Wilhelm is right on or you could also do it at 128 if you don't mind a slower trip, but you should also check the efficiency of your watch crew. If the green bar isn't full then get someone Watch qualified (a little gold star), to get it up as high as you can. Helps if you wacht offizer is also watch qualified.
Get someone Flak qualified as well and you might be able to shoot a few down before they bomb you.
As a shortcut to 1 speed, hit the 'C' key to crash dive immediately you hear them then hit "F2" to get to the control room or 'F7' to get some damage control going.
Don't mess about trying to have a look for them, unless you are going to stay on the surface and fight.
(Hope I'm not telling you how to suck eggs on this but I know what it was like when I first got hit by an A/C attack without warning. sounds like you are playing late in the war when the air attack is your biggest threat you could try for earlier in the war about 41 or early 42 to get some experience dealing with planes before going later in the war because it gets nastier the later you go).