Air search, visual detection, and the submarine mission kill

The earliest airborne ASW sensor was the observer's eyesight because submarines spent so much time exposed. A forced dive was already operationally valuable: the boat became slow, nearly blind, and unable to pursue most targets. Reporting the sighting also allowed surface forces to converge on the area even when the aircraft did not sink the submarine itself. World War II radar later extended the same search principle by detecting surfaced boats and exposed periscopes.