Narrative:

Incident #1: as we flew inbound on the runway 9 localizer at 2000' approach advised of traffic ahead, wbound, altitude unknown (I don't recall the distance given). Usually, this kind of warning results in a sighting, if sighted at all, of traffic well above or below one's flight path. But the horror of seeing another aircraft boring in, spinner to spinner, at about 200 mph (combined closure rate) is, I'm sure, burned into my, and my companion's, memory forever! The intruder, an small aircraft B, was exactly at our altitude and heading straight at us! I dived immediately and the other aircraft sailed less than 100' over us, taking no evasive action. Incident #2: virtually a carbon copy of #1, except that this aircraft (same small aircraft C type) banked to its right after I dived. It went by too fast to read its tail #, but the colors, basic white with aqua stripes, were visible. Later inbound, from the OM, somewhat shell-shocked, I was chided by the tower for not reporting the OM as directed and informed that I was high on the approach. I reported what had happened and asked if he was aware that 2 aircraft had flown a 'bootleg' back course right into my front course approach. He said that he was not aware of said traffic. I think that my companion and I are very fortunate that the OM was not reported by my ELT.

Google
 

Original NASA ASRS Text

Title: PLT OF SMA ON IFR PLAN ON APCH TO SFB HAD ONE NMAC 1 MILE OUTSIDE OUTER MARKER, AND ANOTHER NMAC AT OUTER MARKER.

Narrative: INCIDENT #1: AS WE FLEW INBND ON THE RWY 9 LOC AT 2000' APCH ADVISED OF TFC AHEAD, WBOUND, ALT UNKNOWN (I DON'T RECALL THE DISTANCE GIVEN). USUALLY, THIS KIND OF WARNING RESULTS IN A SIGHTING, IF SIGHTED AT ALL, OF TFC WELL ABOVE OR BELOW ONE'S FLT PATH. BUT THE HORROR OF SEEING ANOTHER ACFT BORING IN, SPINNER TO SPINNER, AT ABOUT 200 MPH (COMBINED CLOSURE RATE) IS, I'M SURE, BURNED INTO MY, AND MY COMPANION'S, MEMORY FOREVER! THE INTRUDER, AN SMA B, WAS EXACTLY AT OUR ALT AND HDG STRAIGHT AT US! I DIVED IMMEDIATELY AND THE OTHER ACFT SAILED LESS THAN 100' OVER US, TAKING NO EVASIVE ACTION. INCIDENT #2: VIRTUALLY A CARBON COPY OF #1, EXCEPT THAT THIS ACFT (SAME SMA C TYPE) BANKED TO ITS RIGHT AFTER I DIVED. IT WENT BY TOO FAST TO READ ITS TAIL #, BUT THE COLORS, BASIC WHITE WITH AQUA STRIPES, WERE VISIBLE. LATER INBND, FROM THE OM, SOMEWHAT SHELL-SHOCKED, I WAS CHIDED BY THE TWR FOR NOT RPTING THE OM AS DIRECTED AND INFORMED THAT I WAS HIGH ON THE APCH. I RPTED WHAT HAD HAPPENED AND ASKED IF HE WAS AWARE THAT 2 ACFT HAD FLOWN A 'BOOTLEG' BACK COURSE RIGHT INTO MY FRONT COURSE APCH. HE SAID THAT HE WAS NOT AWARE OF SAID TFC. I THINK THAT MY COMPANION AND I ARE VERY FORTUNATE THAT THE OM WAS NOT RPTED BY MY ELT.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.