ICAO Doc 4444 permits the application of 4.6 km (2.5 NM) provided several conditions are met. The first listed criterion states that the average runway occupancy time of landing aircraft is proven, by means such as data collection and statistical analysis and methods based on a theoretical model, not to exceed 50 seconds.
It is typically understood and appears implicit in the PANS ATM provisions that if the average runway occupancy time (ROT) criterion is not satisfied by all the arriving aircraft, the reduced 2.5 NM separation minima cannot be applied. This is regardless of the fact that a large part of the traffic might meet this ROT criterion and that all the other criteria from 8.7.3 are met (e.g., suitable surveillance performance).
For a large number of aerodromes and Approach ATS units, and on specific runways, there is usually a part of the traffic/aircraft types for which the average ROT is lower than 50 seconds. This is typically Medium wake category aircraft types, either the case of large single-aisle (e.g. Airbus 320/Boeing 737) models or the case of Regional jets, depending on the specific runway configuration, and these aircraft ‘groups’ can represent more than 50% of the local arriving traffic. These aerodromes could benefit significantly if those aircraft and corresponding flights were able to apply the 2.5NM separation and benefit from the 0.5 NM reduction when all criteria, including the ROT, are met. Based on a unique characterisation of each specific runway, an individual/‘pair-wise’ (based on the lead traffic behaviour) Runway Occupancy Time (‘iROT’ /‘ROCAT-PWS’) per aircraft type can be assigned and with this information ATC could define two aircraft sub-categories based on ROT.
For example, the Medium category could be subdivided in two: one with average ROT lower than 50s for which the 2.5 NM reduced surveillance minima can be applied, and one with average ROT of greater than 50s for which the 3 NM spacing remains necessary. Under SESAR 2020 PJ02.08.03 validated solution, this concept is referred to as ‘ROCAT’ (Runway Occupancy CATegories) and/or ROCAT-PWS spacing.
The application of a varied final approach separation/spacing minima (2.5NM reduced separation or 3NM spacing) would be flexible based on the ROCAT-PWS. This hybrid mode of operations could be applied procedurally in case of ROCAT but this option would rely on specific Controller training. Alternatively, an adaptive final approach separation minima to be applied could be identified with the support of an automation system which would recognise the aircraft types and their ROT category and provide guidance to the controller (or ATC surveillance display system if separation/spacing indicators are provided) on which minimum separation to apply between each arrival pair. This level of automation may also rely on the use of a final approach spacing tool (like used for Time-Based Separation) to support the full ‘ROCAT-PWS’ application.
It is important to note that the ROCAT solution fully relies on existing provisions, and is only based on a differentiated application of ROT criteria for operating the reduced surveillance minima, based on local characterisation specific to a given runway and per pair of individual arriving aircraft types.
Depending on the traffic mix, the runway layout and the currently applied ROT spacing minima, the benefits in runway throughput during peak periods can typically reach up to 8% and even more if combined with optimised wake turbulence schemes.
Also, at equivalent throughput, the ROCAT spacing reduction,
- supports a reduction of the overall flight time for an approach sequence of traffic, which therefore benefits to the whole traffic sequence.
- offers more flexibility for the Air Traffic Controllers to manage the traffic.
- enables more rapid recovery from adverse conditions, helping to reduce the local airport delay and eventually the ATFM delay propagated at network level.