The thing which is hardest to explain here is that we know the copilot looked at the radar screen about 30s before impact and called a small radar return at 6nm/11 o'clock. At this point he was just under 1nm from Blackrock, so what happened to the Blackrock return ?
Assuming the radar return was Duvillaun More then it's about 0.5nm across from the radar's perspective. At 6nm the circumference around the aircraft is about 36nm (6nm*2pi). Let's say the radar sweeps out 60 degrees of that, ie. 1/6th of the circumference or a 6nm arc. It follows that Duvillaun More would have a lateral extent of 8% (0.5/6) of the sweep.
Run the same calculations for Blackrock which presents a lateral extent of about .12nm at about 1nm range and it turns out that it would occupy a similar amount of the sweep. Given the heading and drift of the aircraft those returns might well have been adjacent on the sweep although very different distances away.
It has also been suggested that Duvillaun More and Blackrock would have painted very different pictures on the screen because of the different topography and distances involved. I'm thinking Duvillaun More may have been more of a blob, but Blackrock has some 'stealth' features, sharp edges and flat surfaces angled away from the radar. In addition there might have been some clutter on the screen which might still include some weather in the solid IMC above even though the radar was in ground mapping mode.
I'd love to know from a SAR driver with lots of experience of interpreting weather radar pictures using the fit in this aircraft to speculate on what the radar picture might actually have been. Somehow the Blackrock return was misinterpreted but how ? Would the Carrick Islands just in front of them have appeared on the screen and lulled them into believing that the Blackrock return was a similar low feature ? Could Blackrock have looked like sea clutter or weather somehow ? Could Blackrock itself have created a local weather effect that masked the island ?
Last edited by puntosaurus; 17th Apr 2017 at 13:51.