There's a few places they get stuck at near the factory...I posted screenies over on BFSP.
EDIT: After standing at the main watching a few FW's get shot down, I'd say the AA isn't as accurate as it first appears (unless you're flying straight at it). It's relative to the speed of the target.
I watched one fly toward the main, banking to the right once the AA opened fire (only one gun). The shells were hitting the tail or exploding just behind it (it looks like the bots aren't leading their shots). After a dozen or so shots, the FW made it out of range (smoking) and a moment later came back for more, only to be shot down instantly.
Mostly they get shot down in only 3 or 4 shots due to their bad habit of slowing down to strafe ground targets.
And the Flak 18's are a pain in the ass...I've been shot down by them almost as many times as the AA.