A B727-200 ON TKOF AND CLB TO 4000 FT DECLARED AN EMER AND DIVERTED DUE TO ACFT VIBRATING BADLY CAUSED BY AFT CARGO DOOR OPEN.

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RWY 10 ZZZ BRAKING ACTION FAIR TO POOR. FO WAS PF. AT ABOUT 100 KTS; WE HEARD AN INTERMITTENT FIRE BELL WITH NO VISUAL INDICATIONS. THE THIRD TIME THE BELL CAME ON; THE CARGO FIRE WARNING LIGHT CAME ON BRIEFLY; THEN WENT OUT. DUE TO SLIPPERY RWY CONDITIONS; I ELECTED TO CONTINUE THE TKOF. AT LIFTOFF; WE WERE AWARE THAT WE HAD NO PRESSURIZATION. OUR EARS WERE POPPING AND THE CABIN RAE OF CLB WAS THE SAME AS THE ACFT. WE HAD NO OTHER COCKPIT INDICATIONS THAT ANYTHING WAS ABNORMAL. VIBRATIONS WERE FELT THROUGHOUT THE CABIN AND FLT DECK. AT ABOUT 1200 FT BEFORE THE FLAPS WERE RETRACTED; FLT ATTENDANT CALLED FROM THE BACK OF THE ACFT AND TOLD US THAT THE AFT CARGO DOOR WAS OPEN. ACFT WAS VIBRATING BADLY SO WE KEPT THE SPD TO 210 KTS. CALLED DEP WITH INITIAL CLB TO 4000 FT; THEN CLRED TO 10000 FT. ADVISED DEP CTL OF OUR OPEN DOOR AND ELECTED TO REMAIN AT 4000 FT. WE ASKED FOR WX AND RWY CONDITIONS AT XXX AND ABC TO SEE IF CONDITIONS THERE WERE ANY BETTER THAN ZZZ. WE ELECTED TO RETURN TO ZZZ AND WERE ASKED BY ATC IF WE WANTED THE EMER EQUIP. WE RESPONDED YES. WE TRIED CALLING ACR RADIO TO ALERT DISPATCH -- NO ANSWER. THEN WE TRIED ZZZ OPS -- NO ANSWER UNTIL WE TAXIED IN. AS WE WERE VECTORED TO DOWNWIND AND DSCNT TO 2100 FT; SO ROCKED THE AFT CARGO LIGHT BACK AND FORTH UNTIL 'AFT CARGO' DOOR LIGHT FINALLY CAME ON. WE RAN DOOR OPEN CHKLIST AND RETURNED TO ZZZ. LNDG ON RWY 10 WAS UNEVENTFUL. VISIBILITY WAS GOOD AND BRAKING ACTION WAS RPTED FAIR FOR LNDG; BUT BRAKING ACTION ON LAST PORTION OF RWY WAS POOR AT THE TURNOFF. WE WERE LOOKED OVER BY THE EMER VEHICLES AND THEN TAXIED TO THE GATE. FE DID AN EXCELLENT JOB OF KEEPING FLT ATTENDANTS AND PAX ADVISED OF OUR SIT. FO DID AN EQUALLY OUTSTANDING JOB OF FLYING THE ACFT SAFELY BACK TO ZZZ. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A B727-200 AND THE DOOR WARNING WAS TESTED BEFORE PUSHBACK AND PRIOR TO TKOF AND THE LIGHTS TESTED OK. THE RPTR SAID ON LIFTOFF IT WAS IMMEDIATELY NOTED WE HAD LOST PRESSURIZATION AND THE CABIN WAS CLBING WITH THE AIRPLANE. THE RPTR STATED THE AIRPLANE BEGAN TO VIBRATE BADLY AND THE SPD WAS HELD DOWN. THE RPTR SAID CTL OF THE AIRPLANE EXCEPT FOR THE VIBRATIONS WAS NORMAL. THE RPTR SAID THE CREW WAS ADVISED BY THE AFT CABIN FLT ATTENDANT THE REAR CARGO DOOR WAS OPEN. THE RPTR SAID 'ABOUT THIS TIME WE GOT THE DOOR WARNING LIGHT.' THE RPTR STATED THE TRIP WAS CANCELED AND THE AIRPLANE WAS MAINT FERRIED TO A MAINT FACILITY TO CORRECT THE DOOR PROB. THE RPTR SAID PRIOR TO THE FERRY FLT THE MAINT PEOPLE HAD DIFFICULTY CLOSING AND LOCKING THE DOOR.