Potomac Airfield

This airport is one of the "DC-3", along with Hyde Field [W32] and Hyde Field [W32]. It lies inside the inner Flight Restricted Zone (FRZ) of the Washington DC ADIZ, and access is allowed only to pilots who have been pre-screened and obtained a special Personal Identification Number (PIN). This is in addition to the normal ADIZ procedures. As a result, it won't usually make sense for non-local pilots to try to use this airport.

http://www.aopa.org/whatsnew/air_traffic/frz.html