||
|---|---|---|---|---|---|---|---|---|
| Attribute | Description | Property | ENR 3.1 Lower ATS routes | ENR 3.2 Upper ATS routes | ENR 3.3 Area navigation routes | ENR 3.4 Helicopter routes | ENR 3.5 Other routes | Remarks |
| upperLimit | The vertical position of the route segment ceiling. | Upper limit | x | X | X | X | ||
| lowerLimit | The vertical position of the route segment floor. | Lower limit | x (Note 2) | X | X | X | ||
| minimumObstacleClearingAltitude | Minimum obstacle clearance altitude (MOCA). The minimum altitude for a defined segment of flight that provides the required obstacle clearance. | MOCA | x | |||||
| minimumEnrouteAltitude | Minimum en-route altitude (MEA). The altitude for an en-route segment that provides adequate reception of relevant navigation facilities and ATS communications, complies with the airspace structure and provides the required obstacle clearance. | MEA | x (Note 2) | |||||
| minimumCrossingAtEnd | The lowermost vertical position at the end point, when flying on the route portion in the direction indicated in the RoutePortionUsage. | NA | ||||||
| maximumCrossingAtEnd | The uppermost vertical position at the end point, when flying on the route portion in the direction indicated in the RoutePortionUsage. | NA | ||||||
| NA | Minimum flight altitude | X | ||||||
Note 1
Note 2
Open Question
Isn't the MOCA and the Minimum flight altitude the same "thing? the First is used for ENR 3.1 Lower TAS routes and the latter is used only for ENR 3.4 Helicopter routes ?
Regarding vertical limits of en-route routes different terminologies and concepts are used in the aviation world.
The following paragraphs try to provide an overview of the different terms and definitions used by some of the major aviation organisations.
ICAO Terminology
PASN-AIm requires difefrent ICAO Annex 15 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 Flight Altitudes should 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 type of route different vertical limits may be required by ICAO Annex 15:
ENR 3.1 (Lower ATS Routes):
upper and lower limits or minimum flight 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 (ALT).
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.
In some State AIPs, an additional limit, "overriding" the Lower limit specification is published. To publish this minimum limit, often statements such as " but at least" are used. The Lower limit override always refers to ground e.g. "feet above ground" (see also Rules for SDO_RTE_SEG.
The following figure shows how these attributes can be applied by using some example values:
Coding Examples
More examples TBD in the scope of the DONLON XML.