Fixed Arrival Routes: Early Split – Late Merge

Air traffic volumes and the nature of air traffic using the Dutch airspace has evolved significantly over the past decades and will continue to evolve. This while the airspace structure has remained largely unchanged. To boost efficient use of airspace and to reduce emissions and noise impact, the Dutch Airspace Redesign Programme was established. Redesigns for the Dutch airspace developed in this programme are progressing towards a TMA concept that is based upon a structure with 3D separated and fixed arrival and departure routes. Feasibility of these designs will depend amongst other aspects on whether the desired TMA capacity can be achieved or not. A major factor in this is the route design and more specifically the position of the split and merge point of routes.

To provide guidance on design decisions for the position of merge and split points in the future TMA concept, KDC has initiated a study into the effects of split and merge point locations on airspace capacity. This study has been performed by To70 through the use of fast-time simulation for a set of generic route structures with different split/merge point locations.

There are various factors affecting capacity that are related to the positioning of a split point for departure routes and a merge point for arrival routes. The effects of the position of a split point on capacity are mainly impacted by speed difference between aircraft types. To ascertain that miles-in-trail separation can be maintained on the segment before the split point, when a trailing flight is catching a leading flight, a separation buffer must be applied at take-off. A longer segment before the split point will require a larger buffer at take-off, leading to a reduction in runway capacity. An early split point, leading to a shorter common segment, is therefore preferred to avoid a loss in capacity.

In merge point design for fixed arrival routes, a position close to the threshold(late merge) or a position close to the TMA entry(early merge) may be opted for. Opting for an early merge limits the distance between the initial approach fix(IAF) and the merge point, making it hard to correct for delivery errors at the IAF before a flight pair enters a common path at the merge point. Opting for a late merge decreases the length of a common path where miles-in-trail separation is required, but limits the amount of control space between the merge point and final approach making it hard to correct for turn-in errors at the merge point. Selection of the merge point location will therefore influence how much separation buffer is required and what capacity can be achieved.

For split points of departure routes, the study shows that the optimal split point location is as close as possible to the runway (positioned at 3NM). Positioning the split point further away at 10NM and 20NM from the threshold results in a capacity loss of 1 movement/hour and 3 movements/hour respectively. Introducing a speed restriction can reduce this loss in capacity significantly. For the merge point location the study shows that a merge point should not be positioned too early(close to TMA entry) and not be positioned to late(close to threshold). The optimum position for capacity lies in-between and is influenced by turn-in error, uncertainty in wind conditions and delivery error at the IAF.

More information and all results can be found in the report, which can be downloaded below.

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