GLF4 DEPARTED TYS WITHOUT TKOF CLRNC. AT72 CROSSED RWY IN RELATIVELY SAME TIMEFRAME AS GLF4 DEPS.

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WHILE WORKING LCL CTL/GND CTL/FDCD/CIC COMBINED IN THE TWR; I TOLD ACFT Y (ATR72) TO CROSS RWY 5R AND TAXI TO THE RAMP. ACFT X CALLED READY TO DEPART RWY 5R AND WAS TOLD TO TAXI INTO POS AND HOLD RWY 5R. ACFT X ACKNOWLEDGED TAXI INTO POS AND HOLD INSTRUCTIONS. I TURNED AROUND TO CHK THE WX. WHEN I TURNED BACK AROUND; I OBSERVED ACFT Y CLRING RWY 5R AND TURNING ONTO TXWY A. IN THAT SAME INSTANT; I OBSERVED ACFT X ROTATING IN THE RWY 5R; TXWY A4 AREA (2000 FT APPROX FROM WHERE ACFT Y HAD CROSSED). SUPPLEMENTAL INFO FROM ACN 573666: MYSELF AND A CO-CAPT; TAXIED OUT FOR DEP. KNOXVILLE TWR CALLED US AND I REMEMBER HEARING ACFT Y RWY 5R...(AND I THOUGHT CLRED FOR TKOF). DURING THIS XMISSION I WAS REACHING UP TO SELECT THE EXTERIOR LIGHTS ON. I THEN RAN UP THE ENGS IN POS FOR APPROX 10 SECONDS AND EXPECTED THE TKOF. I ASKED IF WE WERE CLRED ON COURSE OR SHOULD REMAIN ON RWY HEADING. THE COPLT ASKED THE TWR AND I BELIEVE THEY HAD SAID TO REMAIN ON RWY HEADING. APPROX 12 MINS FROM TEB ARPT WE RECEIVED A MESSAGE TO CALL KNOXVILLE TWR. THEY SAID THAT THEY WERE GATHERING INFO FOR A POSSIBLE DEP WITHOUT A CLRNC. I TOLD THEM I WOULD CONTACT THEM AFTER WE LANDED. WE WERE BOTH SURE THAT WE MUST HAVE RECEIVED A TKOF CLRNC AND THAT WE WOULD NEVER HAD DEPARTED WITHOUT ONE. I CALLED AND THEY CONFIRMED THAT WE HAD DEPARTED WITHOUT A TKOF CLRNC. THE CLRNC THAT WE HAD RECEIVED WAS TO TAXI INTO POS AND HOLD. THEY ALSO SAID THAT WE DID NOT ACKNOWLEDGE THE POS AND HOLD CLRNC. THE OBVIOUS CAUSES OF THIS GROSS ERROR WAS THE CREW NOT VERIFYING THAT WE BOTH UNDERSTOOD THAT WE HEARD 'CLRED FOR TKOF' AND NOT 'POS AND HOLD.' THE RUNNING UP OF THE ENGS WAS ALSO A CONTRIBUTING FACTOR AND THE USE OF HEADSETS WOULD HAVE HELPED TO REDUCE THE NOISE LEVEL. ALSO I HAD A MINDSET TO DEPART AS QUICK AS POSSIBLE BEFORE THE SNOW STARTED TO 'STICK' TO THE ACFT. WE HAVE BOTH AGREED NEVER TO BE RUSHED AGAIN NO MATTER WHAT THE CIRCUMSTANCES AND TO USE THE 'ACR' PROC OF NOT TURNING ON THE EXTERIOR LIGHTS UNTIL WE BOTH HEAR AND VERIFY 'CLRED FOR TKOF.' ALSO; FROM NOW ON THE COPLT WILL TURN ON ALL OF THE LIGHTS AND THE PF WILL JUST TAXI THE ACFT AND LISTEN UP CLOSELY. SUPPLEMENTAL INFO FROM ACN 573667: NEITHER PLT PRACTICED THE PROCS WHICH WE HAD PRACTICED ON SO MANY OCCASIONS; AND THAT IS TO CHALLENGE WHEN THERE IS ANY DOUBT ABOUT A CLRNC. I THINK BECAUSE OF THE ACCUMULATION OF SNOW ON THE SURFACE THE (PF) WAS PROBABLY RUSHING AND THOUGHT THAT HE ACTUALLY HAD A TKOF CLRNC; AND THE PNF SHOULD HAVE CHALLENGED SINCE HE HAD NOT HEARD THE CLRNC.