LOW ALT CIVIL FLT BY AN SMA SEL GOV ACFT DURING TAC OBSERVATION AND AN NMAC WITH AN MLT OVER AN UNCTLED ARPT.

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I WAS FLYING AS OBSERVER FOR A CIVIL AIR PATROL COUNTER DRUG FLT (AIRBORNE RELEASE) AROUND THE NORTHERN HALF OF WAYNE COUNTY; NC. ARPT SERVING THIS AREA IS THE GOLDSBORO WAYNE COUNTY ARPT. DURING THESE MISSIONS THE ACFT IS FLOWN AT 500-1000 FT AGL TO SPOT MARIJUANA PLANTS. AT ONE POINT THE ACFT DID OVERFLY AND CIRCLE AN AREA AT APPROX 700 FT AGL. I REMINDED THE PLT WE SHOULD BE HIGHER AS WHICH TIME HE INITIATED A CLB TO 1200 FT AGL. THIS AREA WAS NEAR UNITED STATES HWY 70 ON WESTERN SIDE OF GOLDSBORO; NC. THE ACFT THEN PROCEEDED N. WHILE WORKING THIS AREA N AND W OF WAYNE-SIEGOLDSBORO ARPT; A C-17 GLOBEMASTER III FROM CHARLESTON AFB; SC; ENTERED THE PATTERN FOR LOW APCH AND HIGH SPD PASSES. THE CREW STATED TO THE ARPT UNICOM THAT THEY LIKE TO USE THIS ARPT BECAUSE ITS RWY SIZE GIVES THEM A 'TACTICAL' SIT. THE CREW WAS DOING A GOOD JOB OF COMMUNICATING THEIR INTENTIONS ON UNICOM AS THEY SWITCHED BTWN RWY 5 AND 23. OVERALL I COMMEND THE CREW (C-17) FOR A GOOD JOB OF RADIO WORK AND WORKING WITH A COUPLE OF GA ACFT AS THEY ENTERED THE ARPT AREA FOR LNDG. AS WE APCHED TO LAND FROM THE W; WE ANNOUNCED OUR INTENTIONS TO LAND ON RWY 5. THE C-17 COMING OFF OF AN APCH TO RWY 5 ANNOUNCED THEY WANTED TO DO A HIGH SPD RUN DOWN RWY 23 FOLLOWED BY LOW APCH RWY 5 THEN EXIT TO THE N. WE ADVISED THAT WE WOULD MOVE TO 5 S OF ARPT AND AWAIT THEIR DEP. WHILE MOVING TO 5 S THE C-17 COMING OFF ITS HIGH SPD 23 RUN OVERFLEW OUR ACFT AT AN ALT ESTIMATED AT 500-700 FT. WE IMMEDIATELY ENTERED A DIVE AND TURNED R DSNDING FROM 1500 FT AGL TO 1000 FT. FORTUNATELY; WE DID NOT ENCOUNTER A WAKE FROM THE C-17. HE EXECUTED A 180 DEG TURN AND COMPLETED A LOW APCH TO RWY 5 AND DEPARTED. WE COMPLETED OUR TURN AND ENTERED A L DOWNWIND FOR RWY 5; COMPLETING THE LNDG WITHOUT INCIDENT. IN RETROSPECT FROM OUR RWY 5 DOWNWIND POS MOVING DIRECTLY W TO NW OF THE ARPT WOULD HAVE BEEN SMARTER THAN MOVING S. WE CHOSE S BECAUSE THE C-17 WAS MAKING PATTERNS TO THE N. HE CHANGED PATTERNS TO THE S AND HIS FASTER SPD DID NOT ALLOW US TO COMPLETELY MOVE S ENOUGH TO AVOID HIS FLT PATH. OVERALL; I THINK MIL AND GA CAN OPERATE TOGETHER; BUT IT WOULD HELP IF TFC STAYED TO APCHS ON THE SAME RWY DURING THE PERIODS WHEN THERE ARE MULTIPLE ACFT IN THE IMMEDIATE ARPT AREA.