CAPT ON BE02 ACR FLT AT GRR IS CLRED INTO POS AND HOLD; COMPLETES CHKLIST AND TAKES OFF.

Narrative

WHILE TAXIING TO RWY 17 AT GRR; FLT X (LTT); GRR TO MKE; COMPLETED THE BEFORE TKOF CHKLIST. WHILE THE CAPT TAXIED; THE FO READ THE CHKLIST. AFTER COMPLETING THE CHKLIST; AND WHILE STILL TAXIING; WE WERE CLRED INTO POS AND HOLD (ON RWY 17). IT IS OUR COMPANY PROC TO EXECUTE THE TKOF FINAL ITEMS CHKLIST AS WE TAXI INTO POS; WHICH WE PERFORMED NORMALLY. AT THE COMPLETION OF THE CHKLIST; I BEGAN TO ADD PWR AND BEGIN MY TKOF ROLL. SHORTLY AFTER; MY FO EXPRESSED CONCERN THAT WE MAY NOT HAVE BEEN CLRED FOR TKOF. I REPLIED THAT WE WERE ISSUED A HDG; SO WE MUST HAVE BEEN CLRED. I ELECTED TO CONTINUE THE TKOF NORMALLY. CLBING THROUGH 400 FT; TWR ANNOUNCED 'ACR LTT; I SEE THAT YOU TOOK OFF; I'M NOT SURE THAT I CLRED YOU...MAINTAIN RWY HDG AND 3000 FT.' THE BEST WAY TO HAVE AVOIDED THIS SIT WOULD HAVE BEEN TO EXERCISE BETTER COCKPIT DISCIPLINE IN LISTENING TO CLRNCS AND NOT WHAT I EXPECTED TO HEAR. IN ADDITION; MY COPLT STATED UNCERTAINTY IN MY ACTION; AND I SHOULD HAVE COMMITTED TO A LOW SPD ABORT AND TAXI BACK -- EVEN IF I WAS CORRECT ABOUT THE CLRNC. (I BELIEVE I HEARD '...RWY 17...' AND MISTOOK IT FOR A HDG OF 170 DEGS; WHICH LED ME TO BELIEVE THAT WE HAD BEEN GIVEN A TKOF CLRNC.)