A321 flight crew reported flight control malfunction during departure climb resulting in a return to the departure airport.
Synopsis
A321 flight crew reported flight control malfunction during departure climb resulting in a return to the departure airport.
Narrative
I was the Pilot Flying (PF); First Officer's (FO) take-off. During the take-off phase on XXR out of ZZZ; we took off behind an aircraft. Once airborne; at approximately 800 ft. the aircraft began an uncommanded slow roll to the left. I responded by displacing the FO's flight control to the right with an amount that I believed was equal to counter the left roll. The aircraft did not respond to my flight control input and continued to roll left so I displaced more right stick to counter the left roll. The aircraft continued to roll left; at one point I ran out of right stick input by actually hitting the physical limit to displace the FO side stick. The aircraft began to slowly roll to the right; and I began to think that the aircraft was not responding in kind to my right side stick inputs. I began displacing my side stick to the left slowly moving to center my control stick thinking that the aircraft was responding now. The aircraft had not achieved wings level before the aircraft began another slow roll to the left. I countered with more right side stick and again the aircraft did not respond in kind to my right side stick input to counter the left roll. I hit the physical limit of travel on my control stick again; only briefly before the aircraft began a slow roll back to the right to wings level position. The whole time we were climbing out. By this time we were above 1000 ft. AGL; I was still hand flying. The aircraft seemed to be responding normally to my control inputs by this time. I activated autopilot 2 and the aircraft responded normally. We brought up the flight control page on the monitor to see if there were any unusual inputs on the page. But there was none. We climbed out on the ZZZZZ departure with a normal climb out in managed mode. The aircraft made all the constraints in managed mode. Once we were not as task saturated; me and the Captain discussed what we should do. The decision was made by both of us to not continue the flight all the way to ZZZ1 if there was a possible flight control issue. We decided to coordinate with ATC for a precautionary landing back at ZZZ; we were not overweight; we made an uneventful landing; and taxied the aircraft back to the gate to hand the aircraft over to maintenance. Just a side note; there were no electronic warnings that would've let us know that there was a problem with the flight controls. Not sure what causal factors could've caused this. Again; I don't know what caused this so I don't know how to prevent it from occurring again.
Second reporter narrative
During the take-off phase at ZZZ; passing approximately 800 ft. AGL; the aircraft began an un-commanded roll to the left. The First Officer (FO) who was Pilot Flying (PF) countered the roll with increasing side stick movement to the right until achieving full right side stick displacement. The left roll was arrested at approximately 15 degrees of bank and began a slow roll back to the right at which time the PF began to move the side stick to the left to re-center and match the aircraft attitude. However; prior to achieving center stick; the aircraft again began rolling left causing the PF to again displace the side stick full right to arrest the un-commanded roll. The un-commanded roll to the left was again arrested and reversed to approximately straight and level; at which time the PF matched the side stick position to the aircraft attitude. The PF then explored all axis's of control input with the side stick; to which all seemed normal.The rest of the take-off phases of flight were normal. We then discussed the event in detail to ascertain an external cause; we could not. The wind was constant with no gusts and there were no other weather phenomenon in the area. We were following an aircraft on departure that could not have generated a wake to cause such an event. Considering also that the aircraft generated no system warnings or messages; we were unable to positively identify the cause of the event. We decided that with an unknown flight control issue the safest course of action would be to land. We then coordinated for; and accomplished an uneventful landing at ZZZ. Collect flight data; pilot reports and any other and all sources available from this type of aircraft to ascertain if there is a flight control issue.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.