|This article does not cite any references or sources. (April 2012)|
The term was used during the Battle of Britain, when RAF fighter pilots waited on the ground for Chain Home radar observations to detect oncoming enemy aircraft, at which point a telephone call would reach each airfield (part of the Dowding System) and those air crews available would be scrambled. Every minute lost before take-off would be advantageous to the enemy, as those minutes could have allowed a pilot to gain extra height above the advancing plane formations. Information passed to the scrambling fighters included location and height (Angels - hence phrases such as Angels One Five for aircraft approaching at 15,000ft) and a rough estimate of numbers. Unidentified aircraft were known as bogeys while known enemy ones were called bandits. The scramble order was communicated to alert pilots at the base by the loud ringing of a bell.
During the Cold War, many NATO air forces had crews stationed in Europe on alert and scrambled whenever their airspace was penetrated. The rudimentary bell-ringing communication was eventually replaced by electronic radio communication methods. But many fighter squadrons into the current era would keep a bell at their squadron bar in legacy to the Battle of Britain roots. A common tradition was that if anyone at the bar would ring this bell, they would be required to buy a round of drinks to all present. This tradition was a remnant of the serious consequences of communicating a false scramble order.
Both interceptors and nuclear bomber forces were kept on "Quick Reaction Alert". Crews were kept close to - or in - their aircraft which were expected to be able to take off within a short period eg 15 minutes or less during normal situations, down to two minutes at times of heightened tension between the opposing powers.