Narrative:

While cruising and in contact with and under control of boston approach I saw a large airline jet at 10 O'clock position, converging path, within 1 mi. He seemed to be at my altitude. ATC did not report traffic to me, nor did I hear my position reported to the heavy. After attempting to determine his altitude, I decided I could delay no longer. I knew that if I took evasive action now, I would definitely be able to clear the traffic. I took controls from the PF, turned 45 degree to my right and descended quickly to approximately 5700 ft when I saw the jet behind me and well above. I then returned to 6500 ft and on course and before I could inform ATC of my evasive action I was queried by ATC. ATC then told me the jet was level at 6000 and that my evasive action brought me closer to his altitude. The problem was discovered by adherence to the see and avoid concept. Contributing factors: ATC's failure to inform me of converging path traffic. Even though there was 500 ft vertical separation it was difficult to confirm the jet was below me, and not climbing. Human performance considerations: perceptions: 500 ft separation is difficult to determine with confidence necessary to ensure collision avoidance. Judgements: I judged the jet to be either at my altitude, or climbing since it just departed boston (or it appeared to be departing boston). Decisions: I had no time to further attempt to determine altitude convergence nor to query ATC about the airliner. So I decided to take action that should ensure avoidance, rather than gamble that the jet was lower than me. Actions/inactions: if I was informed by ATC as to the jet's altitude, I would have been more confident about our separation, and maybe evasive action would not have been necessary. The lack of a TA in such a tight situation led me to believe that maybe something was awry with ATC.

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Original NASA ASRS Text

Title: FLT INSTR WITH SPI HAS CLOSE ENCOUNTER WITH JET. TAKES EVASIVE ACTION.

Narrative: WHILE CRUISING AND IN CONTACT WITH AND UNDER CTL OF BOSTON APCH I SAW A LARGE AIRLINE JET AT 10 O'CLOCK POS, CONVERGING PATH, WITHIN 1 MI. HE SEEMED TO BE AT MY ALT. ATC DID NOT RPT TFC TO ME, NOR DID I HEAR MY POS RPTED TO THE HVY. AFTER ATTEMPTING TO DETERMINE HIS ALT, I DECIDED I COULD DELAY NO LONGER. I KNEW THAT IF I TOOK EVASIVE ACTION NOW, I WOULD DEFINITELY BE ABLE TO CLR THE TFC. I TOOK CTLS FROM THE PF, TURNED 45 DEG TO MY R AND DSNDED QUICKLY TO APPROX 5700 FT WHEN I SAW THE JET BEHIND ME AND WELL ABOVE. I THEN RETURNED TO 6500 FT AND ON COURSE AND BEFORE I COULD INFORM ATC OF MY EVASIVE ACTION I WAS QUERIED BY ATC. ATC THEN TOLD ME THE JET WAS LEVEL AT 6000 AND THAT MY EVASIVE ACTION BROUGHT ME CLOSER TO HIS ALT. THE PROBLEM WAS DISCOVERED BY ADHERENCE TO THE SEE AND AVOID CONCEPT. CONTRIBUTING FACTORS: ATC'S FAILURE TO INFORM ME OF CONVERGING PATH TFC. EVEN THOUGH THERE WAS 500 FT VERT SEPARATION IT WAS DIFFICULT TO CONFIRM THE JET WAS BELOW ME, AND NOT CLBING. HUMAN PERFORMANCE CONSIDERATIONS: PERCEPTIONS: 500 FT SEPARATION IS DIFFICULT TO DETERMINE WITH CONFIDENCE NECESSARY TO ENSURE COLLISION AVOIDANCE. JUDGEMENTS: I JUDGED THE JET TO BE EITHER AT MY ALT, OR CLBING SINCE IT JUST DEPARTED BOSTON (OR IT APPEARED TO BE DEPARTING BOSTON). DECISIONS: I HAD NO TIME TO FURTHER ATTEMPT TO DETERMINE ALT CONVERGENCE NOR TO QUERY ATC ABOUT THE AIRLINER. SO I DECIDED TO TAKE ACTION THAT SHOULD ENSURE AVOIDANCE, RATHER THAN GAMBLE THAT THE JET WAS LOWER THAN ME. ACTIONS/INACTIONS: IF I WAS INFORMED BY ATC AS TO THE JET'S ALT, I WOULD HAVE BEEN MORE CONFIDENT ABOUT OUR SEPARATION, AND MAYBE EVASIVE ACTION WOULD NOT HAVE BEEN NECESSARY. THE LACK OF A TA IN SUCH A TIGHT SITUATION LED ME TO BELIEVE THAT MAYBE SOMETHING WAS AWRY WITH ATC.

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.