Narrative:

It was a clear, moonless and very dark night with excellent visibility. I was flying over the water, just west of groton airport. 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 airport at 5500 ft when I listened to the ATIS and called groton airport. At about 15 mi out a started a descent. 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 descend behind him and go under him if need be to reach pattern altitude. On descent, 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 altitude, and could pass well under him. All of a sudden at 9.5 DME from groton, the plane called the tower to request transition through the airspace, which was granted. He said he also was 9 mi out and he was a helicopter. I immediately realized that there was a problem. My eyes were telling me he was still mi ahead of me, yet his position report indicated he was far closer to me than I had thought. Furthermore, I also realized that, being a helicopter, he probably was going significantly slower than I had thought, and that our closure speed was dramatically higher than I had anticipated since I was descending at 170-180 KTS. I had planned to reach his altitude 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 between us would disappear in 20 seconds or so. I decided to increase my rate of descent and fly under him. I did not want to make any sharp turns or altitude excursions which might result in loss of control for either of the aircraft. I passed under the helicopter about 100-200 ft, and on his left side, which scared both of us. The other pilot correctly was very annoyed, I apologized profusely. I believe my complete misjudgement of the distance between us might have resulted from the initial assumption, that he was an airplane traveling at about my speed. I also kept looking at the strobes, and the distance between them, and assuming all the time it was an airplane, judged the other aircraft to be much farther in front than it was. Instead, it was a helicopter, 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 clear him on my descent by 1000 ft, not 100 ft. I was shocked when he called the tower 9 mi out and I was 9.5 DME.

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Original NASA ASRS Text

Title: 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.

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.