Hard to tell for sure, but could be.
This is an image of the controller’s screen of the situation off the south-east coast of Ireland at 19:51 on 23 September last year. Ryanair 907 is heading west and descending through FL286. Flightline 1174 is heading north at FL280. But due to the strong wind, Ryanair has a groundspeed of only 351kt and Flightline has a groundspeed of 517kt. The two aircraft are highlighted in red because the short-term conflict alert (STCA) system has been triggered.What finally happened was that both aircafts’ TCAS systems were triggered – the Ryanair Boeing 737 climbed and the Flightline MD-83 descended. And all was well.
You can read the full report on this from the Irish Air Accident Investigation Unit here, or a quicker version from Flight here. In the words of the investigators: All ATM safety defences, both human and electronic, were breached in this event. The safe resolution of this occurrence was ultimately resolved by the last line of defence, the aircraft’s on board TCAS RA, and not by ATC intervention.
It’s extremely difficult to prove that a collision would have happened without TCAS. After the horrific Lake Constance collison of July 2002, there was much discussion about the alleged ‘dangers’ of TCAS. And there is a grain of truth in those fears – but we shouldn’t throw the baby out with the bathwater – as this incident serves to remind. There were 355 occupants on these two aircraft.