AN MD80; DURING LNDG GEAR EXTENSION; EXPERIENCED YELLOW R MAIN GEAR DOOR WARNING. RECYCLED LNDG GEAR CHKED OK. ON GND FOUND MISSING BOLT.

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ON APCH TO PHX -- VISUAL TO RWY 26; AT APPROX 2000-1500 FT AGL; WE LOWERED THE LNDG GEAR. ALL APPEARED NORMAL; HOWEVER; THE 'GEAR DOOR' YELLOW ANNUNCIATOR LIGHT WOULD NOT EXTINGUISH. WE DISCUSSED A POSSIBLE GAR AND RECYCLED THE GEAR. THIS TIME; THE DOOR LIGHT WENT OUT AND WE LANDED UNEVENTFULLY. I ASSUMED THERE WAS A SPURIOUS SWITCH PROB THAT CORRECTED ITSELF. WE WERE SCHEDULED FOR APPROX 45 MINS OF GND TIME AND WE WENT ABOUT OUR PREFLT DUTIES. PRIOR TO THE FO DOING HIS WALKAROUND; THE AGENT CAME ON BOARD AND WONDERED IF OUR MECHANICAL WOULD DELAY BOARDING. I WONDERED WHAT MECHANICAL HE WAS TALKING ABOUT AND HE INFORMED ME THAT WE HAD A GEAR DOOR PROB. I SAID IT HAD FIXED ITSELF IN THE AIR AND WAS NO PROB. ABOUT THAT TIME; A MECH SHOWED UP AND INFORMED US THAT PIECES WERE MISSING FROM THE EMER LOCK ON THE R MAIN GEAR DOOR. APPARENTLY; WHEN WE INITIALLY LOWERED THE GEAR ON APCH; THE BOLT HOLDING THE LATCH IN PLACE WAS WORKING ITSELF OUT OF POS AND WOULD NOT ALLOW THE DOOR TO FULLY CLOSE. WHEN WE RECYCLED THE GEAR; THE BOLT AND SEVERAL WASHERS AND A NUT HAD DEPARTED THE ACFT (FORTUNATELY; WE WERE OVER THE SALT RIVER BASIN AT THE TIME AND ONLY SNAKES AND LIZARDS WERE PUT IN HARM'S WAY). WITHOUT THE BROKEN PARTS PREVENTING THE LOCKING OF THE GEAR DOOR; IT CLOSED AND INDICATED PROPERLY. IT WAS HELD UP BY HYD PRESSURE AND SAGGED TO THE GND AFTER THE AUX HYD PUMP WAS TURNED OFF DURING THE 'SECURE COCKPIT CHKLIST.'