What was found
(CAN) DURING A TRAINING FLIGHT, ON OVERSHOOT FROM A ABORTED LANDING, FLAPS RETRACTED UN-COMMANDED. PILOT REDUCED POWER AND WAS ABLE TO LAND STRAIGHT AHEAD ON REMAINING RUNWAY. INSPECTION FOUND THE FLAP ACTUATOR STRIPED. ACTUATOR WAS REMOVED AND SENT FOR OVERHAUL. OVERHAUL FACILITY REPORT THAT THE STRIPPED BRASS NUT WAS MADE OF A SOFTER THAN STANDARD MATERIAL. THERE WAS IN EVIDENCE IN THE AVAILABLE LOGS OF THE TIME IN SERVICE SINCE NEW OR OVERHAULED. OVERHAULED ACTUATOR WAS RE-INSTALLED IN ACFT.