Narrative:

I was working local north and south combined when aircraft X called for takeoff. I had no strip on aircraft; VFR or IFR. I asked aircraft to verify VFR. He said negative. I called myself air carrier X with ground control. I did not hear him say negative; but heard how he called himself air carrier X with ground control. I had a VFR strip on an air carrier X and thought if he called ground control as air carrier X then he must be VFR; since the use of air carrier X call sign indicates VFR. I asked if he was going to use the air carrier X call sign or the aircraft X call sign. He said he wanted to use the n-number call sign. This flight school is known to interchange call signs during flight session; so I thought he was VFR and wanted to change call sign in ZZZ. I cleared him for takeoff and approved his direction of flight; not knowing he was IFR. I did not get an IFR release for aircraft. Aircraft went to the IFR approach facility then the operational deviation was discovered. I believe this operational deviation could have been prevented had the flight school followed procedures by using IFR call sign at all times. If he had used the IFR call sign with ground control; ground control would have known he was IFR and would have passed the IFR strip to me as the local controller. I would have called the IFR approach facility for aircraft's IFR release and proper procedure would have been followed all the way through. Though I didn't hear aircraft answer my question; pilot could have helped by saying IFR or not. My focus was shifted by listening to him explain to me his communication with ground control.

Related reports:

Google
 

Original NASA ASRS Text

Title: ZZZ LCL CTLR DESCRIBED OPDEV WHEN DEP ACFT USED TWO DIFFERENT CALL SIGNS CONFUSING IFR VERSUS VFR STATUS; ACFT DEPARTED WITHOUT RELEASE.

Narrative: I WAS WORKING LCL N AND S COMBINED WHEN ACFT X CALLED FOR TKOF. I HAD NO STRIP ON ACFT; VFR OR IFR. I ASKED ACFT TO VERIFY VFR. HE SAID NEGATIVE. I CALLED MYSELF ACR X WITH GND CTL. I DID NOT HEAR HIM SAY NEGATIVE; BUT HEARD HOW HE CALLED HIMSELF ACR X WITH GND CTL. I HAD A VFR STRIP ON AN ACR X AND THOUGHT IF HE CALLED GND CTL AS ACR X THEN HE MUST BE VFR; SINCE THE USE OF ACR X CALL SIGN INDICATES VFR. I ASKED IF HE WAS GOING TO USE THE ACR X CALL SIGN OR THE ACFT X CALL SIGN. HE SAID HE WANTED TO USE THE N-NUMBER CALL SIGN. THIS FLT SCHOOL IS KNOWN TO INTERCHANGE CALL SIGNS DURING FLT SESSION; SO I THOUGHT HE WAS VFR AND WANTED TO CHANGE CALL SIGN IN ZZZ. I CLRED HIM FOR TKOF AND APPROVED HIS DIRECTION OF FLT; NOT KNOWING HE WAS IFR. I DID NOT GET AN IFR RELEASE FOR ACFT. ACFT WENT TO THE IFR APCH FACILITY THEN THE OPDEV WAS DISCOVERED. I BELIEVE THIS OPDEV COULD HAVE BEEN PREVENTED HAD THE FLT SCHOOL FOLLOWED PROCS BY USING IFR CALL SIGN AT ALL TIMES. IF HE HAD USED THE IFR CALL SIGN WITH GND CTL; GND CTL WOULD HAVE KNOWN HE WAS IFR AND WOULD HAVE PASSED THE IFR STRIP TO ME AS THE LCL CTLR. I WOULD HAVE CALLED THE IFR APCH FACILITY FOR ACFT'S IFR RELEASE AND PROPER PROC WOULD HAVE BEEN FOLLOWED ALL THE WAY THROUGH. THOUGH I DIDN'T HEAR ACFT ANSWER MY QUESTION; PLT COULD HAVE HELPED BY SAYING IFR OR NOT. MY FOCUS WAS SHIFTED BY LISTENING TO HIM EXPLAIN TO ME HIS COM WITH GND CTL.

Data retrieved from NASA's ASRS site as of May 2009 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.