Aircraft system functionality is constantly monitored and recorded throughout the complete flight cycle from engine start to engine shut-down.
Systems such as auto-pilot/flight-director, hydraulics, pneumatics, fuel, flight controls, and many others usually have a controller of some type. Controllers use subsystem response and feedback to verify proper operation and if need be, reconfigure or render inoperative a failed component/system.
Flight Control Computers and System Controllers store faults that can be retrieved for troubleshooting by maintenance.
The flight recording system functions in a slightly different manner. It receives and records aircraft data, but plays no role in operation. Flight recorders basically receive data from "all" major systems installed on the aircraft.
The main workhorse in retrieval of this data is the DFDAU or Digital Flight Data Acquisition Unit. The DFDAU can receive digital, analog, or discrete signals.
lists 64 Arinc-429, 54 analog, and 180 discrete inputs are capable on their DFDAU unit.
The DFDAU sends this data to the Flight Recorder which stores 17+ hours of data in a continuous updating loop. The DFDAU can also send data to the
system for real time operation and fault reporting. Aircraft without ACARS often have a QAR or Quick Access Recorder which records data to a retrievable storage card.