PAX ON A COMMERCIAL ACFT OBSERVES ANOTHER COMMERCIAL ACFT APCH HIS ACFT FROM THE SIDE AND PERCEIVES THAT IT MAY BE AN NMAC.

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I WAS A PAX FLYING FROM DTW TO ATL WHEN I NOTICED A VAPOR TRAIL AT SOME DISTANCE OUT THE L (E) SIDE OF THE ACFT. WITHIN 1 MIN OR SO; THE TRAIL BECAME EVIDENT THAT IT WAS COMING FROM A DARK TWIN ENG PLANE APCHING PERPENDICULAR TO OUR FLT PATH. I TOLD MY WIFE TO LOOK OUT THE WINDOW AS THE 2 ROWS IN FRONT OF US AND THE ROW BEHIND US LOOKED OUT AT THE SAME TIME. THE APCHING PLANE MADE A SUDDEN UPWARD CLB AND VEERED TO HIS R TO PASS OVER TOP OF OUR ACFT. THOSE THAT WERE WATCHING EXCLAIMED; 'WOW! DID I REALLY SEE THAT!? THAT PLANE WAS REALLY CLOSE.' THE APCHING PLANE CLBED RAPIDLY TO AVOID COLLISION AND AT SUCH A RATE THAT IT APPEARED ALMOST AS A MIL MANEUVER. IN TOTAL; 8-9 PEOPLE SAW THE PLANE AND THE AVOIDANCE. WE ASKED THE STEWARDS IF THEY HAD SEEN IT OR IF THE PLT OF OUR PLANE KNEW WHERE THE OTHER PLANE CAME FROM. NO ONE KNEW OF THE PLANE AND THE PLT TOLD THE STEWARDS THAT THERE WAS NO OTHER PLANE. UPON LEAVING THE PLANE; SEVERAL OF US ASKED THE PLT IF HE WAS GOING TO RPT THE NMAC. HIS RESPONSE WAS 'YOU MUST HAVE SEEN A UFO!' I'M A BUSY PROFESSIONAL; A FREQUENT TRAVELER AND HAPPY TO BE ABLE TO WRITE THIS RPT. I AM NOT HAPPY WITH THE CAVALIER APPROACH BY A PLT RESPONSIBLE FOR SAFE TRAVEL TO DISREGARD MY SAFETY. SURELY THE NAVIGATOR; ATC; OR SOMEONE RESPONSIBLE FOR TFC SAW THIS ON RADAR -- EVEN IF THE PLT DID NOT SEE IT PERSONALLY. I ASK THAT THIS INCIDENT BE INVESTIGATED AND THAT THE PLT TAKE HIS RESPONSIBILITIES MORE SERIOUSLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE SAW THE CONTRAILS OF THE APCHING ACFT AND AS IT GOT CLOSER IT APPEARED TO CLB AND AVOID THE ACFT HE WAS IN. HE WAS NOT AWARE OF THE CHANGE TO THE SEPARATION RULES ASSOCIATED WITH RVSM AND THE SPDS WITH WHICH ACFT APCH.