AN ACR DHC8 DEPARTED LWS IFR AND FORGOT TO TURN ON HIS XPONDER. ARTCC RADAR CTLR WORKING 4 SECTORS COMBINED FOR THE MID-SHIFT CONFIGN DID NOT GET A DATA BLOCK DUE TO NO XPONDER ON THE DHC8. ACFT FLEW INTO ANOTHER SECTOR'S AIRSPACE AND HAD AN NMAC WITH A B737.

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ON THE EVENING OF AUG/XX/96; I WAS WORKING AIR TFC AT SEATTLE ARTCC. THE AIRSPACE I WAS WORKING WERE 4 SECTORS THAT HAD BEEN COMBINED TOGETHER IN THE 'MID-SHIFT' CONFIGN. AT APPROX XA54 PM LCL A SCHEDULED ACR CALLED THROUGH LEWISTON; ID; TWR FOR HIS IFR CLRNC TO SEATTLE. THE CLRNC WAS ISSUED AT THAT TIME BY MY DATA CTLR. AT APPROX XA58 PM LCL THE ACFT (A DEHAVILAND DH8) DEPARTED LEWISTON AND MADE INITIAL CONTACT WITH ME CLBING FOR 16000 FT. TFC AT THE TIME WAS BUSY AND INCREASING. WHEN THE ACFT CALLED HE WAS PARTIALLY BLOCKED BY ANOTHER ACFT ON ANOTHER OF THE 4 FREQS THAT I WAS USING. I HANDLED THE OTHER ACFT FIRST AND THEN ACKNOWLEDGED THE DEP. NORMAL OPERATING PROCS FOR DEPS OUT OF THE LEWISTON; ID; ARPT IS NOT TO START A MANUAL TRACK (DISPLAYING THE DATA BLOCK ON THE SCOPE) DUE TO THE FACT THAT WE HAVE GOOD RADAR COVERAGE IN THE AREA AND THE COMPUTER AUTOMATICALLY STARTS THE TRACK FOR US WHEN IT RECOGNIZES THE ACFT'S ASSIGNED BEACON CODE. THIS CAN BE A HELPFUL FEATURE WHEN A CTLR IS BUSY EXCEPT FOR WHEN THE PLT FORGETS TO TURN ON THE XPONDER. ON THIS EVENING I DID NOT SEE A PRIMARY RADAR TARGET OF THE ACFT NOR DID I SEE A XPONDER TARGET OF ANY TYPE. AT THIS POINT THE FLT WAS 'OUT OF SIGHT; OUT OF MIND.' AT XB33 PM I WAS FINALLY RELIEVED FROM POS TO GO HOME FOR THE NIGHT. THE MID-WATCH CTLR HAD FAILED TO GET HER RELIEF BRIEFING IN A TIMELY MANNER CAUSING ME TO REMAIN ON MY OWN TIME. THE BRIEFING WAS INTERRUPTED MANY TIMES BY ACFT CALLING AS TFC WAS GETTING BUSIER. THE RELIEVING CTLR WAS ALSO CARRYING ON A CONVERSATION WITH A FELLOW WORKER DURING THE BRIEFING AND THEN FINALLY CUT ME SHORT STATING THAT 'SHE HAD THE SECTOR.' AT THE SAME TIME I WAS BEING RELIEVED MY DATA-SIDE CTLR WAS ALSO BEING RELIEVED AND IN THE PROCESS PUT THE APPROPRIATE (HAD WE RADAR IDENTED; TRANSFERRED CTL AND COM-CHANGED THE FLT IN QUESTION) STRIP MARKINGS ON THE DATA STRIP; PULLED DOWN THE STRIP; AND FILED IT IN THE INACTIVE PILE. AT APPROX XB48 PM THE PLT CONTACTED THE CTR ONCE AGAIN; STATING THAT IT HAD BEEN SOME TIME SINCE HE HAD HEARD FROM US. THE CTLR THEN TOOK MEASURES TO FIND THE ACFT. AFTER ASKING THE PLT FOR HIS POS AND ALT THE CTLR DISCOVERED (MUCH TO HER DISMAY) THAT THE ACFT WAS APPROX 20-30 MI INTO THE NEXT CTLR'S AIRSPACE AND LEVEL AT 16000 FT. IT WAS AT APPROX XB49 PM THAT THE CTLR OF THE ADJACENT AIRSPACE FOR THE FIRST TIME IN THIS ACFT'S FLT SAW ITS XPONDER CODE. AT THAT POINT THE PLT STATED THAT AN ACFT HAD 'JUST PASSED OVER AND IN FRONT OF US.' THE CTLR OF THE ADJACENT SECTOR HAD ALSO REMOVED HIS STRIP ON THE ACFT THINKING THAT IT HAD ALREADY ARRIVED AT SEATTLE. IN STUDYING THE VOICE AND DATA TAPES FOR THE TIME PERIOD IN QUESTION IT HAS BEEN UNOFFICIALLY DETERMINED THAT THE PLT OF THE FLT IN QUESTION FAILED TO TURN ON HIS XPONDER RENDERING THE FLT NEARLY INVISIBLE TO US UNDER THE CIRCUMSTANCES. DUE TO THE BREAKDOWN IN THE SYS THE FLT IN QUESTION CAME WITHIN 100 FT VERTLY AND LESS THAN 1 MI HORIZLY TO THE ACFT THAT WAS DSNDING IN FRONT OF IT. SUPPLEMENTAL INFO FROM ACN 345232: I WAS THE PIC; DEPARTED LWS AND CHKED IN WITH SEA ARTCC. WE WERE BELOW RADAR COVERAGE AND DID NOT RECEIVE VERIFICATION OF RADAR CONTACT. THERE WAS NO FURTHER RADIO COM BTWN OUR FLT AND ARTCC UNTIL XB53 LCL WHEN AN ACFT (LATER IDENTED AS A B737) DSNDED THROUGH OUR ALT. THINKING THAT THE ACFT SHOULD HAVE BEEN VISIBLE ON OUR TCASII; I CHKED THE XPONDER AND FOUND IT STILL ON 'STANDBY.' BECAUSE THE B737 MISSED US BY LESS THAN THE REQUIRED SEPARATION PARAMETERS; OUR COMPANY WAS NOTIFIED AND ACTION TAKEN. THIS WAS OUR THIRD FLT OF THE DAY; IT WAS XB00 HRS; AND WE WERE RUNNING AN HR LATE. THE ACFT WE FLEW IS THE ONLY ONE IN OUR FLEET WITH ITS TYPE OF XPONDER PANEL AND TCASII DISPLAY. SUPPLEMENTAL INFO FROM 346031: WE SAW ANOTHER ACFT DSND IN CLOSE PROX OFF OUR L SIDE. WE CHKED OUR TCASII AND FOUND THAT IT WAS IN THE OFF POS. CTR HAD LOST TRACK OF US DUE TO THIS ERROR. WE DEPARTED AN UNCTLED ARPT; AND DID MAKE RADIO CONTACT WITH ZTL. THIS LED US TO ASSUME WE WERE UNDER RADIO CONTACT. THE ERROR WAS MINE; BY NOT TURNING ON THE XPONDER.