Roseflyer From United States of America, joined Feb 2004, 8746 posts, RR: 52 Reply 3, posted (1 year 3 weeks 2 days 1 hour ago) and read 938 times:
I would expect it to be a charter since judging by the picture the weather is good and SEA rarely has any delays that would necessitate a diversion (BFI is 6 mi from SEA). Sports teams and other charters typically use Boeing Field. It could have been for the Chicago White Sox visiting Seattle.
If you have never designed an airplane part before, let the real designers do the work!
They don't. It was a sports team charter. I forget who it was, but I believe it was for the Oakland A's, who played the Mariners that weekend. I saw it on the schedule that day and thought about going to photograph it, but was out hiking elsewhere. It's not uncommon to see charters like this at BFI.