Surprise attacks plain and simple! No matter where no matter when. Nothing visible on maps or radar.
The type of situation that makes you jump like hell when it hits, a little bit like a first person shooter (although I appreciate that there is a lot of mood music happening there) except it may be something like returning to base after a milk run just before landing, or in a briefing for a new mission. Just so long as there isn't any jerky hard drive loading stuff from triggers, it needs to be seamless.
Also, after taking hits and looking down to see ones legs damaged by gunfire, perhaps visible bone and meat modelling would be really amping!! Also in a Spit or Hurri and seeing visible spillage of fuel into the cockpit would be frightening.
Real physical damage to ground personal. Let's all be honest here, we all enjoy first person shooters as well, with the more realistic effects provided by the latest releases and how personal react to taking gunfire. I say an excellent feature would be to model large calibre gunfire on ground troops. I remember reading in an article some years ago, that a P-47 pilot on a strafing mission late in the war over Germany, was horrified by witnessing his own 50 cal rounds dismembering an enemy soldier manning an anti aircraft gun, he stated that he simply hadn't thought of what his guns were doing outside of shooting down enemy aircraft.
CS.
