What was found
FOUND EXFOLIATION BLISTER ON LT WING SPAR ON LOWER REAR FLANGE OF I-BEAM SPAR; ABOUT 6 INCHES TO 8 INCHES OUTBOARD OF FUSELAGE OR JUST INBOARD OF WS 37.2. CORROSION WAS ABOUT 8 INCHES LONG AND COVERED WIDTH OF FLANGE. THE CRACKED APPEARANCE WAS ACTUALLY LAYERS OF CORRROSION IN THE METAL. SB 789A HAS BEEN C/W TO PROVIDE INSPECTION PLATE AFT OF SPAR. DAMAGE COULD NOT BE SEEN OTHERWISE. RECOMMEND SB 789A BECOME AD TO INSTALL INSPECTION PLATES.