Would love to say that there was a technical issue with this aircraft, can't find any justification for that however. The aircraft at all times responded correctly and promptly to the flight crew input.
Countering the stuck spoiler case, the aircraft yaws on the ground to follow the rudder,
"and the rudder is responding correctly to the rudder pedal"
Not so. Sensing rate for the data stream from the 429 bus is 8Hz. The timestamp is common to both data streams for the rudder and pedal.
The data shows that the rudder position appears to be feeding back to the pedals, in what should be an irreversible control system, yet, there are a number of data points that show the opposite, that the rudder has moved before there was a control input recorded. There is the potential for an artifact from the frequency of the sampling, however, there are a number of data points where it is not possible that the control input could have been made in the time to the record of the position that would result in an artifact, where the control is leading the pedal input.
I am surprised to see that in the data. I doubt that a normal person can make a control input that is so fast that it is recorded out of sequence with the following data.
The blue + is the rudder position, the orange X is the Rudder pedal position recorded in the DFDR. This should not be QAR data, so it should have no sampling errors from 429 to 717 or similar, this should be valid data and valid sampling times. In the expanded view below, there is a rational control position that could have a smoothed line on it, per the DFDR analysis done by the NTSB, but then the position of the control surface is preceding the control input, which is not something that should occur. This occurs in the mid acceleration on the runway, and again appears again in the middle of the upset. Is it an artifact? Probably, but it needs to be discounted before the firing squad lines up.
Can a hydraulic control system feedback to the control input? Normally it does not, however, there are possible internal leak cases that could do that.
Good catch. Strange, this got unnoticed with the investigation.
To me, this looks like the hydraulic accumulator(s) got depleted, due to the extreme high flow/volume requested because of the excessive rudder application. Once the accumulator is depleted, the rudder will no longer rapidly follow the rudder pedals (only at the rate the hydraulic pumps are able to deliver). There is no longer enough hydraulic energy for the rapid "extra" rudder deflection, nor for a forced "return to neutral (or opposite deflection)". And just a few seconds later, without heavy rudder inputs, the accumulator(s) got boosted again, and the sloppy issue is over.
Does it look like that happened at the rotate? No, not immediately, however, if and it is a big if, the rudder system was compromised and feeding into the pedals, then how that would show would be dependent on how it has failed. The rudder pedal is fed back the position of the rudder as well, it also does so on both pedals.
It is possible that a rudder issue did actually occur. I doubt it, but the position of the rudder in the middle of the takeoff acceleration leading the recorded pedal position is concerning, and the same happening in the middle of the lateral excursion is also concerning.
Sensor saturation of the pedal, or rudder or both may be in play, but the data suggests a further look could be warranted. Equally, an error in the followup of the hydraulic actuator to the pedal input could be occurring.
All in all, it is an odd bit of data.
......
I certainly can understand that a rudder, not following the pedals, feels like driving a bicycle/motobike/car with partial flat tires....
Not to say, the ailerons/spoliers may have become sloppy too, given these are having the pressure benefit from the same accumulators.
Q: Do rudder, elevators, ailerons and spoilers use all the same accumulator or are the 3 hydraulic systems working in parallel ? The latter would imply, all accumulators were depleted at the same time.
All in all, it could very well be, the captain was right about the "malfunctioning" of the rudder/ailerons, IE operating in unknown territory, though he did cause this issue himself. The handling transfer to the other side was a decision correct for the malfunctioning experience he encountered.