It's only in 1 frame.

Compare the first 2 images [link|http://news.bbc.co.uk/2/hi/americas/1861977.stm|here] on the BBC. The plane is just above the grass on the right edge of the screen in the first picture, not present in the 2nd. There's less than 1s between frames. Since the plane was going ~ 350 mph, it's not surprising that it's hard to see.

The picture quality isn't very good. I've seen other pictures that have the plane circled, but haven't found them this morning.

More discussion on this topic is in [link|http://z.iwethey.org/forums/render/content/show?contentid=31944|#31944].

(Also note there are other eye-witness reports from people in office buildings near the Pentagon who talked about a jet flying very low and fast over their building just before the impact.)

[Edit:] A better comparison is [link|http://www.vvm.com/~cdhoit/AT_PENTAGONimpact_AAflt77.jpg|here], from [link|http://www.vvm.com/~cdhoit/3AT8.html|this] page (one I imagine that the conspiracy theorists don't like too much).

Cheers,
Scott.