Faculty of air transport engineering the department of «air navigation systems»

Main design options and possible combinations.

This section identifies the main design options for Point Merge and their possible combinations.
The sequencing legs shall be separated in at least one dimension: laterally or vertically. The same dimension may be used all along the legs or a combination of both (e.g. lateral separation in some parts, vertical in others).
Non laterally-separated legs are assumed to be parallel and of same shape (i.e. approximating arcs of circles in the same way), due to iso-distance/equidistance requirements and for the sake of simple/intuitive working method. Vertical separation between legs can be achieved through either levelling off or a constrained descent.
Consequently, the three main options are:
parallel legs with full overlap, with level off (constrained descent all along the legs may also be possible);
parallel legs with partial overlap, with constrained descent for the overlapping part (level off is also possible);
non parallel legs with no overlap (dissociated), with an unconstrained descent (however a vertical separation may be required between the ends of legs in case of leg run-off, subject to further safety assessment).
Considering the three steps of the operating method (before, during and after merging), the three main options only apply to the first step (before, i.e. ‘create spacing’ through path stretching along the legs) and do not affect the other two. Indeed, the direct-to instruction has an invariant and pivotal role in Point Merge as it triggers the actual merging of flows. Subsequent spacing maintenance through speed control is also invariant.
Implications regarding vertical profiles:

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  level off: continuous descents will be possible from the level/altitude of the sequencing legs (when on course to the merge point).
  
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  descent constrained by the other leg(s): aircraft may then follow a ‘gentle descent’ on both legs at least along their first part. Once on course to the merge point, the vertical profile can be adjusted. Uninterrupted CDAs from closer to the cruise level become possible.
  
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  descent unconstrained by other leg(s), only subject to adjustment of the vertical profile when on course to the merge point, assuming the minimum and maximum distances are close enough (i.e. the leg is short enough). CDAs from closer to the cruise level become possible.
  

Regarding entry conditions, traffic may not be pre-sequenced and in particular not longitudinally separated. In case of level-off, this may be catered for by the use of multiple level(s)/altitude(s) on the leg(s), typically two or three. In order to further facilitate adherence to entry conditions (traffic de- confliction and descent), the controller may tactically send aircraft to intermediate points on the legs. For instance, within a given inbound flow, the controller would alternatively give to some aircraft a direct to the second or third point of the leg.
The present section describes the benefits mechanisms, then captures an initial list of benefits, constraints, and human factors related to the new procedure, as anticipated at the time it was first defined. This list shall therefore only be seen as an early snapshot.
High level benefits and constraints mechanisms can be directly inferred from the possible variations in Point Merge route structure design parameters:
Capacity: linked to the dimensioning of the route structure i.e. the length of sequencing legs (directly related to delay absorption capacity through path stretching), and the distance between the legs and the merge point (considered jointly these two parameters influence the maximum number of aircraft in the system at any given time);
Fuel efficiency and environmental impact: linked to the distance between the sequencing legs and the merge point, along with the vertical dimensions of the route structure. These parameters relate to the ability to fly continuous descent approaches; in addition, the overall dimensioning of a Point Merge system is directly related to the containment of trajectories dispersion;
Predictability: reflected by the ratio between the distance from the sequencing legs to the merge point, and the length of legs (the latter represents the maximum uncertainty in actual trajectory flown under nominal conditions).
Enabling the maintenance of FMS lateral navigation, with continuous descents from the legs, even in high traffic density, has the following consequences:
Level off or less optimum descent along the sequencing legs;
Path stretching capability and runway pressure not simultaneously maximised.
Subject to local constraints, the flexibility offered in the adjustment of Point Merge design parameters is expected to enable achieving performance trade-offs according to specific operational objectives.
More generally, it shall be kept in mind that maximum benefits in all Key Performance Areas are generally not obtained simultaneously through a single P-RNAV procedure. The Point Merge route structure itself actually exhibits direct links with KPAs, and related trade-offs.