What was found
DURING 400 HR PROGRESSIVE INSP OF THE LT HORIZ STAB, CRACKS WERE FOUND AT THE LEADING EDGE RIBS OF THE HORIZ STAB AT STATIONS 66.108, 80.608, AND 95.108. CRACKS WERE NOTED WHERE THE RIBS ATTACH TO THE LEADING EDGE SKIN. THIS AC HAS A TT 5151.2 AND 1146.6 TIME SINCE CONVERTED TO A TURBOPROP POWERED FIRE FIGHTING AC. THE LT HORIZ SHOT DAMPER WAS REMOVED DURING TURBOPROP CONVERSION. IT WAS ORIGINALLY INSTALLED TO COUNTERACT THE ORIGINAL RADIAL ENGINE INSTALLATION 3 BLADED PROP HARMONIC VIBRATION AT 2300 RPM. CRACKING COULD BE CAUSED BY VIBRATION FROM TURBO PROP INSTALLATION. ACTUAL CAUSE OF CRACKING HAS YET TO BE DETERMINED. LT HORIZONTAL STABILIZER LE SKIN RIBS SHOULD BE INSPECTED FOR CRACKS.