I think if you backed out all of the CNN feeds at every airport in the country they would actually be below FNC.
Not to mention that as stated, FNC retains it's viewers 4 times as long. Whoever's turning on CNN clearly can't stand it for very long.
The "Airport Channel" is insignificant anyway, since it's target audience are people sitting around at airports the channel is carried in. I've seen that channel at the airport in Minneapolis, and it's nothing more than snippets of CNN and Headline News.