POTENTIAL CONFLICT BTWN A DEP C172 AND A XING HELI DURING A NIGHT OP 5 MI S OF DEP ARPT; N99; PA.

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FLT MISSION WAS A NIGHT LCL FLT WITH A STUDENT PLT (POST SOLO); DUAL INSTRUCTION (I AM A CFI) FOR THE PURPOSE OF FULFILLING THE NIGHT TRAINING REQUIREMENTS FOR THIS STUDENT. THE OBJECTIVE WAS TO PERFORM MULTIPLE NIGHT FULL STOP LNDGS. WE DEPARTED THE BRANDYWINE ARPT (N99); AN UNCTLED FIELD; MAKING ADVISORY ANNOUNCEMENTS ON THE CTAF FREQ OF 123.075 MHZ THAT WE WERE TAKING OFF AND LATER THAT WE WERE DEPARTING SBOUND. WE WERE INTERCEPTING THE 005 DEG RADIAL FROM THE DQO VORTAC (AT THE ILG ARPT) AND WERE INTENDING TO FLY INBOUND ON A 185 DEG TRACK TO ILG. OUR POS WAS CONFIRMED WITH MOVING MAP GPS AS WELL AS DUAL VORS. WE WERE IN CLASS E AIRSPACE WELL BELOW THE 4000 FT MSL FLOOR OF PHILADELPHIA CLASS B AIRSPACE AT OUR LOCATION AND TRACK. AFTER OUR SECOND BRANDYWINE RADIO ADVISORY CALL; WE SWITCHED TO THE WILMINGTON (ILG) ATIS (123.95) TO MONITOR THAT ATIS PRIOR TO CALLING THE TWR AT ILG BEFORE REQUESTING MULTIPLE FULL STOP NIGHT LNDGS TO ACHIEVE THE TRAINING OBJECTIVE FOR THE FLT. WHILE CLBING THROUGH 2000 FT IN THIS NIGHT GOOD VFR CONDITIONS; I NOTICED AN UNUSUAL SET OF ACFT POS LIGHTS AND A RED FLASHING BEACON (THAT IS WHY I SURMISE IT WAS A HELI; BECAUSE WINGTIP RED AND GREEN AIRPLANE POS LIGHTS WOULD APPEAR FARTHER APART FOR AN ACFT AT THE SAME ESTIMATED DISTANCE). I ASSUMED CTL OF THE C172 AND TURNED BEHIND THE APPARENT FLT PATH OF THE OTHER ACFT AND ALSO LEVELED BELOW IT -- ESTIMATED MISS DISTANCE: 1000 FT LATERALLY AND 200 FT BELOW. IT DID NOT APPEAR THAT THE OTHER ACFT TOOK EVASIVE ACTION OR SAW US ANYTIME BEFORE WE SAW THEM AND EVADED THEM. WE HAD POS LIGHTS; ANTI-COLLISION LIGHTS THAT INCLUDED WINGTIP STROBES AND RED FLASHING BEACONS AS WELL AS OUR TAXI LIGHTS ILLUMINATED. WE WERE MUCH BETTER ILLUMINATED THAN THE OTHER ACFT (HELI?). IF IT WAS A HELI; IT SHOULD HAVE MUCH LARGER EXPANSES OF PLEXIGLAS TO LOOK OUT AND SEE AND AVOID OTHER ACFT IF THEY LOOK. ALTHOUGH IT IS THE RESPONSIBILITY OF ALL PLTS TO SEE AND AVOID; ACCORDING TO THE FAR'S FOR CONVERGING ACFT; SINCE WE WERE TO THE R OF THE WBOUND ACFT (HELI?) WE HAD THE RIGHT OF WAY; OF COURSE; THAT WOULD ONLY HELP IF THE OTHER ACFT HAD BEEN SCANNING FOR OUR WELL LIGHTED ACFT IN GOOD VMC!