What was found
DURING 100-HOUR INSPECTION, CRACK WAS FOUND AT THE TOP CENTER FUSELAGE BULKHEAD IN BEND RADIUS AT FS 172.0 WHERE DORSAL FIN ATTACH FITTING IS RIVETED. AIRCRAFT WAS USED FOR SKYDIVING OPERATIONS AND ESTIMATE LONG, FULL POWER OPERATION AND PROPELLER VIBRATION CAUSED CRACK. SUBMITTER RECOMMENDS THOROUGH INSPECTION EACH 100 HOURS OF AREA. AN APPROVED CHANNEL DOUBLER WAS INSTALLED FOR STRENGTH.