My expectation would be for the helicopter do ditch all the weapons and go for the nearest landmass ASAP, assuming it doesn't have a clear contact on a hunted sub. That's why it was so frustrating to see a helicopter following me to the edge of the map (the only way to end the mission with that thing flying around). These days it seems it's a lot easier to loose those pesky helicopters and I did get better at not being detected in the first place
In any case, I really have no clue how a real heli crew would deal with that situation, that's why I'm asking