Helicopters often use Sat phones. You won't be able to monitor those.
Next up is Aviation AM radios. In metro areas look to 123.025 for Helicopter to Helicopter communications - They'll speak to each other when coming in to Helipads.
Quite a few have the State DTRS, with some even having their own talkgroups. Usually they just have the talkgroups for the agencies they'd do scene calls for programmed without any kind of license. But I'd bet more don't, then those that have dedicated talkgroups.