I spent a few years as a Radio and Electric, Non-Routine maintenance mechanic at SFO for United Air Lines. It was a great job but almost all of it was on grave yard shift which quickly grows old.

Seen here is one of the Boeing 727 quick-change aircraft undergoing some routine maintenance in a hanger. Notice the big door forward on the left side. The plane would fly revenue passengers all day, and then in the space of about one hour the seats, galleys, and heads would be removed and the plane loaded with freight. Clever idea but for some reason it didn't catch on.