On modern Boeing aircraft, there is a box called Digital Data Aquisition Unit (short: DFDAU), which is connected to all aircraft systems containing electronics via an ARINC 429 serial datas bus. This unit collects all sorts of information, sorts it and sends part of it (the mandatory data, as required by the authorities) onward to the Digital Flight Data Recorder (FDR) for accident investigation. There exists also the possibility that the operators install an additional non-mandatory software in the DFDAU, which then collects additional data and sends it to a Quick Access Recorder (QAR) to store it on removable disks for maintenance purpose or a transmission encoding device like ACARS or, what we have on our planes, a Wireless Quick Access Recorder (WQAR), which, while on ground only, uses the Vodaphone GSM cellphone network to send the accumulated data back to maintrol.
ACARS in turn converts the data to it's own transmission format and uses either a dedicated VHF radio or SATCOM to transmit it to a ground station while inflight.
The data collected consists of trend monitoring of engine parameters, exceedences and, lately also information about APU use on ground for economical reasons (pilots and ground crew are to use ground power if possible because it is cheaper. Any APU use of more than 20 minutes requires a justification, e.g. no ground power available, which would be an issue between the airline and the ground handler, who by contract has to provide ground power, or maintenance).
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