Short Term Conflict Alert (STCA) is a ground-based safety net intended to assist the controller in preventing collision between aircraft by generating, in a timely manner, an alert of a potential or actual infringement of separation minima.
A Short Term Conflict Alert (STCA) is a system to warn the ATCO of any situation where user defined minimum separation distances between any pair of surveillance tracks is, or is predicted to be violated within a short look ahead time (usually 2 minutes). This is achieved via a visual alert on the radar display, though some systems also provide an audible alert.
STCA is a concept rather than a specific system; there are therefore a number of differing implementations used by ATC providers across the world.
EUROCONTROL recommends that the following principles be placed at the centre of policy making for use of STCA:
- STCA is a safety net; its sole purpose is to enhance safety and its presence is ignored when calculating sector capacity.
- STCA is designed, configured and used to make a significant positive contribution to the effectiveness of separation provision and collision avoidance.
Specific training should be provided to ATC staff for the correct use of STCA.
The alerting performance of STCA is highly dependent on the optimisation of the conflict detection and timing parameters for the local airspace. In complex airspace it is best practice to use STCA volumes, specific to each airspace type, within which specific groups of parameter values will apply.
STCA Eligibility Criteria
STCA systems can vary considerably, not least in the rules used to specify which aircraft will be treated by STCA and which aircraft will be ignored. Eligibility criteria can apply to individual tracks, or to track-pair combinations. The criteria that are applied are usually based on the status of the flight plan, but may additionally (or instead) be based upon user-defined SSR code lists (include lists and/or exclude lists). If SSR code lists are used, it is essential that these are kept up to date with the actual operational SSR code allocation policy adopted by the ATC service provider.
It may be necessary to inhibit alerts for predefined volumes of airspace (e.g. exercise areas) or for specific flights to suppress unnecessary alerts.
Hazardous situations related to aircraft separation can remain unnoticed by the flight crew and the controller. The controller’s workload and priorities may cause an imminent hazardous situation to remain undetected if not alerted by STCA. This is especially likely to occur during heavy workload conditions. For the successful implementation of STCA, it is necessary to tune the function taking into account the specifications and local environment and to provide the relevant training to controllers and engineers.
The operational use of STCA will depend on the controller’s trust in the system. Trust is a result of many factors, such as reliability and transparency. Neither mistrust nor complacency are desirable and training and experience are needed to build trust at the appropriate level. An excessive number of false alarms can reduce the ATCO's confidence in the STCA.
Best practices for using STCA have shown that the increasing complexity of STCA and the environment in which it is used are addressed through appropriate training and competency assessment.
The primary goal of the training is to develop and maintain an adequate level of trust in STCA, i.e. to make controllers aware of situations where STCA is likely to be effective and, more importantly, situations in which STCA will not be so effective (e.g. sudden, unexpected manoeuvres).
Retaining electronic records of all STCA alerts generated by the appropriate ATS authority may facilitate the statistical analyses. The data and circumstances pertaining to each alert should be analysed to determine whether an alert was justified or not. A statistical analysis should be made of alerts in order to identify possible shortcomings in airspace design and ATC procedures as well as to monitor overall safety levels.
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