The best thing you can do if you have the time is give ATC as much info as possible but if you are transponding change your squawk to 7700.....this will then at the very least alert D and D to the fact that an aircraft has a problem and even if no ATC unit is talking to you tracing action can then be initiated....the great thing about the 7700 squawk is that it will remain on the radar screen till manually cancelled....so if you were going down fast and descending quickly below radar cover the SOS label will remain on the radar so an accurate position of the aircraft in distress can be worked out!
