PLT OF AN SMA SEL OVERTOOK AND PASSED ANOTHER ACFT ON THE WRONG SIDE RESULTING IN A NMAC.

Narrative

IT WAS A CLR; MOONLESS AND VERY DARK NIGHT WITH EXCELLENT VISIBILITY. I WAS FLYING OVER THE WATER; JUST W OF GROTON ARPT. THIS PART OF CT IS RURAL; WITH NOT VERY MANY LIGHTS. AS A RESULT MY EYES HAD ADJUSTED TO THE DARKNESS SUCH THAT I HAD TO TURN THE PANEL LIGHTS WAY DOWN. I WAS ABOUT 20 MI FROM MY HOME ARPT AT 5500 FT WHEN I LISTENED TO THE ATIS AND CALLED GROTON ARPT. AT ABOUT 15 MI OUT A STARTED A DSCNT. I NOTICED ANOTHER PLANE DIRECTLY AHEAD OF ME; STROBES FLASHING; WHICH I THOUGHT WAS GOING TO GROTON. I JUDGED THAT THIS PLANE WAS AT ABOUT 3000 OR 3500 FT. IT SEEMED TO BE TRAVELING ONLY SLIGHTLY SLOWER; SO I PLANNED TO DSND BEHIND HIM AND GO UNDER HIM IF NEED BE TO REACH PATTERN ALT. ON DSCNT; THE PLANE SEEMED TO BE GETTING SOMEWHAT NEARER; BUT IT STILL LOOKED TO BE MANY MI AHEAD. I ASSUMED THE PLANE WAS TRAVELING ONLY SLIGHTLY SLOWER THAN ME. IT SEEMED THAT IN A MATTER OF A MIN OR 2 I WOULD BE WELL BELOW HIM; AT PATTERN ALT; AND COULD PASS WELL UNDER HIM. ALL OF A SUDDEN AT 9.5 DME FROM GROTON; THE PLANE CALLED THE TWR TO REQUEST TRANSITION THROUGH THE AIRSPACE; WHICH WAS GRANTED. HE SAID HE ALSO WAS 9 MI OUT AND HE WAS A HELI. I IMMEDIATELY REALIZED THAT THERE WAS A PROB. MY EYES WERE TELLING ME HE WAS STILL MI AHEAD OF ME; YET HIS POS RPT INDICATED HE WAS FAR CLOSER TO ME THAN I HAD THOUGHT. FURTHERMORE; I ALSO REALIZED THAT; BEING A HELI; HE PROBABLY WAS GOING SIGNIFICANTLY SLOWER THAN I HAD THOUGHT; AND THAT OUR CLOSURE SPD WAS DRAMATICALLY HIGHER THAN I HAD ANTICIPATED SINCE I WAS DSNDING AT 170-180 KTS. I HAD PLANNED TO REACH HIS ALT MANY MI BEHIND HIM AND PASS WELL UNDERNEATH. INSTEAD I WAS MUCH CLOSER; POSSIBLY ONLY 1/2 MI AWAY; AND CLOSING IN FAR MORE RAPIDLY THAN I HAD ANTICIPATED. AT A CLOSURE RATE OF 60-80 KTS; THE HALF MI BTWN US WOULD DISAPPEAR IN 20 SECONDS OR SO. I DECIDED TO INCREASE MY RATE OF DSCNT AND FLY UNDER HIM. I DID NOT WANT TO MAKE ANY SHARP TURNS OR ALT EXCURSIONS WHICH MIGHT RESULT IN LOSS OF CTL FOR EITHER OF THE ACFT. I PASSED UNDER THE HELI ABOUT 100-200 FT; AND ON HIS L SIDE; WHICH SCARED BOTH OF US. THE OTHER PLT CORRECTLY WAS VERY ANNOYED; I APOLOGIZED PROFUSELY. I BELIEVE MY COMPLETE MISJUDGEMENT OF THE DISTANCE BTWN US MIGHT HAVE RESULTED FROM THE INITIAL ASSUMPTION; THAT HE WAS AN AIRPLANE TRAVELING AT ABOUT MY SPD. I ALSO KEPT LOOKING AT THE STROBES; AND THE DISTANCE BTWN THEM; AND ASSUMING ALL THE TIME IT WAS AN AIRPLANE; JUDGED THE OTHER ACFT TO BE MUCH FARTHER IN FRONT THAN IT WAS. INSTEAD; IT WAS A HELI; WITH STROBES ONLY 5 FT APART; NOT 35 FT AS IN MOST SMALL AIRPLANES. THIS CREATED AN OPTICAL ILLUSION AND CONVINCED ME I WAS MANY MI BEHIND HIM; AND WOULD CLR HIM ON MY DSCNT BY 1000 FT; NOT 100 FT. I WAS SHOCKED WHEN HE CALLED THE TWR 9 MI OUT AND I WAS 9.5 DME.