GA SMA WITH CLRNC TO OVERFLY IAD ARSA DESCENDED INTO THE ARSA DURING AN AVOIDANCE MANEUVER.

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COMBINATION OF VERY LIMITED VIS CONDITIONS; SEVERE HAZE AND GARBLE ATC/PLT COM OVER 2-WAY RADIO. FLT ORIGINATION: W 66. DEST: LANCASTER; PA. INTENDED ROUTE OF FLT: WARRENTON DIRECT ARMEL VOR; DIRECT WESTMINSTER VOR; DIRECT LANCASTER VOR. VFR CONDITIONS EXISTED AT TIME OF DEP FROM W 66 AND VFR CONDITIONS WERE ALSO PREVAILING FOR OUR ROUTE OF FLT AS PREVIOUSLY CHKED BY PHONE. UPON DEP FROM W 66 AT ABOUT 2000' AND CLBING; IAD APCH WAS CONTACTED ON ASSIGNED FREQ. SINCE DULLES WAS UNABLE TO COPY OUR XMISSIONS WE DECIDED TO TRY AGAIN A BIT LATER. AT A HIGHER ALT AND CLOSE TO IAD; E OF MANASSAS ARPT; WE AGAIN TRIED IAD APCH; THIS TIME SUCCESSFULLY. WE WERE ASSIGNED A TRANSPONDER CODE. AT THIS TIME WE ASKED IAD TO VERIFY MODE C ALT. IAD RPTED 2600' FOR AN INDICATED ALT OF 2550' AS CORRECTED BY IAD ALTIMETER SETTING. APPROX 3-5 MI SSE OF IAD AND WITH IAD IN SIGHT; WE REQUESTED FROM IAD APCH DIRECT WESTMINSTER VOR FROM PRESENT POS. SINCE VIS STARTED WORSENING AS TIME WENT ON (EXTREMELY HAZY) WE REQUESTED TO CLB TO 3500' WITH THE HOPE OF ENCOUNTERING BETTER VIS. FROM THIS POINT ON RADIO XMISSIONS FROM IAD BECAUSE MORE AND MORE DIFFICULT TO UNDERSTAND (BROKEN UP AND SEVERELY INTERFERED BY OTHER PARTIES IN THE FREQ). IAD APCH CALLED FOR TFC AT 11 O'CLOCK. WE ACKNOWLEDGED THE CALLOUT; BUT NEGATIVE TFC CONTACT. WE WERE NEBND; ON HDG AND STILL CLBING; WE LOWERED THE NOSE TEMPORARILY TO KEEP LOOKING FOR TFC AT 11 O'CLOCK AS PREVIOUSLY CALLED FOR. STILL NO TFC. WE FOUND OURSELVES AT THIS POINT WITH NO GND CONTACT AND VIS REDUCED TO ALMOST NOTHING PROBABLY A COUPLE OF HUNDRED FEET ALL AROUND THE ACFT. VERY CONCERNED ABOUT OUR COMPOUNDED SITUATION; VERY POOR VIS; A TARGET CONVERGING ON US; RADIO XMISSIONS UNINTELLIGIBLE IN WHAT APPEARED TO BE IMC CONDITIONS WE DECIDED TO INFORM IAD APCH OF OUR EMINENT PROB. WE CALLED ONCE; NO ANSWER. CALLED TWICE; NO ANSWER. AT THIS POINT SUSPECTING RADIO INOP WE DECIDED TO USE A PORTABLE STANDBY WALKIE-TALKIE. STILL UNABLE TO MAKE SENSE OF RADIO XMISSIONS; WE DECIDED TO CONCENTRATE ON THE RPTED TFC. SUDDENLY AT APPROX 9-10 O'CLOCK; AT A RANGE OF 1500-2000'; WE SPOTTED THE UNMISTAKABLE HVT. IT APPEARED TO BE 200-300' ABOVE OUR FLT PATH IN A CLBING RIGHT TURN. WE AUTOMATICALLY DOVE AWAY. A SECOND LATER WE HEARD DULLES CALLING US. WE RESPONDED; BUT NO ANSWER. DULLES KEPT CALLING AND WE KEPT ANSWERING. FINALLY AFTER 3 OR 4 TRIES; WE ESTABLISHED RADIO CONTACT AGAIN. IAD APCH ASKED WHY WE DSNDED. NO RECOLLECTION OF WHAT WE ANSWERED. WE CLBED TO 3500' ON A NBOUND HDG; STILL IN VERY POOR VIS CONDITIONS AND WITH RADIO XMISSIONS STILL BREAKING UP. USING A WALKIE-TALKIE WE REQUESTED A FREQ CHANGE. WE WERE ASSIGNED A NEW FREQ THAT WE DIALED IN THE ACFT RADIO. FROM THIS POINT ON RADIO CONVERSATION BACK AND FORTH WITH IAD APCH WENT BACK TO NORMAL. RIGHT BEFORE LEAVING DULLES TRSA APCH INFORMED US TO CALL ON THE LANDLINE AS SOON AS WE TOUCHED DOWN; WHICH WE DID AS SOON AS WE LANDED IN LANCASTER ARPT.