LTT SMALL JET CREW WENT OFF THE FAR END OF THE RWY AFTER AN ABORTED TKOF BECAUSE OF AN IMPROPERLY CLOSED FO SLIDING WINDOW.

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START-UP AND TAXI OUT WERE NORMAL WITH NO TFC MOVING AT CGE OTHER THAN OURSELVES. WE STOPPED AT THE DEP END OF RWY 16 TO COMPLETE ALL SYS CHKS AND CHKLISTS AND TO CONDUCT A THOROUGH TKOF BRIEF WHILE THE BRAKES WERE PARKED. AFTER TAKING THE RWY WE PWRED UP TO 100% BEFORE RELEASING THE BRAKES. I CALLED AIRSPD ALIVE AND BEFORE REACHING 80 KIAS NOTICED THAT MY SLIDING WINDOW WAS NOT FULLY CLOSED; ALTHOUGH IT APPEARED TO BE; AND THE LOCKING HANDLE WAS CLOSED AND FLUSH. I CALLED THIS OUT AND THE PIC ELECTED TO ABORT THE TKOF. AS I DIRECTED MY ATTN TO CLOSING THE WINDOW I CANNOT SAY FOR SURE WHAT THE INDICATED AIRSPD WAS WHEN THE ABORT WAS INITIATED. WE DID NOT SUCCESSFULLY COMPLETE THE ABORT ON THE RWY; EXITING ON CTRLINE AT A VERY LOW SPD. WE WERE SUCCESSFUL IN TAXIING THE ACFT BACK ONTO THE RWY VIA THE CTRLINE AND BACK TO THE HANGAR. CONTRIBUTING FACTORS: 1) ALTHOUGH I HAVE A TYPE RATING ON THE ACFT; I HAD A TOTAL OF 10 HRS (ON A DIFFERENT MODEL) AND ONE SHORT TEST FLT IN THIS MODEL; NEVER HAVING OPENED OR CLOSED THE SLIDING WINDOW. 2) EARLY HR: I HAD GOTTEN UP AT XA00; DRIVEN 2 HRS TO MAKE AN XD00 LAUNCH. 3) PIC HAD BEEN ON VACATION AND HAD NOT FLOWN IN SEVERAL WKS. 4) WE WERE BOTH CONCERNED ABOUT THIS ACFT BECAUSE; WITH THE EXCEPTION OF THE TEST FLT I HAD FLOWN; IT HAD NOT FLOWN FOR SEVERAL WKS. ABSENT THIS THOUGHT; WE MIGHT HAVE CONTINUED THE TKOF AND RESOLVED THE WINDOW ISSUE AFTER WE WERE SAFELY AIRBORNE.