NMAC IN THE TFC PATTERN BTWN A PVT TWIN TURBOJET AND AN EXPERIMENTAL GA ACFT 2 NM N OF RWY 15 AT ESN; MD.

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I WAS ON AN IFR FLT PLAN; AND WAS UNDER CTL OF BALTIMORE APCH AS I WAS NEARING ESN; MD; AN UNCTLED FIELD. THE AWOS WAS RPTING CEILING GREATER THAN 12000 FT AND VISIBILITY 4 OR 5 MI AS I RECALL. HOWEVER; AT 3000 FT I WAS IN AND OUT OF CLOUDS AND THE LATERAL VISIBILITY WAS SO POOR THAT I WAS GIVEN THE ILS APCH FROM A POINT ABOUT 10 MI S OF THE ARPT. I ADVISED THE CTLR THAT I WOULD EXECUTE THE ILS RWY 4 AND CIRCLE TO LAND ON RWY 15; WHICH I HAD DETERMINED TO BE THE ACTIVE FROM MONITORING UNICOM. I SAW THE RWY ABOUT 3 MI OUT AT WHICH TIME APCH TERMINATED RADAR SVC. SINCE THE FIELD IS UNCTLED; THE CTLR TOLD ME TO SWITCH TO ADVISORY WHICH I DID; BROADCASTING MY POS AND INTENTIONS. I ATTEMPTED TO IDENT THE CALLS OF THE TFC IN THE PATTERN WITH THE DISPLAY ON MY TCAS AND DETERMINED THAT THE TFC WAS EITHER ON BASE OR FINAL OR ABOUT TO TURN BASE. I INTENDED TO CROSS THE FIELD AT ABOUT 1000 FT AND ENTER A L DOWNWIND FOR RWY 15 AND COULD SEE NO TFC VISUALLY OR ON THE TCAS. I HAD BROADCASTED MY POS AND INTENTIONS SEVERAL TIMES WHILE ON THE ILS. AS I WAS ROLLING OUT ON DOWNWIND; I SAW A SMALL 2 SEAT TAILWHEEL ACFT ABOUT 300 FT AHEAD AND MAYBE 50 FT ABOVE. I OBSERVED NO BEACON OR OTHER LIGHTING AND THE TCAS SHOWED NO TARGET; SO I CAN ONLY ASSUME THE ACFT HAD NO XPONDER. I DETERMINED FROM LATER XMISSIONS THAT THE ACFT WAS AN EXPERIMENTAL. THE REST OF THE FLT WAS UNEVENTFUL AND I CANCELED IFR AFTER LNDG. AN APCH TO AN UNFAMILIAR FIELD IN REDUCED VISIBILITY IS A HIGH WORKLOAD SIT; AND THE PLT IS NOT ONLY TRYING TO FIND THE FIELD; BUT OTHER ACFT WHICH HE OFTEN CANNOT SEE. A TFC DISPLAY SUCH AS THE TCAS HAS PROVEN TO BE EXTREMELY USEFUL IN SUCH SITS; WHICH IS THE MAIN REASON I PURCHASED IT. IT WILL NOT DISPLAY NON XPONDER EQUIPPED ACFT. TO THE BEST OF MY DETERMINATION; IT WOULD ONLY HAVE BEEN LUCK FOR ME TO HAVE SEEN THE ACFT MUCH SOONER IN THOSE CONDITIONS. I CONTINUE TO FEEL THAT HIGH PERFORMANCE ACFT ARRIVING IFR AT A NON-CTLED FIELD WITH MIXED GA TFC IS A POTENTIALLY DANGEROUS SIT.