STUDENT PLT ON TRAINING FLT; FLYING BTWN 2 CTL TWR AIRSPACES SAW A MIL HELI DEPARTING THE DEST ARPT AND HAD TO MANEUVER TO AVOID IT.

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FOLLOWING A TRANSITION E OVER SEATAC; HDG TOWARDS DOWNTOWN KENT (APPROX 130 DEGS); SEA TWR TERMINATED RADIO CONTACT. WHILE ESTABLISHING RADIO CONTACT WITH RNT TWR FOR LNDG; BEGAN L TURN; N APPROX 80 DEGS. UPON LEVELING MY WINGS TO COMPLETE THE TURN; A HELI WAS SPOTTED AT ABOUT THE SAME ALT (A LITTLE HIGHER) AT MY 10 O'CLOCK MOVING TOWARDS MY 12 O'CLOCK. THE HELI JUST THEN BEGAN TO CLB TO HIS R AND I DOVE TO MY L TO AVOID A COLLISION. PROBABLY CAUSE: CONTRIBUTING FACTORS INCLUDE CONGESTED TFC AREA (FROM SEA; RNT; BFI; AND AUBURN (S50)); LACK OF RADAR SVC. ALTHOUGH APPROPRIATE PERHAPS FOR THE ALT OF 1700 FT TO NOT HAVE RADAR SVC; IT MAY HAVE ALERTED THE 2 PLTS OF THE OTHER'S PRESENCE; AND OBSTRUCTION OF MY VIEW BY THE WING DURING MY TURN. SUGGESTION: HAVE SEATAC ADVISE OTHER TWRS IN THE AREA (AND VICE VERSA) OF TFC LEAVING THEIR AIRSPACE TO APCH FOR LNDG AT RNT OR BFI -- OR SOME SUCH COORD -- OR POSSIBLE MONITORING OF THE UNCTLED AIRSPACE TO ADVISE ACFT OF POTENTIAL COLLISION COURSES; ESPECIALLY DURING SUCH ROUTINE TRANSITIONS BTWN SEA; RNT; AND BFI.