RETURN LAND MANDATED IN A DO328-100 WHEN THE MAIN CABIN DOOR BECOMES UNLATCHED AND OPENS INFLT DURING INITIAL CLBOUT FROM ABE; PA.

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I SERVED AS CAPT ON FLT FROM ABE. THE BOARDING; TAXI OUT; AND TKOF WERE NORMAL. WE DEPARTED RWY 6 ABE. AT APPROX 100 FT AGL (DURING GEAR RETRACTION); WE HEARD A LOUD POP OR BANG; FOLLOWED BY A RUSH OF AIR. THE RED WARNING PANEL; MASTER WARNING; AND TRIPLE-CHIME ALERTED US TO AN UNSAFE 'DOORS' CONDITIONS. FURTHER REF TO THE DOORS SYS PAGE CONFIRMED THAT THE MAIN CABIN DOOR WAS OPEN. THE FO DECLARED AN EMER AND WE IMMEDIATELY BEGAN A L-HAND TFC PATTERN FOR RWY 6. ONCE LEVEL; I XFERRED CTL OF THE ACFT TO THE FO. THE FLT ATTENDANT WAS CONTACTED AND INFORMED OF AN IMMEDIATE EMER LNDG. WE MADE NORMAL LNDG CONFIGNS AND CAME TO A COMPLETE STOP ON THE RWY. THE L ENG WAS SHUT DOWN AFTER SETTING THE PARKING BRAKE. THE FO VISUALLY CONFIRMED THAT THE MAIN CABIN DOOR WAS STILL ATTACHED AFTER QUICKLY CHKING THAT ALL CREW MEMBERS AND PAX WERE NOT INJURED. WE TAXIED BACK TO THE GATE WITHOUT INCIDENT AND SECURED ACFT. FIREMEN AND MEDICAL PERSONNEL STABILIZED THE DOOR ENOUGH TO DEPLANE PAX AND ASSISTED IN HELPING THEM OFF. NO INJURIES OR ILLNESSES WERE RPTED AT THE TIME OF DEPLANEMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PIC STATED THAT THE FLT CREW HAD BEEN CLRED OF ANY WRONGDOING IN THE EVENT. THE FAA HAD INTERVIEWED THE FLT ATTENDANT AND; ACCORDING TO THE FAA INSPECTOR; THE FLT ATTENDANT HAD NO MEMORY OF THE FLT AFTER THE TKOF ROLL. SHE COULD NOT RECALL WHETHER OR NOT SHE MAY HAVE INADVERTENTLY RESTED HER ARM ON THE DOOR HANDLE WHILE SEATED IN THE JUMPSEAT. EXAMINATION OF THE DOOR AND ITS LOCKING MECHANISMS REVEALED NO MECHANICAL DEFICIENCIES. PRELIMINARY DATA DOWNLOADED FROM THE FDR REVEALED THE 'DOORS' PARAMETER INDICATED 'OPEN' APPROX 1.9 SECONDS AFTER THE AIR-GND PARAMETER INDICATED 'AIR.' THIS INCIDENT IS OF SOME CONCERN SINCE A SIMILAR EVENT OCCURRED IN EUROPE ON ANOTHER DORNIER TURBOPROP. THE DOOR DROPS DOWN FROM THE TOP; BEING HINGED ON THE BOTTOM AND IS USED AS AN AIR STAIR FOR LOADING AND UNLOADING. THE DOOR WARNING SYS IS 'VERY SOPHISTICATED' ACCORDING TO THE RPTR.