Re STCA’s, I’ve revisited the ATSB Report on the Mangalore tragedy. It says, with my bolding added:
In the time between JQF taking off and the collision, there were three STCA alerts generated.
…
On the basis of analysis conducted by an ATC subject matter expert and technical detail provided by Airservices, it was assessed that:
The first STCA, at 1122:42, was a nuisance alert generated by JQF conflicting with VFR traffic in the Mangalore circuit area.
A second STCA, at 1122:49, occurred as the controller passed traffic information to JQF (Figure 13). At that stage, indications were that the aircraft would pass abeam each other. The STCA was assessed by the controller but not cleared from the screen at this point.
The controller re-inspected the two aircraft at 1123:30 after JQF had turned towards the planned outbound track. The velocity vectors indicated that lateral displacement would be maintained, with JQF passing behind AEM in about one minute. At that time, the controller’s display showed AEM at 4,800 ft while JQF was at 3,400ft.
A final STCA alert occurred at 1123:51. The controller zoomed in to inspect the aircraft flight paths and altitudes again and acknowledged the STCA at 1124:09. The controller identified that JQF was going to pass across the track of AEM, but at that time, 11 seconds prior to the collision, indications on the controller’s display showed AEM at 4,500 ft and JQF at 4,000 ft, with 0.9 NM lateral separation between the aircraft.
So I’ll correct my ‘4’ to ‘3’ (though I’m still digging to find the provenance of my original ‘4’).
Perhaps your ‘2’ comes from Figure 13, described as “Recreation of STCA display at 1122:49”, which shows only 2 STCA ‘boxes’?
Of course, a recreation of the circumstances at 1122:49 excludes the “final STCA” which “occurred at 1123:51”. It’s very hard for me to fathom the logic behind the ATSB’s decision not to include the closest STCA in time to the collision in the recreation of the STCA display. The ATSB went to the trouble of recreating the display showing the velocity vectors as not intersecting at the time of the 1122:49 STCA but decided not to recreate the display showing the velocity vectors at the time of the STCA immediately preceding the collision. Go figure.
Of course, all of this is OK for IFR aircraft because there is no separation standard in non-controlled airspace. Meanwhile, some VFR pilot has an attack of the vapours after seeing another aircraft in close proximity in a published flying training Delta, and the ATSB uses that as basis for another advert for ADS-B IN, which advert included a recreation of the STCA. Go figure.