i think that aircraft will keep returning to your last spotted position. I think they even estimate where you _will_ be based on your course and speed. After being detected by planes, I went under and travelled with the observation scope up for a day. For the first few hours, planes returned to my position, even though i had moved quite a bit since the first sighting. So I think they guess your course. Once you've had a good fight, travel underwater for half a day and you will probably lose them.
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