Depends on the flight altitude of aircraft. It would be best to assume ~1 km distance (diagonal distance) from the submarine yielding a 500 meters flight and 100~ meters depth and some more distance spared for horizontal distance.What is MAD maximum depth?
Helicopter based MAD equipment can be more handy in this case. But MAD is pointless for this case, because it is already explicit there is a submarine to be looked for and approximate locations are known.