Sperrle's air fleet received "Fliegerkorps" I in late August 1940. The corps contained specialist units equipped with Y-"Verfahren" (Y-Control), a night navigational aid which precipitated the Battle of the Beams. Several of Greim's air corps were assigned pathfinder units from KG 26 and KG 55 known as "Beleuchtergruppe" (illumination group). Sperrle had spent the last week of August and first week of September gearing up for large–scale night operations. Sperrle's air fleet assisted in the beginning of The Blitz which began in earnest on 7 September 1940. This night approximately 250 aircraft dropped 300 tons of high explosive and 13,000 incendiaries on the centre of London.