Originally Posted by
Gary Brown
Thanks - I'd missed that sentence about the CVR, Para 19:
19.The CVR recording was found to be of good quality and covered complete flight. However, FDR included the data of the entire flight except after the time when electric power was not available for the FDR. This is as per its designed limitation.
So I guess Airbus and PIA had ?retro?-fitted the 2108 recommended independent, 10 minute-after-main-power-failure battery pack, which I believe had been preferred to RAT-derived power.
No firsthand knowledge of how Airbus does it, but on most Boeing aircraft the FDR data path is fairly complex, with several systems involved. In short, you don't need to just keep the FDR powered, you need to keep the entire data path powered or you'll simply be recording garbage (I spent plenty of time over the years reviewing FDR data as part of investigating various incidents - I could usually tell when the IDG dropped or came on-line because there would be a few frames of garbage data associated with the electrical power transfer). The CVR is straight forward - basically just the microphones and the recorder - so adding a separate battery to that system would be relatively simple.
BTW, again no first hand knowledge, but I'd be a bit surprised if the A320 RAT has electrical power generation capability (at most a simple PMA dedicated to the FBW). For all engine out, most aircraft depend entirely on the battery to power the critical electrical systems. Adding
anything to the battery load pretty much requires an act of god - battery power is simply to precious.