Unarguably, sequencing and metering arrivals during peak traffic at hubs are two of the most challenging tasks Approach Controllers have to cope with during their session. The absence of additional metering tools and sequencing aids often leades to inefficient traffic flows.
But how can controllers integrate valuable pieces of information accessible to every controller into specific situations to determine the most efficient way of managing arrivals?
The notion of runway capacity is crucial for understanding the principle behind metering. The number of planes, which can land on a runway in a given time safely at minimum risk of causing a go–around is referred to as runway capacity, a figure typically ranging from 1 landing/150s (back–taxi, no rapid exit taxiways) up to 1 landing/50s (mainly rapid exit taxiways at optimized distance from runway threshold). As this number depends on the location and the characteristics of runway exits, capacity is space–invariant. This means in order to guarantee a smooth traffic flow a given capacity holds for every single point arrivals pass in sequence. A smooth flow requires little to none controller intervention. As soon as demand exceeds capacity a delay is caused.
The process of tactically limiting the amount of planes which enter a certain airspace in a given time is therefore referred to as metering.
The reciprocal value of the runway capacity is known as spacing. So for a runway capacity of 1 landing/50s for example, the spacing required for a smooth flow all the way down from en–route to approach phase would be approximately 50s. If the spacing falls below this value, delay vectors or holdings are put in place by Approach Control as part of terminal delay to compensate for the reduced separation.
Note that, the spacing for a smooth flow is measured in units of time, therefore called time–based separation, which results in the distance between two aircraft to increase at higher true airspeed and high altitudes and subsequently decrease at low altitudes and low true air speeds.
The fact that this optimized spacing decreases with descending altitudes is called compression. Thus spacing compression is the reason why radar controllers should not vector aircraft onto descending tracks or approaches at the lateral separation minimum, the spacing between two aircraft is the much likely to drop below the separation minimum.
Well, how is this applied to a standard arrival flow into an airport with Approach Control and Center being staffed? Center control’s main task would be to deliver arrivals at an optimized spacing, which can be approximated by the time–in–trail part of the drag & vector–tool, according to the requirements for a smooth flow by employing speed control. The workload for Approach Control would see a tremendous shortcut as potential terminal delay is compensated through reduced cruising/descending speeds issued by Center for the latter portion of their flight in order to achieve optimized spacing even before arrivals reach Approach Control‘s airspace.
With this, let me conclude. Spacing and sequencing are two very important terms when operating a radar facility at a hub. To reduce the amount of workload for Approach Controllers the sequence and the spacing is set according to metering rules, which are determined by the runway capacity. As optimized spacing is measured in units of time, the time measurement tool of the drag & vector does come into handy when controlling Center.