Time stamps and DFDR/CVR synchronisation
Quote from me:
"...I'm still inclined to the theory that TGEN 000.0 seconds is most likely to be the instant that TOGA was selected on T/O."
Quote from CONF_iture:
"It would be surprising that TOGA or FLEX has been selected before any takeoff clearance..."
Yes, I should have done the arithmetic! That puts TGEN 000.0 seconds at 1240:05z, and T/O clearance was not issued until 1240:40z.
We know that movement of the throttle levers through the various thrust-limit gates, as necessary for the selection of TOGA from idle, is audible on the CVR. My assumption was that TGEN 000.0 would have marked a point of synchronisation between the CVR and the DFDR, but there is nothing in the CVR transcript - such as a lever movement, or the start of a transmission on the R/T - to suggest any audible event at 1240:05z that would have been simultaneously recorded on the DFDR.
So it may be that TGEN 000.0 is a purely arbitrary point. The Airbus 1991 document explains that the CVR is not time-stamped, but that the CVR records at a steady speed. I infer that a minimum of two synchronisation points some minutes apart would be necessary, to enable the CVR and DFDR recordings to be synchronised for the whole flight. The selection of TOGA a few seconds before impact would appear to be one ideal example. **
BTW, I'm still reading Capt Asseline's account. My PDF is missing the Annexes. Can anyone steer me towards obtaining them, please?
** The selection of T/O thrust on take-off would be another.
Last edited by Chris Scott; 23rd Jan 2014 at 18:40.
Reason: ** added.