"The tweet was deleted after approximately an hour but not before it had been retweeted hundreds of times."
I'm linking to BBC.com, where the story is ranked as #1 on the sidebar "Most Popular" list under the teaser "US Airways apologises for porn tweet."
Come on, the tweet was incredibly foolish — and a good reminder of the inherent danger of relying on some lower-down writer-drudge to man the Twitter feed — but it was not porn. A photograph of a naked person isn't porn.
Wait! I was relying on the BBC description of the photograph: "a naked woman and a toy plane." Then, I went looking for the photograph. I must say that it is porn. (NSFW: here.) The woman is holding the plane in a very special place.
ADDED: And by NSFW, I mean not safe for anybody.