Much has been written on the subject of aircraft collision risk and avoidance over the past 50 years. As a particularly interested aviator (a military and civilian pilot for the last 32 years), with a limited memory for the mathematics I studied at school, I have read several of these treatises with interest. I do, however, feel that the advent of modern, accurate navigation systems has changed the distribution of collision causes radically. We should be concentrating, not on the risks of aircraft accidentally flying into each others' paths, but on the risks of them deliberately doing so.

That statement requires explanation, of course. There is a risk of one pilot deliberately and willingly putting his aircraft in the path of another (a terrorist or suicide for example), but the risk is considerably less than that of a pilot who does it unwillingly, but equally deliberately.