Side-scroller, ok graphics and gameplay. Must have played it sometime between 92 and 94.
It's not too easy to explain how this looked, but here goes anyway:
You flew around shooting things, all seen from the side. The levels was something like a hill, with things both up in treetops and down on the ground to shoot at. Think a level was typically around 4x4 screens large or so. Quite large, anyway.
There was all sorts of enemies to shoot at, probably even other helicopters (this is a bit vague).
The enemies could have been vietnamese, I seem to remember straw huts and stuff. Please tell me I'm not the only one who saw this game, because I've been searching review pages and games databases all evening, and none of the names ring a bell :-(