Narrative:

While in descent for landing at sacramento metropolitan airport on the wraps 5 arrival, at 5300 ft, north of the elkoe intersection, on a heading of 340 degrees, at XX09Z (XX09 local), at an airspeed of 250 KTS, we encountered this near midair collision. My first officer and I were scanning for traffic when he said and simultaneously pointed to 'traffic.' I instantly started an evasive right bank to avoid an aircraft that appeared to be on a collision course with us. The aircraft was initially at our 11:30 O'clock position, hidden from my view by the vertical structure at the left side of my forward windshield, and probably below the glareshield. The aircraft appeared to be an AT6, white with red-orange stripes, similar to coast guard markings. The aircraft did not have an operating transponder, hence we did not have any indication from our TCASII or smf approach that another aircraft was in our vicinity. I was flying on the autoplt, and we had briefed that smf is not class B airspace, so extra traffic watch was necessary. We both were visually scanning the area for traffic, but a combination of ground clutter, scattered clouds and the fact that the aircraft was on a very similar course, below our altitude made it hard to see until the last few seconds. I observed the aircraft banking left just as I started our right bank. I believe that we passed less than 100 ft horizontal and at the same altitude as the other aircraft. It basically passed through where our left wing would have been had no evasive bank been used. Seconds later we were cleared for a visual approach and given a frequency change. Frequency congestion precluded reporting of the problem until on the ground. I briefly questioned the tower who checked with approach, but they were unaware of any aircraft in our near vicinity. Discussions with the supervisor later on in the evening (sac approach) as they reviewed the radar tapes indicated only a primary radar return for 3 sweeps while we leveled off at 5350 ft. This was probably when the aircraft was 'wing up' in his left turn. As we were using FAA recommended scan techniques and all available equipment, I believe that they only way to avoid this type of event is a requirement for mode C altitude encoding xponders, onboard interceptor grade radar or drastically improved ground radar having the capability to provide processed azimuth and elevation from primary radar returns.

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Original NASA ASRS Text

Title: NMAC. INTRUDER ACFT HAD NO XPONDER SO RPTR ACFT DIDN'T PICK IT UP ON TCASII. RPTR ACFT WAS IN CTL OF APCH CTLR AND POST FLT REVIEW OF THE TAPES FROM SAC APCH CTL SHOWED THE TARGET ONLY FOR 3 SWEEPS. RPTR RECOMMENDS EITHER ALL ACFT BE EQUIPPED WITH MODE C OR GND RADAR IMPROVED TO PRECLUDE THESE CLOSE CALL EVENTS.

Narrative: WHILE IN DSCNT FOR LNDG AT SACRAMENTO METRO ARPT ON THE WRAPS 5 ARR, AT 5300 FT, N OF THE ELKOE INTXN, ON A HDG OF 340 DEGS, AT XX09Z (XX09 LCL), AT AN AIRSPD OF 250 KTS, WE ENCOUNTERED THIS NMAC. MY FO AND I WERE SCANNING FOR TFC WHEN HE SAID AND SIMULTANEOUSLY POINTED TO 'TFC.' I INSTANTLY STARTED AN EVASIVE R BANK TO AVOID AN ACFT THAT APPEARED TO BE ON A COLLISION COURSE WITH US. THE ACFT WAS INITIALLY AT OUR 11:30 O'CLOCK POS, HIDDEN FROM MY VIEW BY THE VERT STRUCTURE AT THE L SIDE OF MY FORWARD WINDSHIELD, AND PROBABLY BELOW THE GLARESHIELD. THE ACFT APPEARED TO BE AN AT6, WHITE WITH RED-ORANGE STRIPES, SIMILAR TO COAST GUARD MARKINGS. THE ACFT DID NOT HAVE AN OPERATING XPONDER, HENCE WE DID NOT HAVE ANY INDICATION FROM OUR TCASII OR SMF APCH THAT ANOTHER ACFT WAS IN OUR VICINITY. I WAS FLYING ON THE AUTOPLT, AND WE HAD BRIEFED THAT SMF IS NOT CLASS B AIRSPACE, SO EXTRA TFC WATCH WAS NECESSARY. WE BOTH WERE VISUALLY SCANNING THE AREA FOR TFC, BUT A COMBINATION OF GND CLUTTER, SCATTERED CLOUDS AND THE FACT THAT THE ACFT WAS ON A VERY SIMILAR COURSE, BELOW OUR ALT MADE IT HARD TO SEE UNTIL THE LAST FEW SECONDS. I OBSERVED THE ACFT BANKING L JUST AS I STARTED OUR R BANK. I BELIEVE THAT WE PASSED LESS THAN 100 FT HORIZ AND AT THE SAME ALT AS THE OTHER ACFT. IT BASICALLY PASSED THROUGH WHERE OUR L WING WOULD HAVE BEEN HAD NO EVASIVE BANK BEEN USED. SECONDS LATER WE WERE CLRED FOR A VISUAL APCH AND GIVEN A FREQ CHANGE. FREQ CONGESTION PRECLUDED RPTING OF THE PROB UNTIL ON THE GND. I BRIEFLY QUESTIONED THE TWR WHO CHKED WITH APCH, BUT THEY WERE UNAWARE OF ANY ACFT IN OUR NEAR VICINITY. DISCUSSIONS WITH THE SUPVR LATER ON IN THE EVENING (SAC APCH) AS THEY REVIEWED THE RADAR TAPES INDICATED ONLY A PRIMARY RADAR RETURN FOR 3 SWEEPS WHILE WE LEVELED OFF AT 5350 FT. THIS WAS PROBABLY WHEN THE ACFT WAS 'WING UP' IN HIS L TURN. AS WE WERE USING FAA RECOMMENDED SCAN TECHNIQUES AND ALL AVAILABLE EQUIP, I BELIEVE THAT THEY ONLY WAY TO AVOID THIS TYPE OF EVENT IS A REQUIREMENT FOR MODE C ALT ENCODING XPONDERS, ONBOARD INTERCEPTOR GRADE RADAR OR DRASTICALLY IMPROVED GND RADAR HAVING THE CAPABILITY TO PROVIDE PROCESSED AZIMUTH AND ELEVATION FROM PRIMARY RADAR RETURNS.

Data retrieved from NASA's ASRS site as of July 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.