No I don't think you understand. It's a performance issue, as soon as you have ice on the airframe then operations into LCY is prohibited and this is due to the steep approach, this can even be a trace of ice. The ATR isn't certified to fly into severe icing just the same as the Q400, infact most planes in the sky aren't certified. You'll most likely find this was only Q400 operations into LCY and not Q400 operations into any other airport. I've flown it in both the left and the right seat for over 5,000hrs. I know it's capability. The Q400 is very resilient in icing conditions and it flies as good as any other turboprop, if not better. Google ATR and icing and it'll be very enlightening.