What was found
DURING A ROUTINE TRAINING FLT FOR A-CLASS IV INSTRUCTOR RATING, WINDSHIELD CRACKED AND BLEW IN. PRIOR TO WINDSHIELD BREAKING, INSTRUCTOR CANDIDATE HAD BEEN PERFORMING SPIRAL DIVE LESSON DEMONSTRATION WHEN THE WINDSHIELD BROKE, AND HAD JUST COMPLETED A RECOVERY. DURING FLIGHT, AIR SMOOTH & MAXIMUM SPEED ESTIMATED AT 140 MPH (WELL BELOW THE VNE OF 162 MPH) AND AT NO TIME WERE ANY EXCESS G FORCES EXERTED ON THE AIRFRAME. PRIOR TO THE BREAKAGE, THERE WAS NO EVIDENCE OF A CRACK OR DAMAGE TO THE WINDSHIELD. AIRCRAFT WAS INSPECTED FOR OVERSTRESS OF THE AIRFRAME WITH NO FAULT FOUND. PIECES RECOVERED OF THE WINDSHIELD SHOWED NO SIGN OF HEAVY CRAZING OR STRESS. WINDSHIELD REPLACED AND AIRCRAFT RETURNED TO SERVICE.