Narrative:

I had been cleared to descend from 8000 ft; my en route IFR altitude for over 2 hours; to 4000 ft as I neared my destination. I reduced power and started my descent at about 500 FPM. Fairly quickly I received a 'traffic; traffic' audio alert from the skywatch traffic alert system operating through my garmin 530. When I looked at the 530; the small traffic screen had activated and indicated a target at my 9 O'clock position; slightly below me and it was well inside the 2 mi ring. I activated the large traffic screen and tried to figure out what the traffic was doing -- crossing; opposing; climbing; descending; or what. Within seconds; I saw that it was moving directly toward me at 9 O'clock position and now at my altitude and the target was almost touching my aircraft symbol on the screen. I pushed the nose down hard and banked hard right. When I looked over my left shoulder; I saw the target plane extremely close passing directly overhead. I don't believe the other pilot was aware of the near miss as he never deviated from his straight and level flight. The WX was clear with good visibility. When I queried ATC if they saw that aircraft that had just passed over me; they hesitated and then said; yes they now see the traffic and it is at 7500 ft. At this time I was at 7000 ft and continuing my descent. One of the shortcomings I have seen with the skywatch is that traffic that is below the shadow of my aircraft is not always shown until it is quite close -- sometimes less than 1 mi. At the low 9 O'clock position; I believe my wing was blanking out the target's transponder response until he was very close. I firmly believe that without the alert that there was a good likelihood of bent metal. Experience has shown me in over 45 yrs of flying that other aircraft are at times extremely hard to see -- even in good WX at fairly close range. The skywatch frequently shows aircraft within 1 mi or so and within 500 ft that ATC may or may not alert me to that I or my passenger are unable to visually pick out. It hasn't stopped me from trying my best to watch for other aircraft -- as ATC occasionally misses or does not call a target that might be a potential conflict.

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Original NASA ASRS Text

Title: AN IFR C310 PILOT DESCRIBES A VMC NMAC DURING A DESCENT TO HIS DESTINATION. AN ADVANCED GA NAV TRAFFIC ALERT ALLOWED FOR AN AVOIDANCE MANEUVER.

Narrative: I HAD BEEN CLRED TO DSND FROM 8000 FT; MY ENRTE IFR ALT FOR OVER 2 HRS; TO 4000 FT AS I NEARED MY DEST. I REDUCED PWR AND STARTED MY DSCNT AT ABOUT 500 FPM. FAIRLY QUICKLY I RECEIVED A 'TFC; TFC' AUDIO ALERT FROM THE SKYWATCH TFC ALERT SYS OPERATING THROUGH MY GARMIN 530. WHEN I LOOKED AT THE 530; THE SMALL TFC SCREEN HAD ACTIVATED AND INDICATED A TARGET AT MY 9 O'CLOCK POS; SLIGHTLY BELOW ME AND IT WAS WELL INSIDE THE 2 MI RING. I ACTIVATED THE LARGE TFC SCREEN AND TRIED TO FIGURE OUT WHAT THE TFC WAS DOING -- XING; OPPOSING; CLBING; DSNDING; OR WHAT. WITHIN SECONDS; I SAW THAT IT WAS MOVING DIRECTLY TOWARD ME AT 9 O'CLOCK POS AND NOW AT MY ALT AND THE TARGET WAS ALMOST TOUCHING MY ACFT SYMBOL ON THE SCREEN. I PUSHED THE NOSE DOWN HARD AND BANKED HARD R. WHEN I LOOKED OVER MY L SHOULDER; I SAW THE TARGET PLANE EXTREMELY CLOSE PASSING DIRECTLY OVERHEAD. I DON'T BELIEVE THE OTHER PLT WAS AWARE OF THE NEAR MISS AS HE NEVER DEVIATED FROM HIS STRAIGHT AND LEVEL FLT. THE WX WAS CLR WITH GOOD VISIBILITY. WHEN I QUERIED ATC IF THEY SAW THAT ACFT THAT HAD JUST PASSED OVER ME; THEY HESITATED AND THEN SAID; YES THEY NOW SEE THE TFC AND IT IS AT 7500 FT. AT THIS TIME I WAS AT 7000 FT AND CONTINUING MY DSCNT. ONE OF THE SHORTCOMINGS I HAVE SEEN WITH THE SKYWATCH IS THAT TFC THAT IS BELOW THE SHADOW OF MY ACFT IS NOT ALWAYS SHOWN UNTIL IT IS QUITE CLOSE -- SOMETIMES LESS THAN 1 MI. AT THE LOW 9 O'CLOCK POS; I BELIEVE MY WING WAS BLANKING OUT THE TARGET'S XPONDER RESPONSE UNTIL HE WAS VERY CLOSE. I FIRMLY BELIEVE THAT WITHOUT THE ALERT THAT THERE WAS A GOOD LIKELIHOOD OF BENT METAL. EXPERIENCE HAS SHOWN ME IN OVER 45 YRS OF FLYING THAT OTHER ACFT ARE AT TIMES EXTREMELY HARD TO SEE -- EVEN IN GOOD WX AT FAIRLY CLOSE RANGE. THE SKYWATCH FREQUENTLY SHOWS ACFT WITHIN 1 MI OR SO AND WITHIN 500 FT THAT ATC MAY OR MAY NOT ALERT ME TO THAT I OR MY PAX ARE UNABLE TO VISUALLY PICK OUT. IT HASN'T STOPPED ME FROM TRYING MY BEST TO WATCH FOR OTHER ACFT -- AS ATC OCCASIONALLY MISSES OR DOES NOT CALL A TARGET THAT MIGHT BE A POTENTIAL CONFLICT.

Data retrieved from NASA's ASRS site as of January 2009 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.