The focus here is on the performance of and interaction between the Traffic Alert and Collision Avoidance System (TCAS) and the controller's short-term conflict alert (STCA) system. The data source used is UK Airprox Board Reports of close encounters between aircraft, and the focus is on commercial air transport aircraft using UK controlled airspace with a radar service. Do the systems work well together? Are controllers surprised when they find out that a pilot has received a TCAS resolution advisory? What do TCAS and STCA events say about collision risk? Generally, the systems seem to work together well. On most occasions, controllers are not surprised by TCAS advisories: either they have detected the problem themselves or STCA has alerted them to it. The statistically expected rate of future mid-air collisions is estimated by extrapolation of Airprox closest encounter distances.

A new ICAO Policy on Airborne Collision Avoidance Systems is needed, which recognizes it to be an integrated part of the air traffic management system's safety defences; and that should be fully included in hazard analyses for the total system's design safety targets.

A review of safety targets for en route ATC radar separation suggests that the existing target level of safety (TLS) is over-cautious. If risk budgeting principles are followed consistently, a ‘radar TLS’ of 1·0×10−9 fatal aircraft accidents per flying hour is appropriate. This rate is consistent with Joint Aviation Authorities (JAA) guidance on system failure conditions leading to catastrophic accidents. Dynamic and static calculations using published data are compared. The new methodology shows where there are problems with the traditional static calculations, and how to improve the estimation. A further improvement introduces a simple robust model of the controller's decision processes. The focus is not on describing what controllers would generally do, but on setting criteria based on what they could not reasonably be expected to do. This additional ingredient into the calculation adds realism and ensures that attention is focused on hazardous correlated errors. Focused data collection would be an essential component of new risk estimates. The key information required would be on radar performance and the nature and frequency of use of radar separation, including the relative velocities for proximate events at closest point of approach and the frequency of correlated gross errors (through a conditional probability factor). If this factor is not properly taken into account, then the data collection and analysis could be inefficient.

Air traffic controllers use a radar separation minimum when keeping aircraft safely apart. The choice of minimum needs to ensure, to a very high degree of confidence, that radar inaccuracies are not putting the aircraft into a risk of collision – controllers can of course be required to use larger minima for operational reasons. A particular separation minimum necessarily feeds back into requirements on radar design criteria, data processing and performance. The estimation of the radar separation minimum is significantly improved by the use of a new methodology. Previous derivations were a major step forward, but were ‘static’, in that they did not attempt to take properly into account how controllers use separation minima dynamically. An analysis on the use of separation minima in en route airspace by controllers concludes that they are generally likely to use the radar minimum to pass aircraft, rather than to use the minimum in a ‘strategic’ sense (e.g. ‘in trail’ separation). The new ‘Event Model’ methodology recognises that collisions cannot happen instantly: the aircraft pair concerned must move from a safe configuration to a hazardous one. Collisions are therefore events caused by flawed flight paths, dependent on both the initial positions of aircraft and their subsequent movements and velocities.