What was found
AT 10500 FT IN CRUISE FLIGHT, GOT AN AUDIBLE "DING" ON THE G1000 WARNING AND THEN A LIGHT SAYING THAT A DOOR WAS OPEN. BACK DOOR HAD COME OPEN IN FLIGHT. THE DOOR HAD BEEN SHUT AND THERE HAD BEEN NO WARNING DURING PRE-FLIGHT CHECK AND RUN-UP. WAS IN THE RT SEAT AND COULD NOT REACH THE DOOR UNTIL I UNDONE MY SEATBELT. TRIED TO SHUT THE DOOR, BUT DUE THE DESIGN OF THE HANDLE, THERE IS NO WAY TO SHUT THE DOOR. THE SAFETY LATCH WAS ABOUT TO LET GO AND I HELD ONTO THE HANDLE AS LONG AS I COULD. IT FINALLY GOT INTO THE AIR SCREAM AND WAS JERKED OUT OF MY HAND AND THEN THE DOOR DEPARTED THE AIRCRAFT. DECLARED AN EMERGENCY AND LANDED.