What was found
(CAN) DURING NORMAL TRAINING FLIGHT, VIBRATION WAS FELT AND AUTOROTATION LANDING CARRIED OUT. UPON SHUTDOWN AND INSPECTION OF BLADES, A FIVE (5) FOOT SECTION OF BLADE COVERING WAS DISCOVERED MISSING FROM THE UNDERSIDE OF THE BLADE FROM THE TIP EXPOSING THE WOOD BLADE CORE CAUSING THE ABNORMAL VIBRATION. BLADES WERE REMOVED AND AIRCRAFT TRANSPORTED BACK TO THE MAINTENANCE FACILITY. NTSB WERE ON SITE SHORTLY AFTER AND REQUESTED THAT THE BLADES BE SHIPPED OFF FOR A DETAILED EXAMINATION. (TC NR 20071026006)