Narrative:

Law enforcement aircraft is on station 10 miles southwest of dca; orbiting at 5;500 ft VFR. Air carrier X is over eldee on the eldee 5 arrival at 8;000 ft when he asks for lower due to turbulence. I descended air carrier X to 6;000 ft and advised him of the VFR traffic working southwest of the airport. When air carrier X got closer to the traffic I advised him again of the position; the aircraft responded 'not in sight.' I called the traffic to [the] law enforcement aircraft and he had the air carrier traffic in sight. I instructed the law enforcement aircraft to maintain visual separation from the traffic. I don't know the exact sequence because I was very busy and it happened so very fast. I turned the law enforcement aircraft northbound after air carrier X requested he be moved because he was border line with severe turbulence and he needed lower. Visual separation was applied.air carrier X asked for lower but because they were in a head on situation I didn't feel comfortable descending air carrier X until the targets had passed. Air carrier X advised he was in IMC and that he was receiving a TCAS RA. Target passed and I promptly descended air carrier X. I continued working my traffic because I was very busy. Air carrier X asked for a number he can call to discuss what happened. The pilot questioned the legality of law enforcement aircraft maintaining visual separation because air carrier X was in IMC conditions. Air carrier X was switched to the tower. I asked the law enforcement aircraft if he was VMC. He didn't respond in time. About 5 minutes later I asked again and the law enforcement aircraft said he has been in VMC conditions the entire time. The law enforcement aircraft said he had air carrier X in sight the entire time until air carrier X took evasive action responding to a TCAS RA. I believe air carrier X was right in the [cloud] bases; law enforcement aircraft saw him and lost him when and if air carrier X climbed.the turbulence may have affected air carrier X's altitude triggering a TCAS RA. Either way; I believe everything was legal and I did the best that I could to assist both pilots. I'm filing this report because it's an unusual event that could have led to an unsafe situation. I wish I could say don't allow official surveillance aircraft to operate so close to the airport. I'd also like to use more than 500 ft; but we are one of the busiest areas in the country and there are times where we have to use the 500 ft requirement in class bravo airspace. We use all our altitudes. We always have special operations because of our locations.

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Original NASA ASRS Text

Title: PCT Controller described a conflict event between a DCA air carrier arrival and a law enforcement aircraft on station; visual separation was applied but questioned by the air carrier.

Narrative: Law enforcement aircraft is on station 10 miles southwest of DCA; orbiting at 5;500 FT VFR. Air Carrier X is over ELDEE on the ELDEE 5 arrival at 8;000 FT when he asks for lower due to turbulence. I descended Air Carrier X to 6;000 FT and advised him of the VFR traffic working southwest of the airport. When Air Carrier X got closer to the traffic I advised him again of the position; the aircraft responded 'not in sight.' I called the traffic to [the] law enforcement aircraft and he had the air carrier traffic in sight. I instructed the law enforcement aircraft to maintain visual separation from the traffic. I don't know the exact sequence because I was very busy and it happened so very fast. I turned the law enforcement aircraft northbound after Air Carrier X requested he be moved because he was border line with severe turbulence and he needed lower. Visual separation was applied.Air Carrier X asked for lower but because they were in a head on situation I didn't feel comfortable descending Air Carrier X until the targets had passed. Air Carrier X advised he was in IMC and that he was receiving a TCAS RA. Target passed and I promptly descended Air Carrier X. I continued working my traffic because I was very busy. Air Carrier X asked for a number he can call to discuss what happened. The pilot questioned the legality of law enforcement aircraft maintaining visual separation because Air Carrier X was in IMC conditions. Air Carrier X was switched to the Tower. I asked the law enforcement aircraft if he was VMC. He didn't respond in time. About 5 minutes later I asked again and the law enforcement aircraft said he has been in VMC conditions the entire time. The law enforcement aircraft said he had Air Carrier X in sight the entire time until Air Carrier X took evasive action responding to a TCAS RA. I believe Air Carrier X was right in the [cloud] bases; law enforcement aircraft saw him and lost him when and if Air Carrier X climbed.The turbulence may have affected Air Carrier X's altitude triggering a TCAS RA. Either way; I believe everything was legal and I did the best that I could to assist both pilots. I'm filing this report because it's an unusual event that could have led to an unsafe situation. I wish I could say don't allow official surveillance aircraft to operate so close to the airport. I'd also like to use more than 500 FT; but we are one of the busiest areas in the country and there are times where we have to use the 500 FT requirement in class Bravo airspace. We use all our altitudes. We always have special operations because of our locations.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.