Okay, speaking as someone who wants to add a dose of realism to the unit without changing its essential nature, nor overly complicating things, how about this:
...
2.) Leave unit attack strength/ability/basing as-is, but change movement range to TWO (2). Basically, just like a hot-air balloon, it can move a little bit, but no more of this long-range recon. The attack value will remain for hitting land/naval units that are adjacent to its base (and to leave the 'fun' playability value of the unit as-is), but the units primary role will be that of recon in border/sea areas. No more long-range bombing runs on incoming macemen and knights.
3.) Once COMBUSTION is discovered, restore the range to its original value (what is it -- 7?) ...
Opinions?
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I have not have the chance to use airships yet, but here's a wild suggestion:
Considering how vulnerable airships are to weather/winds, give them not X range, but a randomly generated 1 to X+1 range each turn. The total would be hidden from the user. This would make them still useful for recon work but unreliable for combat operations. Once combustion is discovered, give them a range of X-1 to X+1, thus making them a little more useful. As far as the AI goes, provide it with the randomly generated range (for that turn) so it can plan its attacks better.
An even simpler to code revision might be to have "high winds" ground them every few turns, again making them unreliable.