So after a couple of hours of tearing my hair out I've discovered something that makes no real sense and thought I'd share so hopefully you don't go crazy like I almost did.
I was testing all my support choppers to make sure the landing areas I'd given them at base where all ok and every single one was fine, except for the MEDEVAC blackhawk. I spent hours adjusting his landing area outside the hospital trying to work out why he would come in to land, drop down then abort and try again, and again, and again and... well we've all been there with ARMA.
Then for some reason I decided to try setting it up using the Transport Module instead of placing the heli and linking it (I've done this for ages with ACE medical vehicles so you can define them as such in the init)
Low and behold as soon as I switched to the module instead of a preplaced helo the pilot suddenly learnt how to land again and all was right with the world.
I have no idea why it would make a difference, but it did. So if you're having issues with helo's not landing this may be something to check. As I say it makes no sense to me, and I've had pre placed chopper land fine before, but all I know is that all I did was switch out a pre placed chopper for the module and it solved my issue.
Note this was with an RHS chopper, VCOM and shitload of other mods. YMMV
