An SA227 in cruise at FL240 experienced loss of all communication audio caused by the left and right...

Narrative:

After departing yqr (regina, sk) and leveling FL240, radio chatter tapered off on winnipeg center. Crossing swift current VOR, chatter ceased entirely. Before crossing mepicino hat VOR, center should have handed off to edmonton center. When no handoff was received, we tried to contact winnipeg/edmonton/FSS etc. Only once, very faintly, we received a frequency switch from winnipeg to salt lake 133.4. No other contact was accomplished. We continued the flight to mso (missoula, mt) according to procedures set forth in far part 91 and made an uneventful landing in mso. After landing and clearing customs we call mso tower who informed us there was no problem with center or mso controllers. The only question was from center, wondering why the squawk was not changed to 7600. Not being able to determine the cause of the problem, 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 right and left sides of the glareshield, had rubbed on the glareshield and generated a lot of filings. The filings shorted out the receiver circuits. This problem may be present on other SA227 aircraft and may be worth issuing a service letter to operators. Callback conversation with reporter revealed the following information: the reporter stated the aircraft was delivered from fairchild with the audio 'baker' boxes installed to the left and right of the glareshield. The reporter said the corners of the boxes had been trimmed to fit the glareshield allowing ann opening into the boxes. The reporter 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 left box which is the master. The reporter said the radio shop in missoula, mt, knew exactly what the problem was and the corrective action required having seen this failure before on SA227 aircraft. The reporter stated the 'baker' boxes were cleaned out with dry nitrogen and the boxes sealed.

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	427109
Time
Date	199901
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : yxh
State Reference	AB
Altitude	msl bound lower : 24000 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	general aviation : corporate
Make Model Name	SA-227 AC Metro III
Flight Phase	cruise other
Route In Use	enroute airway : czwg
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : instrument
Experience	flight time last 90 days : 60 flight time total : 12000 flight time type : 320
ASRS Report	427109
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : commercial
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Air Traffic Incident	other

An SA227 in cruise at FL240 experienced loss of all communication audio caused by the left and right audio boxes shorted by metal filings and dust rubbed from the glareshield mountings.