Definition
A numbering system used in digital electronics in which each decimal digit (0 through 9) is represented by its own four-bit binary group. For example, the decimal number 47 is encoded as 0100 0111, where 0100 represents the 4 and 0111 represents the 7.
Plain English
A way for computers and avionics to store regular decimal numbers by writing each digit separately as a small group of four ones and zeros, instead of converting the whole number into pure binary.
Context Anchor
Seen in avionics, electronic equipment manuals, and maintenance information for digital displays, controls, and data signals.
Derivation
From 'binary' (Latin bini, meaning 'two together') referring to the base-2 system of ones and zeros, and 'coded decimal' meaning the familiar base-10 digits have been encoded into that two-state form. The name describes exactly what it does: decimal digits, coded in binary.
Why Pilots Care
Many older transponders, navigation receivers, and cockpit displays use BCD internally. Knowing the term helps when reading avionics manuals or troubleshooting interface issues between equipment that expects BCD signals and equipment that does not.
Analogy
It is like writing each digit of a number on its own separate label instead of treating the whole number as one large value. In Binary Coded Decimal, 25 is handled as a coded 2 followed by a coded 5.
Intuition Check
Binary Coded Decimal is not the same as converting the whole number into pure binary. It codes each decimal digit separately.
Example Sentence 1
The transponder receives the assigned squawk code from the control head as a Binary Coded Decimal signal.
Example Sentence 2
Technicians verify the binary coded decimal format before installing the new air data module.