The Blackburn Blackburn's work included reconnaissance, spotting and directing gunfire. Flying high above the fleet also allowed its crew to search for approaching aircraft with binoculars, extending the reach of the shipboard lookout. This early form of airborne warning relied on an aircraft flying figures of eight over the task group while an observer searched the sky. The quoted exercise result is effectiveness about 40 percent of the time. An estimated 30 percent in real operations is a judgment about likely performance, with no measured operational result supplied. Even intermittent early warning could give the fleet time to send fighters against some attacks before they reached the ships.
Chapter · 39:02-42:59
The Blackburn Blackburn as visual airborne early warning
An aircraft circling above the fleet could extend the lookout horizon and sometimes give fighters enough warning to meet an attack.