RETURN LAND MANDATED AFTER LGT'S AFT CARGO DOOR OPENS JUST AFTER LIFTOFF.

Narrative

BEFORE START CHKLIST COMPLETE. BEFORE PUSHBACK TAXI CHKLIST COMPLETE EXCEPT FOR AFT CARGO DOOR LIGHT AND WAS SO STATED AFT CARGO DOOR LIGHT TO GO 30 TO 40 SECONDS THE FLT ENGINEER STATED DOOR LIGHTS OUT. I BEING THE FO STATED BEFORE PUSHBACK/TAXI CHKLIST COMPLETE. WE PUSH BACK AND STARTED THE ENGS; RECEIVED TAXI CLRNC; THE ENGINEER RECEIVED THE WTS AND THERE WERE NO CHANGES. THE BEFORE TKOF CHKLIST WAS COMPLETED AND WE WERE CLRED FOR TKOF ON RWY 22R. AFTER THE GEAR WAS CALLED FOR AND RETRACTED THE ENGINEER STATED THAT THE AFT CARGO LIGHT CAME ON. THE CAPT AND I ACKNOWLEDGED THAT FACT AND THE CAPT CALLED FOR FLAPS 5. UPON THE FLAPS REACHING POS 5 WE FELT A BUFFETING AND THE CAPT FELT AND NOTICED THE CABIN CLBING AND AT THE SAME TIME TWR TOLD US TO GO TO DEP. WE TOLD DEP WE HAD TO LEVEL OFF AND DID SO AT 2500 FT. THE CAPT CALLED FOR FLAPS 1 AND WE MADE A DECISION TO RETURN TO THE FIELD AND STAY SLOW. AN UNEVENTFUL LNDG FOLLOWED. IN MY OPINION THE CREW HANDLED THE ENTIRE SITUATION VERY PROFESSIONALLY AND EFFECTIVELY AND SOP WAS USED THROUGHOUT.