At 06:45 on 6 April 1941, the "Luftwaffe" launched Operation Retribution, a series of concerted bombing attacks on Belgrade that coincided with air and ground attacks throughout the country. Several waves of German aircraft approached Belgrade during the day, initially Junkers Ju 87 "Stuka" dive-bombers escorted by fighters. About 08:00, Hurricanes of the 52nd Fighter Group engaged the second wave as it departed after bombing the city; one of the dive-bombers was shot down by three pilots from the 163rd Squadron. For the remainder of the first day of the invasion, the Hurricanes of the VVKJ saw little action, despite constant patrolling between Čačak, Kraljevo, and Kragujevac. Few of the aircraft had radio sets, so the fighters usually arrived too late to take part in the fighting. Two 4th Fighter Regiment machines were tasked with escorting Bristol Blenheim Mk I light bombers to attack targets in Austria, but they lost sight of the bombers in cloud cover. One of the pilots then attempted to intercept some German Messerschmitt Bf 109 fighters.