NMAC BTWN AN ARR F100 1 NM SW OF SHAKI INTXN AT ABOUT 4000 FT AND A DEP; CLBING MIL DC9 OUT OF ANDREWS AFB ON 2 DIFFERENT FREQS WITH PCT; VA.

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ON THE BUCKO 6 ARR INTO REGAN NATIONAL; WE RECEIVED A HEADING OF 150 DEGS AFTER SHAKI FOR THE MT VERNON VISUAL TO RWY 1 AT DCA. APPROX 1 MI ON THAT 150 DEG HDG; DSNDING THROUGH APPROX 4000 FT; WE RECEIVED A TA IN OUR 11 O'CLOCK POS; APPROX 3 MI AWAY; SLIGHTLY BELOW OUR ALT AND CLBING. AS WE LOOKED FOR THE TFC VISUALLY; THE TA QUICKLY TURNED TO AN RA; WITH INSTRUCTIONS TO 'MONITOR VERT SPD.' WE SLOWED OUR DSCNT. WHEN THE TCASII COMMANDED US TO 'CLB;' WE VISUALLY OBTAINED THE TFC. IT WAS CLR THAT WE WOULD COLLIDE WITH THE TFC IF WE DID NOT ALTER OUR COURSE; SO I; THE PF; USED APPROX 45 DEGS OF BANK IN A L CLBING TURN HEADING TO THE E. AS THE TFC VISUALLY WENT PAST; WE ASSUMED A TURN BACK TO THE S. AS WE TURNED BACK; I OBSERVED A MIL DC9 AT OUR ALT WITHIN 2000 FT; HEADING W. THE CAPT INFORMED POTOMAC APCH OF OUR ACTIONS AND NOTIFIED THEM WE WOULD BE FILING A NEAR MIDAIR COLLISION RPT. THE REMAINDER OF THE FLT WAS UNEVENTFUL. THE CAPT CALLED POTOMAC AFTER LNDG AND WAS TOLD THAT MIL TFC DEPARTING ANDREWS HEADING WBOUND AND VECTORED OVER DCA CAN GO RIGHT OVER THE RADAR ANTENNA AND NOT BE PICKED UP IN THE RADAR SWEEP. IT SEEMS THAT THIS COULD BE A POTENTIALLY SERIOUS SAFETY PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: WHEN CALLED; THE FO RPTR STATED THAT HE HAD HEARD BACK FROM THE COMPANY REGARDING HIS RPT. THAT RPT INCLUDED INFO FROM AN ATC INVESTIGATION WHICH SAID THAT THE APCH CTLR HAD FORGOTTEN ABOUT THE WBOUND MIL DC9. THE EXCUSE OFFERED BY THE CTLR ON THE PHONE ABOUT THE SUPPOSED 'BLACK HOLE' OVER THE ANTENNA WAS CHALLENGED AND WAS ALLEGED TO BE A FALSE STATEMENT AND A NON EXISTING SIT. THE VERT DISTANCE BTWN THE 2 ACFT WAS ABOUT 200 FT WITH A VARIABLE HORIZ DISTANCE OF BTWN 200 FT TO 2000 FT. THE F100 WAS ACTUALLY IN A LEVEL TURN; BANKING OVER 45 DEGS. THERE WASN'T TIME FOR THE FLT TO CLB IN ACCORD WITH THE TCASII RA COMMANDS. SUPPLEMENTAL INFO FROM ACN 588730: I ACQUIRED THE TARGET VISUALLY AND IT APPEARED AS THOUGH THE OTHER ACFT WAS CLBING INTO OUR ALT AND WE WERE DSNDING INTO IT; SO I ASKED THE FO; PF; TO 'LEVEL OFF A LITTLE BIT.' I BELIEVE I ASKED THE CTLR IF HE WAS PAINTING A TARGET FOR US AT THAT APPROX POS; HAVING RECEIVED NO NOTIFICATION OF PERTINENT TFC FROM HIM. I NOTIFIED THE CTLR THAT WE WERE RESPONDING TO AN RA. IF WE DID NOT ALSO ALTER OUR COURSE WE WOULD COLLIDE WITH THE OTHER ACFT WHICH WE COULD VERY PLAINLY IDENT AT THIS POINT AS A MIL DC9 CLBING THROUGH ABOUT 4000 FT HEADING WBOUND; AT LESS THAN 1/2 MI AWAY. AS WE CLBED AND TURNED TO THE E TO AVOID THE OTHER ACFT; THE FO LOOKED BACK AT THE TARGET AND OBSERVED IT TO BE AT OUR ALT AND WITHIN LESS THAN 2000 FT. I ASKED THE CTLR IF HE WAS IN CONTACT WITH THE OTHER ACFT AND HE RESPONDED THAT HE WAS NOT DUE TO THE FACT THAT THE OTHER ACFT WAS ON ANOTHER FREQ AND ADDITIONALLY; 'HE HAD NOT SEEN THE TARGET ON RADAR.' I CONTACTED POTOMAC APCH CTL VIA TELEPHONE AFTER LNDG AND DISCUSSED THE PARTICULARS OF THE EVENT WITH THE INDIVIDUAL WHO RECEIVED MY CALL AND WAS TOLD (TO PARAPHRASE HIS COMMENTS) THAT SOMETIMES WHEN MIL TFC DEPARTS ANDREWS ARE WBOUND AND IS VECTORED OVER OR NEAR DCA; THAT THEY CAN FLY RIGHT OVER THE RADAR ANTENNA AND MOMENTARILY NOT BE 'HIT' BY THE RADAR SWEEP. APCH CTL HAD NO COMMENTS OR QUESTIONS FOR ME. ALTIMETER SETTING WAS 30.28 INCHES. LENGTH OF TIME UNTIL EVASIVE ACTION WAS 20-25 SECONDS.