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What is a HEX code?
Mode S equipped aircraft are assigned a unique ICAO 24-bit address or (informally) Mode-S “hex code” upon national registration and this address becomes a part of the aircraft’s Certificate of Registration. Normally, the address is never changed.
However, the transponders are reprogrammable and are occasionally moved from one aircraft to another, either by maintenance or by changing the appropriate entry in the aircraft’s Flight management system.
As an example, the following is the ICAO 24-bit address assigned to the aircraft with the registration N905NA.
- Hexadecimal: AC82EC.
- Octal: 53101354.
- Binary: 101011001000001011101100 (Note: occasionally, spaces are added for visual clarity, thus 1010 1100 1000 0010 1110 1100 {Hex little endian} and 001 101 110 100 000 100 110 101 {Octal big endian}).
- Decimal: 11305708.
An issue with Mode S transponders arises when pilots enter the wrong flight identity code into the Mode S transponder.