Narrative:

I was the trip captain (pilot not flying) in the right seat and the co-captain (pilot flying) was in the left seat. We departed runway 26 at fty on an IFR flight plan. Our IFR clearance was to climb to 3;000 ft on a 290 heading. After departure we were handed off to atl approach and cleared to 4;000 ft heading 300 degrees. Upon reaching 4;000 ft; ATC issued a traffic alert (10 o'clock at 5;000 ft. I don't remember the distance). We started scanning for traffic and about the same time we picked up two targets on TCAS at about 10 o'clock and less than 6 miles. Aircraft Y was 1;000 ft above; traffic aircraft Z was 200 ft below. Both were almost on top of each other and appeared to be heading towards us. I asked ATC about the aircraft Z traffic at the same time that we got a TA on aircraft Z. Before ATC could respond we received a climb RA from aircraft Z. The pilot flying began a slow but intentional climb at about the time we obtained visual contact on aircraft Y above us. The TCAS RA profile showed red below with a green climb that was capped at approximately 500 ft vs. We climbed approximately 400 ft (to 4;400 ft MSL) which gave us 600 ft clearance below aircraft Y and 600 ft above aircraft Z. As the traffic passed overhead the RA resolved itself and we descended back to 4;000 ft. During our RA climb I informed ATC we were climbing in response to an RA. We never saw aircraft Z below us. Immediately after the RA resolution I informed ATC that we were sandwiched between the two aircraft which were at 3;800 ft and 5;000 ft. After processing what had just happened for a few moments I asked ATC if he was talking to the aircraft Z below us. The controller responded that he was not and that the target was shadowed by the 5;000 ft IFR target and that is why he did not call that traffic to us. A few sectors later atl center notified us to call atl approach upon landing to discuss a possible pilot deviation. The problem arose from the controller not noticing the VFR traffic and alerting us to it. Even though it was masked by the IFR traffic above it; it seems there should have been some warning since there was an operable transponder (we received the altitude readout from the VFR traffic on our TCAS). Had the controller seen it I'm sure we would have been vectored away until clear. Also contributing was the pilot of the VFR aircraft who was not on a VFR altitude and was not communicating with ATC. This occurred under the floor of the busiest class B airspace in the world (class B floor where this occurred was 6;000 ft) and the VFR traffic was headed east towards the heart of the class B airspace. While it may not be a requirement to talk to ATC; it's a good idea in such a high traffic area; as anyone flying in that region should know. The corrective actions that we as a crew took were performed flawlessly. We were right on speed (200 KTS); heading (300); and altitude (4;000 ft) when the initial traffic alert was issued. We immediately began to scan visually for traffic and also began monitoring the TCAS. We saw the traffic below us on our TCAS before we received a TA from it and proactively queried ATC about it. The response to the RA was textbook; immediate but controlled enough so that we did not receive a secondary RA from the traffic above us. We also alerted ATC to our RA climb in a timely manner. We were in total compliance with far 91.123 (a) and (c); which provide guidance on a TCAS resolution advisory. I don't feel that there is anything more we could have done to avoid this situation; and if I were placed in the same situation again I would respond in the exact same manner.

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Original NASA ASRS Text

Title: HS125 Captain experiences a TCAS RA at 4;000 FT after departure from FTY with VFR traffic at 3;800 FT and IFR traffic at 5;000 FT. The TCAS split the difference with a 600 foot climb. ATC had not seen the the VFR traffic which was shadowed by the IFR traffic directly above.

Narrative: I was the trip Captain (pilot not flying) in the right seat and the Co-Captain (pilot flying) was in the left seat. We departed Runway 26 at FTY on an IFR flight plan. Our IFR clearance was to climb to 3;000 FT on a 290 heading. After departure we were handed off to ATL Approach and cleared to 4;000 FT heading 300 degrees. Upon reaching 4;000 FT; ATC issued a traffic alert (10 o'clock at 5;000 FT. I don't remember the distance). We started scanning for traffic and about the same time we picked up two targets on TCAS at about 10 o'clock and less than 6 miles. Aircraft Y was 1;000 FT above; traffic Aircraft Z was 200 FT below. Both were almost on top of each other and appeared to be heading towards us. I asked ATC about the Aircraft Z traffic at the same time that we got a TA on Aircraft Z. Before ATC could respond we received a climb RA from Aircraft Z. The pilot flying began a slow but intentional climb at about the time we obtained visual contact on Aircraft Y above us. The TCAS RA profile showed red below with a green climb that was capped at approximately 500 FT VS. We climbed approximately 400 FT (to 4;400 FT MSL) which gave us 600 FT clearance below Aircraft Y and 600 FT above Aircraft Z. As the traffic passed overhead the RA resolved itself and we descended back to 4;000 FT. During our RA climb I informed ATC we were climbing in response to an RA. We never saw Aircraft Z below us. Immediately after the RA resolution I informed ATC that we were sandwiched between the two aircraft which were at 3;800 FT and 5;000 FT. After processing what had just happened for a few moments I asked ATC if he was talking to the Aircraft Z below us. The Controller responded that he was not and that the target was shadowed by the 5;000 FT IFR target and that is why he did not call that traffic to us. A few sectors later ATL Center notified us to call ATL Approach upon landing to discuss a possible pilot deviation. The problem arose from the Controller not noticing the VFR traffic and alerting us to it. Even though it was masked by the IFR traffic above it; it seems there should have been some warning since there was an operable transponder (we received the altitude readout from the VFR traffic on our TCAS). Had the Controller seen it I'm sure we would have been vectored away until clear. Also contributing was the pilot of the VFR aircraft who was not on a VFR altitude and was not communicating with ATC. This occurred under the floor of the busiest Class B airspace in the world (Class B floor where this occurred was 6;000 FT) and the VFR traffic was headed east towards the heart of the Class B airspace. While it may not be a requirement to talk to ATC; it's a good idea in such a high traffic area; as anyone flying in that region should know. The corrective actions that we as a crew took were performed flawlessly. We were right on speed (200 KTS); heading (300); and altitude (4;000 FT) when the initial traffic alert was issued. We immediately began to scan visually for traffic and also began monitoring the TCAS. We saw the traffic below us on our TCAS before we received a TA from it and proactively queried ATC about it. The response to the RA was textbook; immediate but controlled enough so that we did not receive a secondary RA from the traffic above us. We also alerted ATC to our RA climb in a timely manner. We were in total compliance with FAR 91.123 (a) and (c); which provide guidance on a TCAS Resolution Advisory. I don't feel that there is anything more we could have done to avoid this situation; and if I were placed in the same situation again I would respond in the exact same manner.

Data retrieved from NASA's ASRS site 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.