PVT PLT OF AN SMA DSNDED FROM ASSIGNED ALT TO AVOID FLYING INTO AN OVCST RESULTING IN A CONFLICT WITH ANOTHER ACFT.

Narrative

ON A RETURNING FLT FROM MIDLAND AIRPARK TO ADDISON ARPT ON APR/SUN/94; REGIONAL APCH ASSIGNED ME AN ALT OF 3500 FT MSL AND TO FLY DIRECT DFW. I WAS APCHING FROM THE SW. THEY DETERMINED THE MODE C ON THE PLANE'S XPONDER WAS XMITTING AN INCORRECT ALT; SO APCH TOLD ME TO TURN THE MODE C OFF AND LEAVE THE XPONDER ON. I DEVIATED FROM 3500 FT MSL TO 3100 FT MSL TO MAINTAIN VFR BECAUSE I AM NOT INST RATED; THE AIRPLANE WAS NOT INST CERTIFIED AND THERE WAS NOT AN IFR FLT PLAN FILED. I INTENDED TO MAKE MY INTENTIONS KNOWN AS SOON AS THERE WAS A BREAK IN THE CONVERSATION ON THE RADIO. BEFORE I COULD CALL APCH; AN ACR TWIN TURBOPROP PASSED MY L SIDE HEADED W AND CLBING TO HIS ALT. HE ACKNOWLEDGED ME IN SIGHT AFTER ATC ADVISED HIM OF MY PRESENCE. I IMMEDIATELY CUT INTO THE CONVERSATION AND GAVE MY ALT. APCH THEN TOLD ME TO TURN L TO 270 DEGS. I FOLLOWED THE DIRECTION AND THEY VECTORED ME AROUND DFW INTL TO THE N AND INTO ADDISON ARPT. THE ONLY WAY I BELIEVE THIS SITUATION COULD HAVE BEEN PREVENTED IS IF APCH HAD VECTORED ME AROUND THE CLASS B AS SOON AS THE MODE C FAILURE HAD BEEN IDENTED; ESPECIALLY SINCE THE DENSITY IN AIR TFC WAS SO HVY FOR THAT MOMENT. AS FOR MYSELF; IF I HAVE THIS TYPE OF FAILURE AGAIN; THAT WILL BE MY REQUEST. ONE FACTOR THAT MIGHT PLAY IN THE HUMAN PERFORMANCE ON MY PART IS FATIGUE. I ONLY HAD 2 HRS OF SLEEP THE NIGHT BEFORE WAITING FOR TSTMS TO CLR.