I think Husar's probably right. Go to the hitpoints overlay and check how many HPs the different modules have. Then find the room to slap a large (or maybe grand?) keel in there somewhere and see how much the hitpoints go up. I had a similar problem designing a rather large landship. Adding a large keel took it from "significantly" reduced integrity to "slightly" reduced. I've never built anything that rated "heavily" reduced.
Also note that the number of Air Sailors says "264 (max 120)". I'm pretty sure 120 is the maximum you can have in the current game. That design recommends 164 crew to staff it, so the fortress is probably underperforming with 120 airsailors. Maybe getting rid of those extra ammo stores helped though.
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