The Airship is going to be activating a temporary speed boost it has (essentially working the engine into Overdrive). It's also flying fairly low so it's marksmen can get a good shot in. So the PC's can certainly hit the shooters, or at least make them keep their heads down. I'll probably have the PC's make ride checks each round to outpace it, jump obstacles, or run serpentine to make them easier to hit. Call it moving slightly faster than their horses.
People onboard the airship arn't using nets (They might try a little). They're going to be shooting. They're not very nice people, and an ally the PC's are with has been giving them lots of trouble, so the guy/guys (Gunslinging Ettin with a higher-than-average int score) in charge is/are not playing nice. They want them alive, which is why they aren't just chucking dynamite at the PC's, but they don't need them in very good shape. They're hoping to put a couple holes in them to convince them to surrender,or shoot their horses out from underneath them. Either way works, though I guess they could start with Bolos and Nets.
One of the PC's is a Beguiler with a cunning streak a mile wide, so he'll probably come up with some nifty way to confuse the airshippers.
terrain is a good idea. Maybe have some stands of tree's the PC's can try to use to lose the Airship crew.
Edit: My play (Lawl, yeah right, like that's going to happen) is to end with the PC's sheltering in an old farmhouse, forcing the Airship to land and send in people to try to capture them so I can have a nice climactic battle/realization that it was all a hilarious misunderstanding.