BE-C99 CARGO FLT HAD PLT HATCH DOOR COME OPEN AFTER TKOF. RPTR THOUGHT HE HAD CHKED THE DOOR CLOSED AND LATCHED; BUT APPARENTLY IT WASN'T SECURED WELL ENOUGH. RPTR RETURNED TO ARPT AND LANDED. NO DAMAGE ON THE GND FROM THE DOOR FALLING.

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BEFORE PERFORMING THE ENG START CHKLIST I CLOSED AND LOCKED PLT HATCH; LOOKING AT THE HOOK ENGAGEMENT FROM ABOVE THE HOOKS APPEARED TO BE COMPLETELY OVER THE HATCH PINS. THEN THE D-HANDLE WAS TURNED TO THE STOP; INDICATING A CLOSED AND LOCKED DOOR; REAFFIRMED AFTERWARD BY HITTING THE DOOR WITH MY L ELBOW. DURING CLBOUT; I NOTICED A HISSING NOISE COMING FROM THE DEFROST VENTS. UPON FURTHER INVESTIGATION OF BLEEDS AND WINDOWS I NOTICED A GAP ON THE BOTTOM OF THE HATCH. I PULLED THE STRAP TO CLOSE THE GAP AND TRIED TO TURN THE D-HANDLE TO CLOSE AND LOCK THE DOOR. THE HANDLE DID NOT MOVE AND I TRIED TO UNJAM IT BY JIGGLING IT. THEN THE HATCH CAME OPEN; I MADE MULTIPLE ATTEMPTS TO CLOSE IT; AND WAS HAVING TROUBLE CTLING THE ACFT WITH MY KNEES (1 HAND ON THE STRAP AND 1 HAND ON THE D-HANDLE). THEN THE HATCH FLEW OPEN COMPLETELY WITH MY L ARM STILL ON IT. I GRABBED IT WITH BOTH HANDS AND CLOSED IT BUT I COULDN'T HOLD IT AND FLY THE PLANE. I SLOWLY LET THE HATCH OPEN AND IT DEPARTED THE ACFT; WITHOUT HITTING THE ACFT AND WITHOUT DAMAGING ANYTHING ON THE GND. I CALLED THE TWR AND RETURNED TO THE ARPT AND MADE A NORMAL LNDG. THE LATCH APPEARED TO BE CLOSED UPON INSPECTION; BUT IT STILL CAME OPEN.