TRIAL AT SELECTED UK AERODROMES - USE OF SSR IN THE AERODROME TRAFFIC PATTERN - Background:
Current policy requires Air Traffic Controllers to instruct aircraft, which are below 3000ft in the vicinity of an aerodrome traffic pattern where transponder operation is not mandatory, to select SSR transponders to STANDBY. This policy originates from a requirement to prevent degradation of ATC facilities, which could have an adverse effect on safe ATC provision.
A negative aspect of this policy, however, is that the benefit which could be provided to aircraft in the aerodrome traffic pattern by the, last-ditch, Airborne Collision Avoidance System (ACAS) is effectively denied. Modern ATC displays have, however, become more capable of dealing with multiple SSR responses and the fitting of ACAS systems is much more widespread and mandated for certain classes of aircraft. Accordingly, the balance of risk is shifting away from protecting ATC systems toward retaining the ‘last-ditch’ collision avoidance function provided by ACAS.
The CAA is considering a change in policy toward widespread SSR use in the vicinity of the aerodrome traffic pattern and, as a precursor activity, the trial is being conducted at a small number of selected aerodromes. The aim of the trial, which is sponsored jointly by the CAA Directorate of Airspace Policy (DAP) and Safety Regulation Group (SRG) is to gather evidence of the relative benefits and drawbacks of such a change in policy.