What The 4-Digit Code Every Commercial Aircraft Broadcasts Actually Tells Air Traffic Control

Every commercial flight in the world is assigned a four-digit code before it pushes back from the gate. The code appears on air traffic control radar screens wherever that aircraft travels, and three specific combinations (7500, 7600, and 7700) trigger automatic visual alerts on every controller display within range the instant they appear, without the pilot saying a word. But the code is not a decimal number. It never uses the digits 8 or 9. Which means the four digits on the transponder dial do not generate 10,000 combinations but only 4,096.