A FLT OF 2 HOMEBUILTS FLYING JUST UNDER ATL CLASS B AIRSPACE TRIGGERED AN RA ON AN INBOUND CRJ-200.

Narrative

DEPARTED BROOKBRIDGE ARPT AS A FLT OF 2 (RV4; RV8). BOTH ACFT ALTIMETERS SET PRIOR TO DEP. CONTINUED ENRTE TO FULTON COUNTY ARPT (FTY) BENEATH 3500 FT FLOOR OF ATL CLASS B AIRSPACE. AT APPROX THE ATL 265 DEG RADIAL; 17 NM; A CRJ-200 PASSED OVER US ON HIS DOWNWIND AND THEN TURNED BASE AND FINAL WHILE STILL IN SIGHT. WX CONDITIONS WERE MOSTLY CLR; 8-10 MI IN HAZE. AT THIS TIME BOTH ACFT INDICATED 2900-3000 FT ON THE ALTIMETERS WITH THE LEAD ACFT XPONDER ON 1200. ALTS WHILE UNDER THE CLASS B VARIED FROM 2700 FT TO 3100 FT MSL -- WITH THE FLOOR AT 3500 FT. AFTER LNDG AT FTY; TRACON ASKED FOR A CALL AND TOLD US THAT THE CRJ HAD A TCAS RA AND CLBED TO 5000 FT EVEN THOUGH THEY HAD US IN SIGHT. XPONDER RETURNS SHOWED A POSSIBLE RETURN OF 3700 FT AT A TIME BOTH ALTIMETERS (ONLY 10 MINS AFTER BEING SET ON GND) SHOWED APPROX 3000 FT. 1) DUE TO GOOD VISIBILITIES AT NO TIME DID WE FEEL SAFETY WAS COMPROMISED. 2) ENCODER IN LEAD ACFT MAY HAVE MALFUNCTIONED (ALTHOUGH 700 FT STEADY READOUT IS UNLIKELY) AND ENCODER WILL BE CHKED PRIOR TO BEING USED AGAIN. 3) SUSPECT HIGH DSCNT RATE DURING APCH (NOT UNUSUAL IN ATLANTA) BY CRJ CREATED A CLOSE RATE THAT TRIGGERED TCAS.