[Mustang]

How about you just have them retreat out of LOS of the front line or 20 hexes, whichever is greater? 20 hexes is only 1000 meters, which is usually enough but sometimes you have a very large open area to retreat across.

About helicopters and landing ships, they shouldn't be reatreated unless they're unarmed. It's SOP in most armies for transport helos to stick around and provide cover fire. Besides, the AI might need them later to withdraw, which is what happens a lot in airmobile assaults. About ships, landing ships are usually pretty beat-up on contact with the beachhead (this is why in training everoyone goes to shore in zodiacs because the real transports are too expensive to waste) and their hulls can only take 3-4 landings on a beach, which is usually rocky and/or muddy. So they're designed to be expendable and are usually left where they are unless they're needed later.