Date: 201707
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Dusk
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Aircraft component: Aileron
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Distraction; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Returned To Gate; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Following an uneventful pushback we accomplished the After Start Checklist. Once the flaps were in the takeoff position I accomplished the flight control check. Starting with the Ailerons in the neutral position I went to throw in full left aileron followed by full right. About 30 to 45 degrees to the left I felt as if something gave way in the ailerons just as the aileron abruptly snapped to full left deflection and stuck there. Normal switch positions for hydraulics; autopilot; etc for this point of the flight. We called maintenance on the radio and explained the situation. They asked us to confirm our switch positions and clarify what happen. They then asked us to see if we [could] move the ailerons back to neutral. It took both hands and considerable force to move the ailerons to neutral. As I went pass neutral to approximately 30 to 45 degrees to the right the ailerons abruptly snapped to the full right position and stayed there. We then confirmed that we had normal nose wheel steering which we did so taxied back to the gate. Maintenance then came on board and confirmed our findings as well as took pictures of the yolk and videotaped the movement since this incident was unusual and not a single mechanic up to the supervisor had ever seen such an event. On inspection of the flight control cables it was discovered that the aileron centering mechanism had failed.  I must point out we could still move the ailerons using considerable force but as the mechanic pointed out if this happen in flight and this piece was flapping around it could have entangled in the flight control cables in which case we would have been done. Weather in the area was rain showers following thunderstorm passage. This failure if it had occurred in flight could have been a serious incident. I believe at the very least we would have been looking at an upset recovery followed by a structural controllability check. In order to move the ailerons to neutral it would have taken both hands which would have meant it would have taken both pilots to land the airplane safely. We either would have had to switch controls due to fatigue or possibly have one fly the control wheel and the other the throttles or combinations thereof. We no doubt would have had our hands full. I would hate to think what would have happened if the failure occurred low to the ground either on takeoff or landing. This also highlights the need to do a thorough flight control check on the ground. We as a crew were very lucky.




Synopsis: A B757 Captain conducted the flight control check after pushback and discovered after the ailerons was moved about 30°-45° left or right the yoke snapped full deflection in that direction. Maintenance determined the aileron centering mechanism had failed.