There are bomber planes (Sunderlands?) which are very performant in AOD, they can drop 3 bombs in 1 overfly. Recently I was spotted by one even when going only half speed, I was too late to dive, if you dive when the plane drops its bomb you may sink. My deck man manged to destroy the plane, but its 3 bombs were dropped anyway, I had a direct hit that destroyed my aft diveplane, my diesel machines were damaged for 11 hours in 2 different sections.
All in all I was very fortunate, a direct hit could have ended the game for me
The most dangerous airplane, at least for me, is the Catalyna. This plane will drop acoustic torpedos that you will have a very hard time to evade, even if you can see them on the map. I have tried with a Bold but stupidly I didn't change my depth quick enough, so I got hit. Your turn rate is not quick enough at 7 knots...It was during my experiments, so I got no damage, but with realistic settings it would have been the end, or out-of-action for my sub...