FLT CREW OF MD80 EXPERIENCED VIBRATION ON TKOF AND RETURNED TO DEP ARPT. PREVIOUS TKOF ENCOUNTERED IDENTICAL SIT.

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THE ACFT WE WERE GIVEN HAD FLOWN EARLIER THAT MORNING; BUT HAD TO RETURN IMMEDIATELY AFTER TKOF BECAUSE OF A MODERATE TO SEVERE VIBRATION. MAINT HAD FOUND AND REPLACED A BAD WOBBLY R-HAND ENG DRIVEN HYD PUMP AND RETURNED THE ACFT TO SVC. AFTER OUR TKOF; AND ABOUT 10-15 SECONDS AFTER THE LNDG GEAR HAD BEEN RAISED; WE ALSO ENCOUNTERED A MODERATE VIBRATION -- ESPECIALLY LOUD AND NOTICEABLE IN THE COCKPIT. I REQUESTED AN IMMEDIATE RETURN FOR LNDG AT SEA FROM THE TWR. THE APCH AND LNDG WERE NORMAL AND THE VIBRATION SEEMED TO STOP OR RADICALLY DECREASE ONCE THE LNDG GEAR WERE LOWERED. WE LANDED 7 MINS AFTER OUR TKOF; OVERWT AT 141700 LBS. OUR TOUCHDOWN WAS SMOOTH AND THE RATE OF DSCNT WAS DECREASED TO 200 FPM OR LESS AT TOUCHDOWN. I DO NOT RECALL IF I EVER 'OFFICIALLY' DECLARED AN EMER; WHICH IS REQUIRED BECAUSE OF OUR OVERWT CONDITION -- BUT CFR WAS ON THE SCENE FOR OUR LNDG. AFTER PULLING IN TO THE GATE; WE WERE ADVISED BY THE GND CREW THAT THE R-HAND NOSE LNDG GEAR DOOR WAS OPEN AND MAINT LATER DISCOVERED THAT THERE HAD BEEN A FAILURE IN ITS LINKAGE SYS. THAT PROB WAS ALSO CORRECTED AND WE LATER PERFORMED A FUNCTIONAL CHK FLT AND FERRY BACK TO DFW WITH NO FURTHER VIBRATIONS NOTED. WE WERE BEING GIVEN EMER PRIORITY FOR LNDG AND IT DIDN'T DAWN ON ME UNTIL AFTER WE HAD LANDED THAT I PROBABLY NEVER DID USE THE WORDS 'I AM DECLARING AN EMER' WHICH SHOULD HAVE BEEN STANDARD LITANY AFTER ALL THE SIMULATOR TRAINING I HAVE BEEN THROUGH OVER THE YRS.