Aye terrain against your specific target is usually hard to utilize in an aircraft, but it can often be used to cover yourself from some (eternal) AA sources at least. You don't have to be visible to every single enemy in a 600m radius at all times, that is the point I'm trying to make I am not much for flying in PS2 (have flown plenty in other games, it just never caught on here), but I do love to fly through AA nests on cliffs (through as in roadkilling them), and in performing those maneuvers I have been able to use much of the experience I've gathered using terrain to my advantage in a tank. Nothing beats flying through 5-10 people I was actually having a discussion with some people earlier about improving the capability of tanks to defend themselves against aircraft (either via the secondary as you suggested, or simply altering elevation), which should of course be complimented by a buff to aircraft in general, so as to not make tanks dominate everything. It would simply help to have more counter-play, and be able to defend yourself in most situations (but of course not as effectively as dedicated AA-options). I at least think the concept is intriguing, but balancing it out might be difficult.