That's no bug, that's a large obnoxious buzzard!
Still I contend that no number of aircraft in the game are actually a danger to a properly handled submarine.
Don't get spotted--these guys love to call in their little buzzard friends for the party and you're the cake.
Stay surfaced for every second you can--survival depends on combat awareness. You can see aircraft long before you can spot them on sonar (little joke there). Draw that 5 mile diameter danger circle around your sub. If the plane will enter that circle, submerge, hit the timer when you pass 40 feet, submerge to 100' in TMO, hit the surface button at five minutes and get back up on top NOW! Then resume cruise speed as usual. Rinse and repeat as necessary.
Without radar you will be occasionally spotted. This will make planes more frequent until nightfall. Let 'em come. They're harmless.Tomorrow will be a better day.
In any event, stay on the surface for every second you can, charging those batteries. This is doubly important in an S-Boat with its super-slow charge cycle. Hiding may feel safe, but I assure you it is needlessly endangering yourself, your boat and crew.