Anti-aircraft fire was often referred to as 'ack-ack'. This is generally thought to come from the WW1 phonetic alphabet for the initials A.A. for anti-aircraft (or anti-aircraft artillery), although it was later claimed to come from the German Acht-acht, meaning 8-8, because most of the anti-aircraft guns in WW2 fired 88mm shells.
When the shells from these guns exploded they brought down planes either by direct hits or by the blast and fragments of shrapnel. An exploding shell within 30 yards would usually bring down an aircraft but serious damage could be inflicted within 200 yards and many airmen were killed by flak in aircraft that returned relatively undamaged. A fuselage made from aluminium alloy** less than three quarters of a millimetre thick would have given no protection to the crew inside - the only armour plating protecting the crew was behind the pilot's seat and head . A direct hit on a full bomb load would have left very little of the aircraft or crew.
German searchlights were fitted with sound locators and the guns had increasingly sophisticated devices to improve their accuracy, including shells fitted with transmitters so they detonated when within a pre-determined distance from an aircraft. One particularly lethal method was to fire ‘box barrages’ – up to 40 guns would fire 15 to 20 shells per minute to explode in a sphere about 60 yards in diameter at the height that radar or ‘predictors’ (primitive computers) indicated the bombers were flying.
Anti-aircraft guns brought down more aircraft than fighters did, particularly in 1944 and 1945 when the Luftwaffe was at its weakest, but it has been estimated that the Germans fired 3000 shells for every plane that was shot down. Allied success rates during the blitz were considerably poorer than this.
|A Lancaster on its bombing run facing heavy flak|
**The material used was 'Alclad', a composite that was stronger and more resistant to corrosion than previous light-weight alloys. Alclad is formed from high-purity aluminium surface layers metallurgically bonded to a high-strength alloy core made from heat-treated aluminum, copper, manganese and magnesium. It has the corrosion resistance of pure metal at the surface and the strength of the strong alloy underneath (see these Technical Notes from the US National Advisory Committee for Aeronautics). The first aircraft to be constructed from Alclad was the all-metal Navy airship ZMC-2, constructed in 1927. The Alclad sheets covering most of the airframe of a Lancaster were 22 SWG (0.71 mm thick), although some were 16 SWG (1.63 mm).