A TWIN COMMANDER 690 HAD AN ULTRASONIC INSPECTION OF THE FRONT SPAR LOWER SPAR CAP PER SVC BULLETIN. CORROSION FOUND ON STEEL STRAP.

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ON JAN/MON/05; I PERFORMED AN ULTRASONIC INSPECTION OF THE FRONT SPAR; LOWER SPAR CAP ON ACFT X IN ZZZ. THE WORK WAS ACCOMPLISHED PER TWIN COMMANDER SVC BULLETIN 208A. THE PURPOSE OF THIS INSPECTION IS TO DETECT CORROSION BTWN THE SPAR CAP LOWER SURFACE AND A STEEL STRAP. THE LOWER WING SKIN IS ATTACHED BELOW THE STRAP. THERE WERE AREAS WHERE CORROSION WAS DETECTED; THE MOST SEVERE BEING INBOARD OF THE L NACELLE; WHICH WILL REQUIRE A RE-INSPECTION EVERY 12 MONTHS. VISIBLE EVIDENCE OF CORROSION WAS NOTED IN ONE AREA BTWN THE SPAR AND THE STRAP ALSO INBOARD OF THE L NACELLE. THERE WERE ALSO VISIBLE SIGNS OF CORROSION ON BOTH WINGS INBOARD OF THE NACELLE BTWN THE STRAP AND THE SKIN. THIS AREA IS NOT EVALUATED DURING THE ULTRASONIC INSPECTION. THESE CONDITIONS DO NOT RENDER THE ACFT UNAIRWORTHY; HOWEVER; I BELIEVE THE PROPER WAY TO ADDRESS THE CORROSION IS TO DISASSEMBLE THE SKIN AND STRAP IN THE AFFECTED AREAS; REMOVE THE CORROSION AND TREAT WITH A CORROSION INHIBITOR. IT IS MY UNDERSTANDING THAT MY CUSTOMER IS SPRAYING THE AREAS WITH ACF 50 AND RE-INSPECTING IN 12 MONTHS. I DON'T BELIEVE THIS WILL EFFECTIVELY ARREST THE ADVANCEMENT OF THE CORROSION. THIS ACFT HAD BEEN OTS FOR AN EXTENDED AMOUNT OF TIME AND FLOWN IN FROM ANOTHER COUNTRY. ALTHOUGH I WAS CONTRACTED ONLY TO INSPECT THE SPAR CAP; I NOTICED OTHER AREAS THAT REQUIRE MAINT AND FEEL IT WOULD BE A GOOD IDEA FOR AN ASI TO TAKE A LOOK AT THIS ACFT. FINALLY; THIS ACFT HAS NO PROVISION (ACCESS) TO THE SPAR CAP JUST OUTBOARD OF THE FUSELAGE FOR APPROX 5 INCHES ON EITHER SIDE AND COLD NOT BE INSPECTED. THIS CONDITION WAS SO NOTED ON THE CUSTOMER'S RPT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE ACFT OWNER IS PRESENTLY SPRAYING THE AREAS OF CORROSION WITH CORROSION INHIBITOR AND WILL RE-INSPECT IN 12 MONTHS. THE RPTR BELIEVES THIS WILL NOT BE EFFECTIVE AND THE CORROSION WILL SPREAD. AFTER THE ULTRASONIC INSPECTION WAS COMPLETE; THE OWNER WAS GIVEN A RPT ON THE AREAS OF CORROSION AND THE RECOMMEND REPAIR. THE RPTR IS CAUGHT BTWN CUSTOMER LOYALTY AND THE FUTURE OF FLT SAFETY.