AN ACR WDB IN A RETURN LAND SITUATION AFTER BEING UNABLE TO RESOLVE A GEAR PROBLEM DURING CHKLIST USE.

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I WAS THE CAPT OF FLT ON CARGO ACFT THAT DEPARTED ORD. SHORTLY AFTER DEPARTING ORD WHILE PERFORMING THE AFTER TKOF CHKLIST; DURING THE UP-LATCH CHK EVERYTHING WAS NORMAL UNTIL THE GEAR HANDLE WAS MOVED FROM THE UP-LATCH CHK POS TO THE UP POS. WHEN THIS WAS DONE THE RED GEAR UNSAFE LIGHT CAME ON. I HAD THE GEAR CYCLED DOWN THEN BACK UP. AFTER THIS WAS DONE THE RED UNSAFE LIGHT REMAINED ON. I THEN CALLED FOR THE AFTER TKOF CHKLIST. AFTER THIS WAS COMPLETED I CALLED FOR THE FAILED UP-LATCH ABNORMAL CHK. DURING THIS ABNORMAL THE FE STATED THAT THE R MAIN LNDG GEAR DOOR WAS NOT UP-LATCHED. THE CHKLIST FOR THIS ABNORMAL STATED THAT IF THERE WAS NO PAST HISTORY OF GEAR PROBLEMS OR LOUD NOISE DURING RETRACTION THE GEAR COULD BE CYCLED AT THE DISCRETION OF THE CAPT. I CALLED FOR THE GEAR TO BE LOWERED; AT THIS TIME THE GEAR HANDLE WOULD NOT MOVE TO THE DOWN POS. THE HANDLE WOULD MOVE JUST PAST THE UP-LATCH CHK POS BEFORE JAMMING. I CALLED FOR THE GEAR HANDLE TO BE RAISED. WHILE THIS WAS BEING ACCOMPLISHED; THE FE CALLED HYD PRESSURE AND FLUID LOSS. THE FE PERFORMED THE IMMEDIATE ACTION ITEMS. THE HYD CHKLIST WAS ACCOMPLISHED THROUGH PRESSURE CHK TERMINAL AREA; DOWN TO ABNORMAL HYD CHKLIST. DURING THE ABNORMAL CHKLIST WHILE MOVING THE GEAR HANDLE TO THE DOWN POS THE FE CALLED OUT TOTAL HYD PRESSURE AND FLUID LOSS. I PROCEEDED WITH THE EMER HYD CHKLIST. THE GEAR HAD LOWERED AT THIS TIME WITH 3 GREEN LIGHTS. WHILE RETURNING TO LAND AT ORD WE NOTIFIED ATC THAT WE WERE RETURNING AND DECLARED AN EMER AND REQUESTED THE EQUIP STANDING BY AT THE END OF RWY 27L. AFTER LNDG AT ORD ALL BRAKE PRESSURE WAS LOST DURING ROLL-OUT; FROM ABOUT 30 KTS THE ACFT WAS STOPPED WITH AIR PRESSURE. I CLRED THE RWY AND TOLD ATC THAT THE EMER WAS OVER BUT THAT I WAS GOING TO HOLD MY POS UNTIL THE COMPANY SHOWED UP TO TOW THE ACFT TO THE GATE. WHEN OUR MECH ARRIVED PLANE SIDE HE TOLD US THAT THE R MAIN LNDG GEAR DOOR HAD LEFT THE ACFT. I ALSO CALLED GND CTL AND NOTIFIED THEM THAT THE RWY MIGHT BE CONTAMINATED. THEY ADVISED THAT THE RWY HAD BEEN CHKED AND NOTHING WAS FOUND. ATC INFORMED ME AT THIS TIME THAT A RESIDENT HAD CALLED IN AND RPTED THAT SOMETHING HAD FALLEN THROUGH THE ROOF OF HER HOME. AS FAR AS I KNOW NOBODY WAS HURT. THE ACFT WAS TOWED TO THE GATE. I INSTRUCTED THE FE TO PULL THE COCKPIT VOICE RECORDER CIRCUIT BREAKER. UPON LEAVING THE ACFT I WENT TO THE NEAREST PHONE AND NOTIFIED THE COMPANY ABOUT EVERYTHING ABOVE. I TALKED TO MR X. I FILLED OUT A CAPT IRREGULARITY RPT; ALONG WITH A FLT DEBRIEF RPT ATTACHED TOGETHER AND GIVEN TO MR X WHEN WE ARRIVED IN WILMINGTON; OH. I PERSONALLY DO NOT KNOW WHAT MIGHT HAVE CAUSED THE R MAIN GEAR DOOR TO SEPARATE FROM THE ACFT. THE HYD ACTUATOR WAS FOUND JUST AFTER TKOF AND THE GEAR DOOR WAS FOUND 2 OR 3 DAYS LATER IN AN INDUSTRIAL LOT MI APART. I INSPECTED THE ACFT AFTER THE INCIDENT AND FOUND THAT THE R GEAR DOOR HAD PULLED OUT OF ALL THE HINGES AND THE ACTUATOR WAS ALSO MISSING. I SAW NO INDICATION THAT THE TIRES HAD IMPACTED THE GEAR DOOR BUT I WAS TOLD BY PEOPLE THAT SAW THE DOOR THAT THERE WERE TIRE MARKS ON THE DOOR. SINCE I HAVE NO IDEA AS TO THE CAUSE OF THE PROBLEM I CAN'T RECOMMEND ANYTHING THAT WOULD PREVENT THIS FROM HAPPENING AGAIN.