[Vince82]

U-flak is not realistic in silent hunter.

From the GWX manual:

The U-flak proved disappointing: the extra-large anti aircraft battery meant the U-boat had to carry 20 extra men and several tons of ammunition in order to operate them, which left little room for food, torpedoes, and sleeping space. This in turn limited these boats to employment in the Bay of Biscay, where Allied aircrews soon learned to coordinate their attacks, coming in from different directions in order to split the boat‘s fire.

I believe that the allies would radio for planes with rocket or surface forces aswell when a U-flak was spotted.