AN SA227 IN CRUISE AT FL240 EXPERIENCED LOSS OF ALL COM AUDIO CAUSED BY THE L AND R AUDIO BOXES SHORTED BY METAL FILINGS AND DUST RUBBED FROM THE GLARESHIELD MOUNTINGS.

Narrative

AFTER DEPARTING YQR (REGINA; SK) AND LEVELING FL240; RADIO CHATTER TAPERED OFF ON WINNIPEG CTR. XING SWIFT CURRENT VOR; CHATTER CEASED ENTIRELY. BEFORE XING MEPICINO HAT VOR; CTR SHOULD HAVE HANDED OFF TO EDMONTON CTR. WHEN NO HDOF WAS RECEIVED; WE TRIED TO CONTACT WINNIPEG/EDMONTON/FSS ETC. ONLY ONCE; VERY FAINTLY; WE RECEIVED A FREQ SWITCH FROM WINNIPEG TO SALT LAKE 133.4. NO OTHER CONTACT WAS ACCOMPLISHED. WE CONTINUED THE FLT TO MSO (MISSOULA; MT) ACCORDING TO PROCS SET FORTH IN FAR PART 91 AND MADE AN UNEVENTFUL LNDG IN MSO. AFTER LNDG AND CLRING CUSTOMS WE CALL MSO TWR WHO INFORMED US THERE WAS NO PROB WITH CTR OR MSO CTLRS. THE ONLY QUESTION WAS FROM CTR; WONDERING WHY THE SQUAWK WAS NOT CHANGED TO 7600. NOT BEING ABLE TO DETERMINE THE CAUSE OF THE PROB; I ELECTED NOT TO CHANGE ANYTHING MORE THAN NECESSARY. THE AVIONICS SHOP AT MSO DETERMINED THAT THE BAKER BOX AUDIO PANELS; INSTALLED AT THE TOP R AND L SIDES OF THE GLARESHIELD; HAD RUBBED ON THE GLARESHIELD AND GENERATED A LOT OF FILINGS. THE FILINGS SHORTED OUT THE RECEIVER CIRCUITS. THIS PROB MAY BE PRESENT ON OTHER SA227 ACFT AND MAY BE WORTH ISSUING A SVC LETTER TO OPERATORS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS DELIVERED FROM FAIRCHILD WITH THE AUDIO 'BAKER' BOXES INSTALLED TO THE L AND R OF THE GLARESHIELD. THE RPTR SAID THE CORNERS OF THE BOXES HAD BEEN TRIMMED TO FIT THE GLARESHIELD ALLOWING ANN OPENING INTO THE BOXES. THE RPTR STATED THAT OVER THE YRS THE BOXES RUBBED ON THE SOMEWHAT FLEXIBLE GLARESHIELD ALLOWING METAL FILINGS AND DUST TO ACCUMULATE IN THE ELECTRICAL COMPONENTS SHORTING OUT THE L BOX WHICH IS THE MASTER. THE RPTR SAID THE RADIO SHOP IN MISSOULA; MT; KNEW EXACTLY WHAT THE PROB WAS AND THE CORRECTIVE ACTION REQUIRED HAVING SEEN THIS FAILURE BEFORE ON SA227 ACFT. THE RPTR STATED THE 'BAKER' BOXES WERE CLEANED OUT WITH DRY NITROGEN AND THE BOXES SEALED.