AN F15S FLC CONSISTING OF A CIVILIAN PRODUCTION TEST PLT AND A MIL PLT HAS AN NMAC WITH A TWIN ENGED PROP ACFT.

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A NEAR MISS OCCURRED AT APPROX XA10 LCL TIME ON APR/MON/96 AT APPROX 12500 FT IN VMC CONDITIONS APPROX 45 NM DUE S OF STL BTWN A F15S AND A TWIN ENG PROP DRIVEN ACFT; POSSIBLY A CESSNA; PIPER OR BEECH. AVOIDANCE WAS TAKEN BY THE F15. OTHER ACFT DID NOT APPEAR TO HAVE TAKEN ANY EVASIVE ACTION. NEAREST DISTANCE WAS APPROX 500 FT. F15S ACFT PILOTED BY A CIVILIAN PRODUCTION TEST PLT AND A MIL PLT; ON A PRODUCTION SUPPORT TEST FLT AND INST CHKFLT. THIS ACFT IS USED FOR PRODUCTION SUPPORT AND TARGET WORK AND IS BASED AT STL. SIT: AS WE WERE HDG N; CLBING TOWARDS 12500 FT; WHICH IS THE AGREED UPON RETURN ALT (PER LOA) TO RETURN TO ST LOUIS FROM THE S OPERATING AREA; AS WE LEVELED OUT; I NOTICED A MOTIONLESS SPOT ON THE UPPER R CORNER OF THE HUD. I DID A MOMENTARY DOUBLE TAKE AND REALIZED THAT IT WAS A TWIN ENG PROP ACFT HDG TOWARDS US. REALIZING THAT WE WERE ON A NEAR COLLISION COURSE; I ROLLED RAPIDLY TO APPROX 135 DEG L BANK; AND PULLED THE STICK RAPIDLY AFT TO GENERATE SEPARATION. I ESTIMATE THAT THE MINIMUM SEPARATION MAY HAVE BEEN ABOUT 500 FT; ALTHOUGH THE GEOMETRY CAUSED ME TO LOSE SIGHT IN THE PULL. AFTER I ROLLED OUT; WE SAW THE OTHER ACFT CONTINUING S IN WINGS LEVEL FLT. WE ASKED ZKC; WHO USUALLY PROVIDES US WITH TA'S; IF THEY HAD SEEN A CIVIL ACFT PASS US. THEY SAID THAT THEY HAD; BUT DIDN'T HAVE ANY ALT INFO ON HIM; BUT THAT HE WAS ON A CLB TO 14000 FT. ALTHOUGH WE HAVE A PREFERRED RECOVERY ALT OF 12500 FT INTO OUR NBOUND S RECOVERY CORRIDOR; I HAVE OFTEN FELT THAT A BETTER RECOVERY PROFILE WOULD HAVE US PROCEED INBOUND AT 8500 FT (AT LEAST IF WE WERE INITIALLY BELOW 10000 FT); RATHER THAN CLB UPWARDS THROUGH OCCASIONAL NBOUND ARR TFC WHICH IS NORMALLY AT 10000 FT ON THE QBALL ARR. ALTHOUGH THE OTHER ACFT WAS HEADED SBOUND; SBOUND TFC IS RATHER INFREQUENT IN THIS AREA. HUMAN FACTORS: IT IS AN INTERESTING NOTE; THAT WHEN ON A COLLISION COURSE; THERE IS NO ANGULAR MOVEMENT OF THE OPPOSING ACFT WHICH TENDS TO MAKE IT APPEAR MOTIONLESS; WITH ONLY BARELY PERCEPTIBLE SIZE GROWTH. DURING THE TIME OF MY DOUBLE TAKE; IT SEEMED AS IF IT WAS IN SLOW MOTION AND THAT THERE WAS VERY LITTLE PERCEPTION OF IMPENDING CONFLICT. OF COURSE; THE COMBINED SPDS WERE QUITE HIGH AS WE WERE AT ABOUT 280-300 KCAS AND I ESTIMATE THAT THE OTHER ACFT WAS AT PERHAPS 150 KCAS FOR A COMBINED CLOSING OF 450 KCAS (A TRUE CLOSING SPD OF 500 KTS); WHICH WOULD BE ROUGHLY 850 FT PER SECOND. LITTLE TIME REMAINS TO SPOT A FAIRLY SMALL OBJECT HEAD-ON; ESPECIALLY WHEN IT HAS NO ANGULAR RELATIVE MOTION TO AID RECOGNITION. IF WE HAD TAKEN NO EVASIVE ACTION; I ESTIMATE THAT WE WOULD HAVE PASSED WITHIN 50 FT. IF I HAD MADE A PURE PULLUP; WE COULD HAVE MADE THE SIT WORSE.