POTENTIAL CONFLICT WITH OVERTAKEN ACFT COMPLAINING ABOUT TFC SEP ON ACFT X VFR CLB THOUGH OCCUPIED ALT.

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DURING CLB OUT FROM LAGUARDIA ARPT; WE HAD BEEN CLRED TO CLB AND MAINTAIN 15000 FT. DEP CTL THEN REVISED THE CLRNC; AND ASKED US TO MAINTAIN 11000 FT DUE TO TFC AHEAD. THE CTLR CALLED THE TFC AT 2 O'CLOCK AND 7 MI. THE ACFT WAS AN MDT Y ALSO ON CLB OUT. WE VISUALLY SIGHTED THE ACFT; AND RPTED IT TO THE CTLR. HE THEN CLRED US TO MAINTAIN VISUAL SEPARATION; AND CLB TO 15000 FT. THE COPLT WAS FLYING; AND HE BEGAN A GRADUAL CLB. I ESTIMATE THAT WHEN WE PASSED LEVEL WITH THE MDT AT ABOUT 12000 FT; WE WERE 1/2 MI BEHIND AND SLIGHTLY L OF HIS COURSE. AS WE PASSED ABEAM OF THE MDT; THE PLT OF THAT ACFT REMARKED THAT HE THOUGHT WE PASSED TOO CLOSE TO HIM; AND HE DIDN'T APPRECIATE THAT WE CAME THAT CLOSE. I ESTIMATE THAT AT THE TIME WE PASSED; WE WERE 800 FT VERTICALLY ABOVE AND 2000 FT HORIZLY FROM HIM. I MADE AT LEAST 2 ERRONEOUS ASSUMPTIONS IN THIS ENCOUNTER. FIRST; I ASSUMED THE MDT Y WAS ON THE SAME DEP FREQ; THEREFORE HE WAS AWARE THAT WE WERE APCHING FROM BEHIND AND WE WOULD BE RESPONSIBLE FOR MAINTAINING SEP. AS IT TURNED OUT; ONCE WE WERE GIVEN THE CLB CLRNC; WE WERE THEN GIVEN A DIFFERENT FREQ. APPARENTLY; THE MDT CREW WERE UNAWARE OF OUR PRESENCE AS HE WAS ON THE SECOND FREQ ALL ALONG. THE SECOND BAD ASSUMPTION WAS THAT I WAS STILL ABLE TO ACCURATELY FIGURE ACFT CLOSURE RATES. HAVING LAST FLOWN FORMATION IN PLT TRAINING SOME 18 YRS AGO; I DISCOVERED I AM NO LONGER ABLE TO ACCURATELY GAUGE CLOSURE RATES BTWN ACFT. THE MIDAIR COLLISION THAT KILLED SENATOR 'A' IS A GRAPHIC EXAMPLE OF WHAT CAN HAPPEN WHEN UNSKILLED PLTS ATTEMPT TO USE FORMATION FLYING TECHNIQUES. I MADE AN ERROR WHEN I DID NOT INSTRUCT THE COPLT TO FLY PARALLEL TO THE MDT UNTIL WELL ABOVE HIM; OR TURN TO PASS WIDE TO THE R. THE MDT PLT'S NOT KNOWING OF OUR PRESENCE MAY HAVE CONTRIBUTED TO AN INCIDENT; AS HE MAY HAVE BEGUN A TURN TO THE L; TOWARD OUR FLT PATH.