Originally Posted by
CrazyJ82
I thought I had read here on FT that there can be issues sometimes with ATC when new routes start -- the authorities can weigh in on flight numbering to avoid similar numbers on different carriers operating in the same airports at the same time. No idea if that would explain some of the exceptions you point out.
That's exactly the reason. Most European flights depart within 1-2 hours before/after midnight, while for the North American flights, they depart at different time in a day. It is unlikely that two (or more) flights with similar flight numbers (and hence call sign) are flying at the same time. CAD Hong Kong has guidelines on this (published in AIP).