UPON LEVELING AT FL270; A PREMIER 1 JET EXPERIENCES LOSS OF CARGO DOOR.

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I WAS SEATED IN THE COPLT'S SEAT OBTAINING OUR ATC CLRNC WHILE THE CAPT LOADED BAGGAGE AND BOARDED THE PAX. THE CAPT BRIEFED THE PAX; SEATED HIMSELF IN THE COCKPIT AND WENT THROUGH THE CHKLIST AND ENG START PROC. WE TAXIED TO RWY 29 AND DEPARTED IN VFR CONDITIONS. WE COMPLETED ALL CHKLISTS AND NOTED NO ABNORMALITIES; NO CAUTION (YELLOW) AND NO WARNING (RED) ANNUNCIATOR LIGHTS. THE CAPT WAS AT THE CTLS ON DEP AND THE ONLY LIGHTS ILLUMINATED ON THE ANNUNCIATOR PANEL WERE TWO GREEN IGNITION LIGHTS WHICH IS THE NORMAL CONDITION FOR TKOF. AT ABOUT 15 MINUTES INTO THE FLT; WE WERE BEGINNING TO LEVEL OFF AT FL270 OVER A SPARSELY POPULATED AGRICULTURAL AREA. SUDDENLY WE HEARD A LOUD BANG AND FELT SOMETHING SLAM AGAINST THE REAR PORTION OF THE FUSELAGE. WE NOTED A CAUTION (YELLOW) LIGHT AND A 'BAGGAGE DOOR OPEN' (YELLOW) LIGHT ILLUMINATED. THE CAPT IMMEDIATELY DISENGAGED THE AUTOPLT AND BEGAN HAND-FLYING THE AIRPLANE. I DID NOT NOTE ANY CTL PROBLEMS AND THE ACFT REMAINED IN STABLE FLT. THE CAPT CONTINUED FLYING THE AIRPLANE; TOOK RESPONSIBILITY FOR THE RADIOS; AND DIRECTED ME TO THE ABNORMAL CHKLIST FOR A 'BAGGAGE DOOR OPEN' CONDITION. THE CHKLIST DIRECTED US TO SLOW THE ACFT AND LAND AT THE NEAREST SUITABLE ARPT. THE CAPT COMPLIED WITH THE CHKLIST; CONTACTED ATC; AND DECLARED AN EMER. WE ELECTED TO RETURN TO ZZZ FOR SEVERAL REASONS INCLUDING THE FACT THAT IT HAS A 10000 FT RWY; AMPLE EMER SVCS AND MAINT SVCS WERE AVAILABLE. ATC CLRED US DIRECT TO ZZZ AND WE SET UP FOR THE VISUAL APCH. DURING THE APCH; THERE WAS CONCERN FOR POTENTIAL CTL PROBLEMS AT LOWER AIRSPDS AND WITH ACFT CONFIGURATION CHANGES. THE CAPT KEPT THE APCH AIRSPD A LITTLE HIGHER THAN NORMAL AND MONITORED ACFT CTL PARAMETERS WITH EACH CONFIGURATION CHANGE (FLAPS 10; 20; GEAR DOWN; FLAPS FULL). PRIOR TO THE LNDG; ATC CONTACTED EMER SVCS AND THEY WERE STANDING BY FOR WHAT WAS AN UNEVENTFUL LNDG. UPON CLEARING THE RWY; THE FIRE DEPT CONFIRMED THAT THE L SIDE REAR BAGGAGE DOOR WAS 'OPEN;' BUT IT WAS; IN FACT; MISSING. WE TAXIED TO THE RAMP AND DEPLANED THE PAX AND ACCOUNTED FOR EVERY PIECE OF PAX AND CREW LUGGAGE. UPON INSPECTION; WE NOTED THE L REAR BAGGAGE DOOR HAD SEPARATED IN FLT. ON THE RAMP; WE WERE VISITED BY A REPRESENTATIVE OF THE FSDO. WE PROVIDED PLT AND ACFT INFO AND GAVE HIM A BRIEF VERBAL STATEMENT OF WHAT OCCURRED. THERE IS ABSOLUTELY NO QUESTION IN MY MIND THAT THE REAR BAGGAGE DOOR WAS SECURE PRIOR TO DEP. WERE IT NOT SECURE; WE WOULD HAVE HAD A BAGGAGE DOOR WARNING LIGHT ILLUMINATED IN THE COCKPIT. I'M CONVINCED THAT THE INFLT SEPARATION WAS THE RESULT OF STRUCTURAL FAILURE.CALLBACK CONVERSATION WITH RPTR ACN 694331 REVEALED THE FOLLOWING INFO: THE CAPT WAS ADAMANT ABOUT PROPER CLOSURE OF THE UNPRESSURIZED CARGO DOOR PRIOR TO FLT. HE ADVISED THE FOUR CARGO DOOR LATCHES ARE LARGE; HAVE A BRIGHT FLUORESCENT COLOR THAT IS CLEARLY VISIBLE IF NOT LATCHED; AND HAVE A POSITIVE FEEL ALLOWING THEM TO ONLY BE LATCHED OR UNLATCHED. HE DESCRIBED THE NATURE OF THE DAMAGE AS ONE HINGE TOTALLY MISSING AND THE OTHER STILL ATTACHED TO THE FUSELAGE STRUCTURE WITH AN APPROX 8 INCH SQUARE OF THE CARBON FIBER DOOR STILL ATTACHED. RPTR FURTHER ADVISED THAT A SVC BULLETIN HAD BEEN ISSUED ABOUT PREVIOUS DOOR FAILURES BUT THAT THE ACFT IN QUESTION HAD COME FROM THE FACTORY WITH THE APPROPRIATE MODIFICATIONS DIRECTED THEREIN.