C172 reported severe aileron problem while downwind for runway 28 in Charlottetown. Aircraft landed safely. TSB case close more to follow : Update: The pilot reported that control yoke input to the left was very difficult and occasionally a "snap" would emit from overhead in the cockpit. Simultaneous to the "snap" sound the aircraft would tend to roll to the right. Maintenance completely inspected the Aileron system and no problems were evident. Upon further investigation it was determined that the wing levelling system was switched to the "On" position and the right/left bank knob was adjusted to a right bank. The system was tested as set in the hangar. When the yoke was moved to the left it took considerable force and a "snap" sound could be heard as the manual input was overriding the Roll Servo. The operator suspects the pilot inadvertently and unknowingly activated the wing level system while carrying out the pre-landing checks. TSB file # A00A0011 Update: Private Pilot building experience toward the CPL, so no flight training concerns. Since there is potential for this to happen to a trainee (inadvertent activation of the autopilot in flight) the CFI was asked to address the issue. This FTU presently offers instruction on how to use the autopilot as part of their PPL training syllabus. It seems this pilot did not fall back on that training, so suggestions were made to change the pre-take off and pre-landing checklists to ensure the autopilot is turned off. It was suggested that the CFI look into the development of an emergency procedure checklist that would be initiated when a pilot encounters stuck ailerons, directing them to turn off all autopilot modes. CFI will check the POH and autopilot supplements for procedures and, if possible, will develop a checklist for this emergency situation.