What was found
DURING A ROUTINE 150 HOUR INSPECTION IT WAS DISCOVERED THAT THE COMPOSITE PANEL BELOW THE OIL COOLER BLOWER, AS WELL AS AN AIRFRAME LONGERON AND STRINGER BELOW THE PANEL EXHIBITED STRUCTUAL CRACKING. THE DEFECT OCCURRED DURING UTILITY FIRE FIGHTING OPERATIONS UNDER NORMAL FLIGHT PARAMETERS SOMETIME WITHIN THE LAST 150 FLIGHT HOURS. NO ABNORMAL VIBRATIONS OR ANOMALIES WERE REPORTED BY THE PILOT PRIOR TO DISCOVERY OF THE ISSUE. THE PROBABLE CAUSE IS EITHER AN IMBALACE IN THE OIL COOLER BLOWER OR A FAILED TAIL ROTOR DRIVE SHAFT BEARING.