a few times the total surprise by aircraft has happened to me. but i would say only something like 2 out of 10 times if that!
most of the time the watch crew spots the planes in time for me to at least get to 30 meters depth. I like to run on the surface with just the watch tower above the waves - this reduces the dive time.
I dont like to duke it out with aircraft unless repairs or something of that nature require me to be surfaced. I find that the tactic of waiting until the aircraft starts its dive down on you - hold your fire till then - almost 100% of the time results in a downed aircraft. I played a short late war campaign recently where "testing" this method resulted in 46 enemy planes downed... the price of course over the course of 13 patrols was 5 crewmen
Right now Im playing with GWX and SH3 commander too, March 1940, in base before my third patrol with 84 % realism. (Sorry i like the outside views too much hehe)
Just ran across the HMS Rodney in the middle of a convoy a while ago, closed to within 700 meters undetected- shot a spread of 4 at her, brilliant explosion followed, 36,000 tons to the bottom. managed to get a stern shot off at a large tanker. Both ships went down.
shadowed the convoy for another day, and went in for another attack the next night, got an ammunition ship, and a small tanker.
went deep and got the heck out of dodge after that.