What was found
HORIZONTAL STABILIZER INBOARD LEADING EDGE RIBS CRACKED. REMOVAL OF HORIZONTAL STABILIZER INBOARD RIBS REVEALED ONE PIECE 0.5 SQ INCH OF FLANGE COMPLETELY CRACKED AWAY STARTING FROM APEX RELIEF HOLE. THIS AREA IS NOT NORMALLY VISIBLE FOR INSPECTION. ONE OTHER FLANGE CRACK EXTENDED 12.7MM (0.5IN) FURTHER UNDER SKINS. THIS AREA IS ALSO NOT NORMALLY VISIBLE FOR INSPECTION. IN TOTAL, FOUR CRACKS (THREE RT AND ONE LT) WERE FOUND MAKING THE STABILIZER UNSERVICEABLE.