FO SIDE WINDOW OPEN ON TKOF ROLL.

Narrative

ON PREFLT BOTH CAPT AND FO SLIDING WINDOW WERE OPEN TO COOL COCKPIT. PRIOR TO START BOTH WINDOWS WERE CLOSED AND LOCKING HANDLE WAS ENGAGED. ALL CHKLISTS NORMAL. ON TKOF ROLL PASSING 10-20 KTS FO WINDOW CAME PARTIALLY OPEN. TKOF ABORTED. NORMAL METHODS OF CLOSING WINDOW FAILED; SO ACFT RETURNED TO GATE. MECHS CAME ON BOARD; APPLIED PRESSURE TO AFT FRAME OF WINDOW TOGETHER WITH CRANKING WINDOW CLOSED. FLT PROCEEDED WITHOUT FURTHER INCIDENT. CONTRIBUTING FACTORS: WINDOW CRANK HAS A CLUTCH THAT PREVENTS CABLE FROM BREAKING. CLUTCH USUALLY DISENGAGES ONLY WHEN WINDOW IS FULLY CLOSED. THIS TIME WINDOW WAS NOT OVER CTR WHEN CLUTCH DISENGAGED. WINDOW WAS CLOSED ENOUGH SO THAT COCKPIT NOISE LEVEL WAS NORMAL. WINDOW CLOSING HANDLE WAS CHKED AND PULLED TO ASSURE IT WAS LOCKED BUT THIS FAILED TO DISCLOSE WINDOW WAS NOT FULLY LOCKED. 95% OF TIME I VERIFY ACTUATOR ARM WAS OVER CTR AND THE WORD 'CLOSED' IS PLAINLY VISIBLE IN INDICATOR BUT THIS MUST HAVE BEEN THE OTHER 5% AS I DON'T REMEMBER READING CLOSED. AN AM30 LCL DEP MAY HAVE SOMETHING TO CONTRIBUTE. JUST GLAD IT DIDN'T HAPPEN AT 120 KTS.