What was found
DURING A SCHEDUALED 12 MONTH FUEL SYSTEM INSPECTION IAW MM, THE DEFECT WAS NOTED ON A CUSTOMERS AIRCRAFT. THE BOTTOM INSIDE OF THE AFT FUEL CELL WAS BLISTERED WITH .2500 INCH BLISTER IN DIAMETER, SPACED FROM .5000 INCH TO .2500 INCH APART. THE BLISTERS CONTINUED UP THE SIDE OF THE CELL 6 INCHES WITH THE SPACING BECOMING LARGER BETWEEN BLISTERS. THE FORWARD 2 FUEL CELLS SHOWED NO DEFETS, ALL FILTERS WERE NORMAL, THE 2 INLINE SCREENS AND INJECTOR PUMP WERE CLEAN ALSO. THE AIRCRAFT MFG WAS NOTIFIED AND THE CELL WAS DETERMINED TO BE UNAIRWORTHY AND WILL BE RETURNED TO THE AIRCRAFT MFG. PROBABLE CAUSE IS UNKNOWN, RECOMMENDATIONS ARE TO RETURNE THE CELL TO MFG TO EVALUATE DEFECTS.(K)