FLC OF AN EMBRAER BRASILIA EMB120 (E120) TOOK OFF IN THE OPPOSITE DIRECTION DURING AN INTXN TKOF. IT WAS AT NIGHTTIME; AND THE CAPT BECAME DISORIENTED WHEN TAXIING INTO POS FOR TKOF.

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IT WAS THE LAST LEG OF THE 5 WE WERE SCHEDULED. AFTER TAXI CHKS WERE COMPLETED AND RAMP CTL HANDED US OFF TO GND CTL; WE RECEIVED CLRNC TO TAXI TO RWY 16R. SOMEWHERE ALONG TXWY E; THE CAPT ASKED IF WE WERE LIGHT ENOUGH FOR AN INTXN TKOF; AND REQUESTED THAT I ASK FOR IT FROM GND. I IMMEDIATELY LOOKED UP OUR TKOF WT FOR AN A6 INTXN DEP; CONFIRMED WE WERE LIGHT ENOUGH; AND THEN CONTACTED TWR FOR CLRNC. AS I WAITED FOR ATC CLRNC; I BEGAN THE BEFORE TKOF CHKLIST REQUIRING NOTIFICATION OF THE FLT ATTENDANT; FLAP SETTING AND OTHER VERBAL AND PHYSICAL CHKS. DURING MY CHALLENGE AND RESPONSE FROM THE CAPT; ATC CLRED US FOR 'TKOF; A6 INTXN DEP;' AND WHAT I PERCEIVED AS 'L TURN 280 DEGS.' I READ BACK THE CLRNC; BUT STUTTERED; UNSURE OF THE 'TURNOUT' DIRECTION. THE CAPT THEN ENTERED THE RWY; HDG N AND APPLIED PWR FOR TKOF; CALLED 'SET PWR' AND AS I STRUGGLED TO CLR MY MIND OF THE CLRNC WE RECEIVED; I ADJUSTED MY THINKING TO THE TKOF ROLL; CALLOUTS; AND WATCHING FOR TFC. TKOF WAS NORMAL. AFTER ABOUT 400 FT; TWR REQUESTED 'IMMEDIATE' TURN 280 DEGS. WE NOTED AN ACFT ABOUT OUR 1-2 O'CLOCK POS; AND APPROX 6-10 MI ON OUR TCASII. WE THEN BEGAN TO REALIZE OUR ERROR IN TAKING OFF THE WRONG DIRECTION. NO OTHER INCIDENTS. POSSIBLE CAUSES: 1) DARK COCKPIT; UNABLE TO VIEW ATIS INFO. 2) CONFUSION WITH ATC CLRNC. 3) BEING IN TOO BIG OF A HURRY. 4) BREAKDOWN OF CREW COM/COORD DURING A CRITICAL PHASE OF FLT. 5) FAILURE TO MAINTAIN SITUATIONAL AWARENESS. COMPLACENCY! 6) FAILURE OF ATC TO VERIFY READBACK OR CLARIFY IT.