At what distance between two aircraft will the air traffic controller get an alarm?
The STCA (Short Term Conflict Alert) or pre-alert settings depend on the radar client in use and the ACC's or ARTCC's own standard operation procedures. Some ATC systems give a warning before a loss of separation can occur if criteria are met, informing the controller of a possible conflict. Factors can be clearing aircraft on the same altitude / flight level, aircraft on converging flight profiles, lateral or vertical separation reducing below threshold values, e.g. where 5nm lateral separation is required, a STCA could be set to occur at 7nm lateral separation.



The minimum radar separation between aircraft depends on airspace (TMA area vs enroute area), aircraft equipment and airspace separation requirements (RVSM vs CVSM), flight rules (e.g. IFR to VFR is not separated outside class C airspace in Germany) or local rules imposed by the Civil Aviation Authority.

The general rule of thumb for IFR separation in most countries (as most countries follow RVSM rules):


5nm lateral separation enroute (upper airspace)
3nm lateral separation in TMA area (lower airspace)
1.000ft vertical separation in lower and upper airspace where RVSM is observed and aircraft equipped
2.000ft vertical separation in non-RVSM airspace or between aircraft not equipped for RVSM


Oceanic separation is achieved differently, as non-radar procedures are in place. The separation in non-radar environments is achieved procedurally and through position reports and timing.