What was found
BOLTS ATTACHING THE UPPER SHOCK STRUT ASSY TO THE UPPER GEAR HOUSING HAD SEVERE STRESS IN THE THREAD AREA. THIS WAS CAUSED BY REPEATED OPERATIONS IN FLIGHT TRAINING. BOLTS FAILED ALLOWING GEAR LEG TO EXTEND AND UPON RETRACTION, JAMMED ON LOWER WING SKIN RESULTING IN A GEAR-UP (2 EXTENDED) LANDING. SUBMITTER SUGGESTED REMOVING BOLTS PERIODICALLY TO CHECK THREAD AREA FOR DISTRESS. HAVE FOUND 6 OTHERS IN FLEET TO HAVE SAME DAMAGE TO THREADS.