What was found
(CAN) DURING A TRAINING FLIGHT POWER WAS ADVANCED, THE ENGINE LOST POWER AT THIS POINT AND STARTED TO VIBRATE. THE AIRCRAFT IMMEDIATELY RETURNED TO THE AIRPORT AND LANDED SAFELY WITHOUT FURTHER INCIDENT. UPON INSPECTION OF THE ENGINE IT WAS DETERMINED THAT THE HEAD OF THE EXHAUST VALVE ON NR 2 CYLINDER HAD SEPARATED FROM THE STEM. BECAUSE OF INTERNAL DAMAGE TO THE CYLINDER IT WAS CONSIDERED UNSERVICEABLE, AND A COMPLETE NEW CYLINDER ASSEMBLY PN TBN12-OCA WAS INSTALLED. THE ENGINE WAS TEST RUN AND THE AIRCRAFT RETURNED TO SERVICE.