AN EMBRAER 120 ON DSCNT AT 14000 FT DECLARED AN EMER DUE TO SMOKE IN THE COCKPIT CAUSED BY AN ENG OIL VALVE.

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DURING DSCNT TO MSN; WHEN ENG PWR WAS REDUCED; OILY SMOKE FLOWED FROM THE COCKPIT'S FORWARD GASPER VENTS. THE SMOKE STOPPED IMMEDIATELY WHEN THE ENG BLEEDS WERE CLOSED. THE EMER CHKLIST WAS EXECUTED. THE APU WAS USED TO PROVIDE CONDITIONED; PRESSURIZED AIR FOR THE BAL OF THE FLT. THE PLTS COORDINATED WITH THE FLT ATTENDANT WHO HAD NOTICED AN OILY SMELL; BUT SAW NO SMOKE. THIS WOULD HAVE BEEN A MUCH MORE SERIOUS EMER SIT HAD WE NOT BEEN PRE-WARNED OF IT. I AM A PLT GROUP SAFETY REPRESENTATIVE; AND WAS AWARE THAT THIS PARTICULAR ACFT HAD HAD 2 SERIOUS SMOKE-IN-THE-COCKPIT INCIDENTS DURING THE PRECEDING WK OR SO. MAINT WAS 'UNABLE TO DUPLICATE.' WHEN WE WERE ASSIGNED TO THIS SHIP; I BRIEFED THE FO AND FLT ATTENDANT ABOUT THE POSSIBILITY OF A REPEAT OCCURRENCE. WE ALSO STARTED THE APU PRIOR TO DSCNT 'JUST IN CASE.' CONTRACT MAINT WORKED WITH US FOR ABOUT 7 HRS ON THE GND AT MSN; AND WE WERE UNABLE TO DUPLICATE THE PROB. OUR COMPANY MAINT FOLKS WORKED ON THE ACFT AFTER WE FERRIED IT TO OUR HOME BASE. THEY REPLACED AN ENG OIL VALVE; BUT DID NOT TEST FLY THE ACFT PRIOR TO RETURNING IT TO SVC. IT IS NOW DOING NOMINAL REVENUE SVC. MINE WAS THE THIRD SMOKE INCIDENT IN 2 WKS AND THE ACFT WAS RETURNED TO SVC WITH NO TEST FLT! I HOPE THAT THEY FIXED THE REAL PROB THIS TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS AWARE OF THE ENG BLEED SWITCHING PROBS WHEN AT LOW PWR BUT IS CONCERNED THAT MAINT MAY NOT BE AWARE OF THIS CONDITION. THE RPTR STATED THAT MAINT HAD REPLACED AN ENG OIL VALVE BUT NO MORE DETAILS WERE AVAILABLE. THE RPTR SAID THAT HE HAD BEEN ON VACATION FOR SEVERAL WKS AND WAS UNABLE TO FOLLOW THE TAIL NUMBER WITH THE REPEATED RPTS OF SMOKE IN THE CABIN.