Most radio and electronic equipment on airliners are contained in a "box". Usually a rectangular metal enclosure with one or more electrical connectors.
Technically, these removal boxes are referred to as "Line Replacable Units" or just "LRU
". This essentially means they can be replaced with no special tools or procedures on the operational "line" for maintenance purposes.
The average A & P mechanic or AME or LAME (or whatever and airplane mechanic is called in your part of the world) has absolutely no idea what goes on inside the LRU
at a component level and isn't authorized to do any repair to the box itself. This is reserved for specialized shops.
The "blackness" of the box has historically represented the mystery of the electronics inside the unit. So, not only flight recorders have been tagged with this "black box" name. Pretty much every LRU
is called a black box.
Of course the media likes buzz words, so they latched on to the mysterious "black box" moniker and use it whenever they can. Of course the public loves the trivia associated with the fact that these particular black boxes are orange....or yellow. The standard allows for yellow but as far as I know, only B and D ever made a yellow flight recorder.
Like anything else in aviation, there is no single answer. So the number of boxes representing the flight recorders vary. For example, an airplane like the King Air 200 only needs a CVR. So there is one black box. If you want to include the audio summing unit that mixes the hot microphone signal, you can add another.
Many light aircraft that require a CVR and FDR will have 2 LRUs as the FDR is typically a type that has built in data conditioning circuits. This is an ARINC standard 542(a) unit. So all the data parameters get fed to it directly.
Larger aircraft typically require a CVR and FDR but due to the desire for fleet compatibility, they use a "dumb" FDR that only records data. An FDAU is fed the data from the various places and converts it in a format that the FDR can record. So this type of aircaft will use 3 black boxes.
Still other aircraft have dual FDR and/or CVR installations to allow for dispatch following the failure of one system.
And still another variation, is the dual recorder that houses voice and flight data recorders. So you would think that an airplane that uses one of these wonderful units would have ONE black box. Guess again....the authorities don't really like these single units because it cuts the chances of finding at least one of the black boxes in half. Therefore aircraft using these have to have TWO installed!