PVT PLT OF A C172 DSNDED TO AVOID AN ATR TURBOPROP OBSERVED ON A COLLISION COURSE IN CLASS E AIRSPACE APCHING A VOR WITHIN CLASS D AIRSPACE. ATC ISSUED A TA JUST PRIOR TO THE RPTR SIGHTING THE ATR ACFT.

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WAS IN L SEAT OF A C172 TRACKING THE NE PNE VOR INBOUND AT 3000 FT. ACFT HDG WAS ROUGHLY 130 DEGS AND ACFT WAS ROUGHLY 2-3 MI FROM PNE. I WAS UNDER THE HOOD (SIMULATED INST CONDITIONS) AND THE PVT PLT; IN THE R SEAT WAS ACTING AS SAFETY PLT. THE FLC HAD ORIGINATED AT DECK ARPT (9D2) ROUGHLY 1 HR EARLIER; AND THERE HAD BEEN A STOP AT PTW ARPT. I WAS HANDLING THE RADIOS AND WAS IN CONTACT WITH PNE TWR. I HAD TOLD THEM THAT WE WOULD BE FLYING DIRECTLY TO THE (ON FIELD) PNE VOR AT 3000 FT (THE CEILING OF THE CLASS D AIRSPACE IS 2800 FT); AND THEN WOULD LIKE TO JOIN THEM FOR A 5 MI FINAL TO RWY 24. WE WERE AT SOME POINT ASKED TO SQUAWK IDENT; AND WE WERE IN RADAR COVERAGE. ABOUT THIS TIME ALSO; WE WERE TOLD BY PNE TO LOOK FOR A TWIN ENG ACFT BELOW US EITHER TAKING OFF OR LNDG AT PNE; MOST LIKELY; THAT WAS 'NO FACTOR.' AT ABOUT THIS TIME; THE SAFETY PLT TOLD ME 'DSND NOW!' OR SOMETHING TO THAT EFFECT; IN A STERN VOICE. I THREW OFF THE VIEW-LIMITING DEVICE AND INITIATED AN IMMEDIATE DSCNT TO 2800 FT. LOOKING OUT MY R WINDOW; I SAW THE HEAD-ON VIEW OF A COMMUTER AIRLINER; POSSIBLY AN ATR42. THE COMMUTER PASSED BEHIND US AT APPROX THE SAME ALT (IE; HE WAS NOT 500 FT ABOVE OR BELOW -- HE WAS AT 3000 FT). I AM NOT CONFIDENT IN THIS ESTIMATE; BUT I SUPPOSE THE SEPARATION WAS ABOUT 1/2 MI; DIRECTLY PERPENDICULAR. THE ATR DID NOT APPEAR TO ALTER COURSE AT ALL. I DON'T KNOW IF THAT'S BECAUSE HE HAD US ON TCASII OR VISUALLY OR WHAT. I AM NOT SURE IF ANY FARS WERE BROKEN; AND I'M PRETTY CONFIDENT THAT IF THEY WERE; I'M PERSONALLY NOT RESPONSIBLE (FOR VFR TFC SEPARATION WHILE UNDER THE HOOD). NEVERTHELESS; THIS INCIDENT HAD A NUMBER OF SALIENT FEATURES THAT I FELT SHOULD BE RPTED.