What was found
ABORTED TAKEOFF AFTER PILOTS WINDOW OPENED AT 80-90 KTS. PILOTS REPORTED WINDOW WAS NOT FULLY LOCKED. *S/D* CAPTAIN'S SLIDING WINDOW WAS NOT FULLY LOCKED. RED-COLORED RELEASE BUTTON ON WINDOW HANDLE POPS UP WHEN THE WINDOW IS LOCKED. THIS SERVES AS A VISUAL INDICATION. ENSURING THAT WINDOW IS FULLY CLOSED AND LOCKED PRIOR TO TAKEOFF HAS BEEN AND REMAINS AN ELEMENT OF THE FLIGHT CREW'S PREFLIGHT CHECKLIST.