Plenty of airlines switch equipment around on routes as the demand ebbs and flows. One that seems to do this more than most when it comes to flights to Seattle is Asiana. Their long-haul fleet includes Boeing 777-200ERs, Airbus A350-900s and Airbus A330-300s and all of them seem to appear.  The A330s are not as regular as they used to be but you never really know whether it is going to be an A350 or a 777 at any point. Here is a selection of images I have got over time of the three types as they arrive and depart from SEA.