Pilot reported a rejected take off at 80 knots due to the #2 engine not producing required thrust. This was accompanied by aircraft yaw and N1 fluctuations. The flight crew returned to the gate for maintenance.

Narrative

This was day 3 of a 4 day pairing. Current day was 2 segments: the first out of ZZZ to ZZZ1 and return. We were to utilize the same aircraft for the day; Aircraft X. For the flight we accomplished and completed all checklists; briefings; procedures as per SOP. The Captain was the Pilot Flying. The weather in ZZZ was VFR; departure Runway was XXL. During the takeoff the Aircraft performance did not present any issues until late in the takeoff roll I began to notice that the aircraft was behaving outside my normal expectations. I was not sure of what was presenting itself. Once airborne and at the appropriate altitude I selected the autopilot on to evaluate the aircraft. On a downwind heading I referenced the Upper ECAM DU and realized the N1 for the #2 engine was fluctuating causing the Aircraft to yaw with each oscillation of the N1. Since the engine was producing power and within parameters. I determined we would continue the climb and assess the situation at a safe altitude. Approximately 5;000 ft. AGL the auto thrust was selected off the thrust levers were adjusted to see if that would stabilize the N1. That did not work next airspeed was pulled and again that did not stop the fluctuations. Above 10;000 ft. when the aircraft started to accelerate the N1 stopped fluctuating and stabilized. During this period there was no ECAM caution or warning. The Engine stayed within normal operating parameters. I called back to the inflight to see if they had noticed anything unusual about the flight so far or if any passengers had. I was informed that they had noticed that the aircraft was acting differently but at the present time no passenger had mentioned anything. Next the QRH was referenced for any guidance. Dispatch and Maintenance was contacted via SATCOM. Informed them of what had occurred. Maintenance was able to determine that one of the FADEC channels had received a faulty signal and switched over to the other FADEC channel stopping the N1 fluctuations. Because the aircraft was now performing as expected within normal operating parameters Maintenance said our aircraft was safe to continue to destination. The Dispatcher agreed with Maintenance only if we were comfortable as well. Both I and the First Officer discussed the situation. Agreed to continue the flight to destination based on current performance being within normal operating ranges; with no warnings or cautions ever presenting themselves. The rest of the flight was uneventful; all Checklist; briefings and procedures were accomplished per SOP through the Parking Checklist. Upon shut down the ECAM had a Class II maintenance Message for ENG 2 FADEC. A log book entry was made and Dispatch was contacted Via SATCOM. Maintenance was performed where a reset on the FADEC cleared the message. The return flight XA10 ZZZ1 to ZZZ. Accomplished and completed all checklists; briefings and procedures through the Before Takeoff checklist per SOP. ZZZ1 tower cleared us for takeoff on Runway XY. Verified runway and a clear final; taxied onto the runway and commenced takeoff by setting the thrust levers into the MCT/FLEX power position per SOP. Airspeed alive call was made. By 80 kts. the #2 engine power was fluctuating and would not maintain a flex thrust power; along with slight yaw movements in relation to the fluctuating power. (N1 was fluctuating between 10 - 20% and would not maintain targeted flex thrust). The decision was made to reject the takeoff. The rejected takeoff actions were completed as per SOP. Tower was notified of the rejected take off. PA was made for crew and passengers. We taxied clear of the runway. Evaluated the situation to determine if the aircraft was safe to taxi back to the gate. The landing checklist was completed. With the aircraft determined to be safe we taxied to a gate. No assistance was required. Another PA to inflight and passengers was made regarding the events. ZZZ1 operations were notified. An ACARS message was sent to Dispatchregarding the event. At the gate the aircraft was shut down; parking checklist was completed as per SOP. The logbook was filled out regarding the power fluctuations on #2 engine and subsequent RTO.Dispatch was contacted via SATCOM and a Maintenance Manager was informed of the occurrence. The plane was removed from service and the passengers were deplaned.