Machinbird;
I would like to call attention to some anomalous indications in the DFDR trace.
. . . .
First, look at the position of the R & L elevators. Wouldn't you expect both elevators to be moving synchronously? But just before 14:48:10, the L elevator (brown trace) follows a stick pulse, but the R elevator (blue trace) does not. Then at 14:48:12, the R elevator responds to a stick input, but the L elevator does not. This split behavior continues until 14:48:17 at which point the aircraft has apparently flown out of the turbulence and is in a recovery phase.
There is an alternate explanation which might help. It comes from the way flight data recording works.
Remember how popular strobe lights were? Let's put two such strobes in a dark room where a lot of people are dancing to disco music...(yes, I remember). Let's point one strobe one way and the other in the opposite direction, (left and right); let's pretend they're shielded so they dont' light up the whole room but just where they're pointed.
Let's then fire one strobe a bit later than the other and film the position of the dancers lit as the first strobe fires, and compare their positions with the dancers lit by the second, later-firing strobe and ask, Are the dancers lit by the second strobe in the same position they were in when the first strobe was fired?
We know by now that they're not. Dancers are always moving!
Are the second group of dancers "lagging" behind the first? Again, no, we just see them slightly later because of the "snapshot" way the data was taken and recorded.
The same happens with digital flight data recorders. The process is sequential, not all at once.
So we know each parameter has it's place and time of recording and it is different than all the others.
All aircraft parameters are also generated and recorded at different rates per second.
Fuel quantity is recorded every four seconds because a faster rate isn't needed to notice change. Heading is recorded once per second, roll is recorded at twice a second, pitch at 4x/sec, while vertical 'g', which changes rapidly, is recorded up to 16x per second and sometimes higher.
These "frame rates" are a matter of design and software programming. QARs, (Quick Access Recorders used in FOQA/FDA programs) often record
many more parameters and at far faster frames per second than DFDRs or SSFDRs.
The key point is, parameters can't be recorded all at once. If there are 1800 parameters (in binary form) coming into the system, the system must have a way of "listening, parsing and recording". The data frame software is how that process is handled.
As the second begins at '0' and proceeds to '1', the data frame runs through all parameters, sometimes "flashing the strobe" 16x a second, sometimes less, sometimes not, and then places the binary data received in the data frame cells (much like a spreadsheet...simple data frames are 4 columns, 64 rows, filled each second), at the programmed recording rate.
Without getting more complicated, (because it has to if we go any further and everyone will be asleep), the nature of recording can give the appearance of a 'lag', when there "may, or may not" have been one.
I say "may or may not", because there is one more thing to know.
As with the strobe light, when the lights are "off", no one knows what position the dancers are in, until the next strobe fires. This is equivalent to the position of the aileron or sidestick, etc, not being recorded, even when these devices will always have a certain position.
Further, one can make absolutely no assumptions whatsoever, about the positions of the dancers, while the lights are off. Similarly, in the time between the snapshots of the left aileron position and the right, one can make no assumptions about what these controls were doing while not being recorded, even as they logically had a position at all times.
If the sidestick (recorded on the A319/A320/A321 at 8x second in most frames) is moved so rapidly that the frame rate can't catch every important position, then what we see in the data may be misleading. A rapid sidestick movement full forward then full aft in about a second will not look like a smooth fore and aft movement and, depending when in the entire recording sequence such motion was made, the data may not show full aft or full forward because "the strobe was off at full deflection"...
So it is with all aircraft parameters.
To introduce what will be a reasonable complexity, there may actually exist some lead or lag in the flight controls. But there is no way to tell as slower frame rates and ground testing is the only process that can answer the question.
I haven't studied the Air Canada traces so can't comment in detail but this kind of explanation is the way recording works.
This isn't to say that the recordings aren't still extremely useful, obviously. But it takes experience and training, and in cases as fine-tuned as these it would be up to the maintenance people and the aeronautical engineers to say whether the positions of the flight controls, as they relate to the side-stick and rudder pedals even with the lag, affected control of the aircraft in a certain way vice another way.
This is why I think, even when the recorders are found and read, (which I am confident they will be), that the real discussion will just be starting.