In other such airprox reports eg on approaches, the minima are a lot lower eg 3nm or even 2.5nm.
Could somebody explain why this is the case please? And is there a rough n ready guide to what the minima are in different airspaces?
One of the objectives of ATC is to prevent collisions between aircraft. They acheive that objective through a number of means many of which do not involve radar.
When using radar the objective is still simply to prevent the aircraft from being in exactly the same place at the same time.
Radar is not 100% accurate.
Depending on the radar performance - scan rate, frequency, pulse length, PRF clutter reduction elements etc, the number of radars used, the range at which the aircraft is from the radar among many other considerations lead to a minimum horizontal separation being set that will ensure aircraft which are at that minimum separation will not collide.
This separation can be 10nm, 5nm, 3nm or in special cases some figure less than 3nm.
It must be remembered that when two aircraft are displayed on a controler's screen with the centres of the blips or radar position symbols exactly the minimum appart, the aircraft can actually be less than that distance appart however the system guarantees that they will not be so close as to be in danger of collision.
Thus when at large range from certain radars, 5nm indicated separation may not be suficient and therefore 10nm will be prescribed in those cases.
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With regard to TCAS, it must be remembered that the TCAS system scans for other aircraft which will enter a small zone arround the parent aircraft. If it predicts that another aircraft will enter this zone it issues an alert to the parent aircraft's pilot to either climb or descend to prevent the other aircraft from entering that small zone.
It is important to note that again the TCAS system is very accurate but not 100% so. Therefore since it is important to err on the side of safety in many cases it issues a warning when aircraft will get very (dangerously) close but when they will not actually collide. i.e. they will actually miss by some metres or even 10s of meters.
Thus when reporting this serious incident report, while not detracting from the importance of the report and the serious nature of the incident, it is impossible from the radar or from the TCAS to say 100% that the aircraft were on a collision course. They would have got dangerously close no doubt and TCAS ensured a minimum level of safety but no one can say in certainty that they would have collided.
Many people in the industry will be familiar with the ATC advice of traffic opposite direction straight ahead 1000ft above report when clear and on the ATC radar the indications of the relative positions close and merge to the same position before separating again but the pilots see the aircraft pass a small distance to the left or the right of their aircraft.
So controllers try to prevent colisions. They try to use various techniques to keep aircraft a certain defined indicated distance appart so that they do not collide. TCAS works to keep a small safe zone round the parent aircraft clear of other aircraft who while very close may not be about to collide with the parent aircraft.
Regards,
DFC
PS The report did not dwell on one very important issue. With the display on 250nm range measure the distance on the screen between two tracks 10nm apart. Now change the range to 150nm. Those aircraft appear to move appart. With a tired mind, end of shift distractions, cluttered airspace lines etc it would be easy to allow one's mind to use the same visual on screen distance to represent a minimum of 5nm when now it is actually less than 5nm.
STCA suffers from false indications especially when aircraft are changing level at high rate.
Combine the two and one could see a situation where the controller could think that they had more distance than they actually have and the initial STCA would be initially counted of as unreliable.........but then on checking the controller would realise the error and take corrective action - as in this case.
Is there a 5nm reference available to the controller in that area of the sector?
References to the South German accident may not be relevant since I believe that maintenance was on going and the STCA was not available in that case.