FLC EXECUTED GAR DUE TO PERCEIVED JUDGEMENT OF TFC SPACING OCCUPIED RWY. ACR X NON ADHERENCE TO ATC CLRNC DSCNT FROM OCCUPIED ALT DUE TO EQUIP PROBLEM AUTOPLT ALT HOLD. PLTDEV.

Narrative

ON APCH TO RWY 18R DFW WE WERE CLRED TO LAND. 1 ACFT HAD JUST LANDED AND WAS ON ROLLOUT. WE OBSERVED ANOTHER ACFT TAKING THE RWY. I ASKED MY FO TO ASK THE TWR ABOUT THAT ACFT. WE WERE COMING THROUGH ABOUT 5000 FT. AFTER THE FO REQUESTED INFO ON THE ACFT; THE TWR TOLD THE TFC TO EXPEDITE HIS TKOF. I OBSERVED THE LNDG TFC STILL ON THE RWY. I DECIDED TO TAKE IT AROUND. I EXECUTED THE GAR. MY FO COULDN'T CONTACT THE TWR RIGHT AWAY DUE TO OTHER TRANSMISSIONS. DURING THE GAR; WE WERE DIRECTED BY THE TWR TO A HDG AND FREQ CHANGE. WE WENT TO 5000 FT AS DIRECTED AND 270 DEGS. AT 5000 FT I DECIDED TO ENGAGE #2 AUTOPLT (#1 INOP). THE FMA SHOWED HDG SELECT AND ALT HOLD. THEN I NOTICED THE ALT WAS GOING ABOUT 100 FT LOW. I TRIED TO CORRECT BY ADJUSTING THE VERT SPD WHEEL. I COULD NOT MOVE IT. I DISCONNECTED THE AUTOPLT AND FLEW THE ACFT TO 5000 FT. WE GOT AS LOW AS 4700 FT. THE VERT SPD WHEEL WAS FROZEN AT FULL DOWN POS TO THE GATE. INFO: #1 FLT DIRECTOR WAS PLACARDED INOP. THE CAPT SIDE WAS BOOTSTRAPPED TO THE COPLT'S. AFTER TKOF FROM LAX; I NOTICED THAT WHILE HAND-FLYING USING THE FLT DIRECTOR; THE VERT SPD WHEEL WAS NOT WORKING AS WELL AS IT SHOULD; BUT WHEN THE #2 AUTOPLT WAS ENGAGED (#1 AUTOPLT INOP AND PLACARDED); THE VERT SPD WHEEL WORKED JUST FINE. I WROTE UP THE SYS GOING INTO DFW. AFTER EXECUTING THE GAR AT DFW; THE TWR ASKED IF WE NEEDED ASSISTANCE. I GUESS HE DIDN'T REALIZE WHY I TOOK IT AROUND. SUPPLEMENTAL INFO FROM ACN 225482: WITH #1 AUTOPLT ENGAGED IN ALT HOLD MODE AT 5000 FT MSL; THE ACFT BEGAN A SLOW DSCNT AT WHICH POINT THE CAPT DISCONNECTED THE AUTOPLT AND RECAPTURED THE ASSIGNED ALT. MAX DEV WAS 300 FT. (THERE WERE SEVERAL MEL'ED ITEMS ASSOCIATED WITH THE FMS ON THIS ACFT; BUT DURING THIS HIGH WORKLOAD SITUATION WE WERE UNABLE TO MONITOR THE ACFT AS DESIRED.)