Porter Airlines flight from YYZ to YTZ?

[CathayPacific]

Looking at the flight status on Porter's website, notice that there is a flight 1823 from YYZ (Toronto Pearson) to YTZ (Toronto City). I suppose that's not a regular flight? I don't think PD even serves YYZ? Is that a re-positioning flight after maintenance? Do these YYZ-YTZ flights happen often?

[LOT747-400]

Yeah I wondered too about it, saw it takeoff from rwy 23 when I was standing during the runway run. Weather was fine the day before so it's not weather diversions. My best guess is that one Porter missed the YTZ curfew the night before so they had to divert to YYZ.

[porter guy]

this is a common thing.

as stated above, most likely a plan missed the curfew and the plane was just returning to base.

other possibilities is that the runway was slick with something (water or ice) and was deemed unfit for the plane to land on it at the time. In this case, it would also be a return to base.

last possibility, it was a plane landing for an emergency, or something like that. Again, just an RTB flight.