IFR PA31 ON VISUAL APCH TO MOT RWY 8 EXPERIENCED CONFLICT WITH PRECEDING DC9 ON APCH TO SAME RWY.

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I WAS FLYING A PA31-310 (NAVAJO) ON A VISUAL APCH TO RWY 8. MY FLT PLAN WAS TO FLY V15 FROM BIS TO MOT AT 6000 FT MSL. AT 20 MI FROM MOT; THE CTLR HAD ME FLY A HDG OF 320 DEGS; THEN 310 DEGS; THEN 340 DEGS; THEN 040 DEGS AND INTERCEPT THE FINAL APCH COURSE OF 080 DEGS TO LAND ON RWY 8. AT THE SAME TIME A DC9 WAS VECTORED OVER THE MOT VORTAC AND ENTERED A R DOWNWIND FOR RWY 8. I RPTED I HAD THE DC9 IN SIGHT. I THEN LOST THE DC9 AND ASKED THE TWR WHERE THE DC9 WAS. TWR SAID 'ON BASE.' I TOLD HIM I DIDN'T HAVE THE DC9 IN SIGHT. A FEW MOMENTS LATER; I SAW THE DC9 ABOUT 500 FT BELOW ME AND OFF TO MY 2 O'CLOCK POS. WE WERE BOTH ABOUT 4 MI OUT; ON FINAL. I IMMEDIATELY TURNED FROM 080 DEGS TO 330 DEGS AND CLBED FROM 4000 FT MSL TO 5000 FT MSL. THE DC9 DID A R 360 DEG TURN AND CAME BACK IN TO LAND AHEAD OF ME. HE WAS ABOUT 500 FT BELOW. I CALLED THE TWR AFTER LNDG AND EXPRESSED MY CONCERN AND CONFUSION OVER WHAT HAPPENED. I WONDER IF I DIDN'T UNDERSTAND A VECTOR; OR MISSED A CALL FROM ATC OR WHAT. IT SCARED ME.