ACR MDT TYPE ACFT LOST DOOR HATCH DURING TKOF ROLL AND ABORTED THE TKOF.

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ABORTED TKOF. LOST CREW HATCH ON ACFT THAT CAUSED US TO ABORT AT BOSTON LOGAN. UPON DOING A COMPLETE FIRST FLT (PREFLT) OF THE DAY; WE STARTED OUR TAXI. DURING TAXI; WE NOTICED WATER LEAKING FROM AROUND HATCH; WHICH IS NORMAL FOR AN LTT. THE FO CONFIRMED THE HATCH WAS SECURE BY PUSHING ON IT AND WE BOTH LOOKED AT THE RED HATCH HANDLE AND CONFIRMED IT LOCKED. AS WE STARTED OUR TKOF ROLL; ALL WAS NORMAL UNTIL ABOUT 50 KTS. AT THAT POINT THE HATCH FLEW OFF AND THERE WAS A LOUD 'BANG.' LATER WE FOUND OUT IT WAS THE HATCH HITTING THE VERTICAL STABILIZER. WE INITIATED 'ABORT' OF THE TKOF AND TAXIED BACK TO THE GATE. WHEN THE HATCH WAS FOUND; IT WAS INTACT. THE HANDLE WAS BROKEN IN THE LOCKED POS AND COULD NOT BE MOVED. THE COCKPIT INDICATIONS WERE ALL NORMAL BEFORE THE INCIDENT. THERE ARE NO ANNUNCIATOR LIGHTS FOR THE HATCH. JUST THE RED HANDLE IN THE LOCKED POS. SUPPLEMENTAL INFO FROM ACN 205577. IT WAS LATER BROUGHT TO OUR ATTN THAT A RAMP PERSON HAD BEEN ON THE ACFT WITH A BROOM TO CLEAN OFF SNOW THAT MORNING. HE MAY HAVE MOVED THE TOP HATCH HANDLE. THERE HAD ONLY BEEN ABOUT 2 INCHES OF SNOW ON THE ACFT. THE 'RAMPY' SAID HE SLIPPED AT ONE POINT AND MAY HAVE CAUGHT THE HANDLE. THE ONLY WAY TO INSURE THE HATCH IS BY VISUAL INSPECTION OF THE HANDLE WHICH IS IN A VERTICAL POS. THE HATCH WAS FOUND AND INSPECTED. IT WAS FOUND IN THE LOCKED POS. THERE ARE NO MICRO SWITCHES TO MONITOR THE POSITIONS OF THE LOCKING PINS.