C172 FLT CREW TKOF WITHOUT CLRNC AT HEF.

Narrative

WE WERE BEGINNING A TRAINING FLT FROM MANASSAS; AND WE WERE CLRED BY THE TWR TO 'TAXI INTO POS AND HOLD' ON RWY 16R; SINCE TFC (A MOONEY) HAD JUST LANDED. MY STUDENT TOOK 15-30 SECONDS TO TAXI INTO POS ON THE RWY. I WAS PROMPTING HIM ABOUT CHKING THE WINDSOCK; AND TURNING THE XPONDER TO ALT; AND I DIDN'T STOP HIM WHEN HE ADVANCED THE THROTTLE FOR TKOF. THE RWY WAS CLR; HOWEVER; WE HAD NOT BEEN GIVEN OUR TKOF CLRNC YET. WHEN WE WERE ABOUT 100-200 FT AGL IN THE CLBOUT; THE TWR SAID 'CESSNA (CALL SIGN); IN THE FUTURE; PLEASE LISTEN MORE CAREFULLY TO YOUR CLRNC; YOU WERE CLRED TO TAXI INTO POS AND HOLD.' THAT WAS WHEN I REALIZED MY ERROR. I IMMEDIATELY APOLOGIZED TO THE TWR CTLR AND SAID THAT I WOULD BE MUCH MORE CAREFUL IN THE FUTURE. ONE CONTRIBUTING FACTOR WAS THAT I WAS RUNNING BEHIND FOR THE FLT LESSON; AND I MAY HAVE BEEN SUBCONSCIOUSLY RUSHING THE STUDENT. WHEN THE STUDENT STARTED TO ADVANCE THE THROTTLE; I SHOULD HAVE STOPPED HIM; AND REMINDED HIM THAT WE WERE ONLY CLRED INTO POS AND HOLD. I FEEL I GOT MOMENTARILY FIXATED ON TEACHING THE STUDENT ABOUT WHAT TO DO WHEN LINING UP ON THE RWY; AND I MISTAKENLY THOUGHT WE HAD BEEN CLRED FOR TKOF ALREADY; WHEN IN FACT; WE HAD NOT. USUALLY; I FEEL PRETTY GOOD ABOUT MY ABILITY TO MULTI-TASK; TO TEACH; TO LISTEN; AND RESPOND TO ATC; TO BE AWARE OF AIRSPACE CONSIDERATIONS; ETC. HOWEVER; I HAD A MOMENTARY LAPSE AND ALLOWED THE STUDENT TO BEGIN THE TKOF ROLE PREMATURELY. AFTER WE LANDED; THE STUDENT AND I DISCUSSED WHAT HAPPENED; WHAT SHOULD HAVE BEEN DONE; AND WAYS TO PREVENT THAT SIT FROM HAPPENING AGAIN.