Procedure Leg (Overview)

The trajectory to be flown by an aircraft executing a departure, arrival or instrument approach procedure is defined with “procedure legs”. First, an abstract SegmentLeg class is defined in the model, covering the properties that are common to all phases of a procedure. This is then specialised into DepartureLeg (used by SID), ArrivalLeg (used by STAR) and ApproachLeg (used for IAP). The ApproachLeg is further specialised, as visible on the following diagram, which also shows several other possible associations of a procedure leg.

The trajectory to be flown on each leg may be encoded either using ARINC 424 leg types (attribute legTypeARINC with values such as ‘CF’, ‘RF’, ‘PI’, etc. ) or using a simple path indicator (attribute legPath with values such as ‘STRAIGHT’, ‘ARC’, ‘BASETURN’, etc.) Other attributes are intended for the encoding of the flying direction (course and courseType), altitude (lowerLimitAltitude, lowerLimitreference, upperLimitAltitude, upperLimitReference), climb/descent angle (verticalAngle), etc.

The specialisations of ProcedureLeg come with attributes that are specific to each type of procedure or procedure phase. The requiredNavigationPerformance attribute is common to most. For FinalLeg it can take several values, as part of several approach conditions).

The FinalLeg class has additional attributes, modelled as complex objects, which are not visible on this diagram (they will be discussed later).

The following diagram shows the associations of the ProcedureLeg class, which are inherited by all its specialisations.

A SegmentLeg is designed for one of or more aircraft landing categories, which is modelled by the association isvalidFor with AircraftCharacteristics.

A holding may be used as segment leg, which is modelled by the association establishes with HoldingPattern.

Each segment leg may have a starting point (beginsAt TerminalSegmentPoint), an end point (terminatesAt TerminalSegmentPoint) and an arc centre for certain leg types (has arcCentre). The role attribute of the TerminalSegmentPoint indicates if the point has a special significance, such as ‘IAF’, ‘IF’, ‘FAF’, etc.

The actual location of the point is a SignificantPoint, which is an association (isLocatedAt) inherited from the abstract SegmentPoint class. The same class provides additional attributes such as reportingATC (‘COMPULSORY’, ‘ON_REQUEST’, etc.), flyOver and waypoint indications (with ‘YES’/’NO’ values). If the start or end point of the segment leg has associated “fix” information, this is modelled through the isFoundUsing association to PointReference, which may refer to one or more AngleIndication and/or DistanceIndication. The association uses SignificantPoint is used to indicate the navigation aid on which the fix is based (VOR, DME, etc.)

Certain leg types (such as a procedure turn) may be limited by VOR radial or a DME distance, which may be due to obstacle clearance reasons, noise abatement, etc. This is modelled through the direct associations SegmentLeg isLimitedBy AngleIndication or DistanceIndication.