|Quoting CF105Arrow (Reply 1):|
I think because originally it was a water based facility. In the early days water ports started with Z.
Although what must be one of the busiest water-based "airports" in Canada, Coal Harbour in downtown Vancouver, with frequent floatplane operations, some scheduled, has a code that doesn't begin with either Y or Z. It's CXH
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Photo © Dan Barnes
As already mentioned, there are quite a few smaller airports in Canada with codes that don't start with Y.
As a sidenote, CXH
also has what may still be the world's highest control tower location, on the roof of a 30-storey office building overlooking the harbour.