I think you're missing the point entirely. The aircraft can cope with icing perfectly I was kinda hoping you'd know a little bit about LCY as an airport as that will reveal your answer. LCY has a short runway but more importantly it has a 5.5 degree glideslope, this makes it a very unique airport to fly into. It is a category C airport which means it's captains only landings. When you have ice on the Q400 airframe you have to increase your vref speed by 15 knots. This is very limiting in regards to landing performance due to the nature of LCY operations. The Q400 can land at GCI and SOU in icing just fine so it has no issues with ice apart from LCY. Whether this is a operator stipulation or an aircraft AOM stipulation I'm not sure and neither do I get paid enough to know. I just do as I'm told; if you have ice detected and ice on the airframe then landing at LCY is prohibited. If you are so inclined to find out how other Q400 operators operate into there you may find the answer, but for me, I've got more important things in life to worry about.