Aircraft fatigue is something I know nothing about. Can it decrease without maintenance? (Don't think so, but I really have no idea.) Anything in the manual I've missed? How about the forum?
CV/CVL air group management - I monitor both pilot fatigue and plane damage (time to repair). After prolong activity, I often have to fly off and transfer groups to a base to speed up recovery time. Often you will see 5 days or more time for planes to repair if on CVs. This is a very slow process. So, they go ashore. Your Judy and Jill with SR 2 or 3 need the extra time ashore to repair and recover, IMO.
I've learned to do exactly as 'ny59giants' says. If you are unfamiliar with this I storngly suggest you check as I suspect your CV A/C are quite 'tired'. To do this simply click on the airgroup to bring up its page, and in the lower left corner where you'd check your pilots check the 'planes' heading (is that what it says, at any rate the third choice). My guess is you'll find a bunch of planes >50 fatigue. Remember to fly off you A/C (don't dock) or they'll all need to 'repair'. Put as many as possible 'onshore' and leave the rest on the carrier. If needed I'll even swap the two to get a better 'repair' rate.
Can it decrease without maintenance?
I pulled this out for more clairity. The answer is yes, if the unit is 'stood down', but the process onboard can be slow (as stated above). Also as soon as you 'stand the unit down' you'll notice a bunch with high faitgue will go into maintenance (red). Seems an act of trying to keep everything 'flying' while ops are occurring, rather than bringing half your group down. I guess a fatigued airframe is better than none.
Edit:Not to mention a factor of the planes' SR rating. So an SR1 A/C will stay in action 'til what? Is it as high as 75 fatigue? I forget but is in the manual.
Edit2: I know I've read this somewhere, but doing a cursory search of the manual I couldn't find it. I'll use the search funtion in the manual later as I don't have access to the game currently. Hope I remember.