Narrative:

While cruising at 10;000 feet and assigned 300 knots with rockford approach; approximately 20 miles northwest of fytte; a TCAS traffic advisory (TA) showed an aircraft appearing at our 12 o'clock; same altitude. I took control of the aircraft and began scanning for the traffic. Shortly thereafter; a TCAS resolution advisory (RA) called for a descent of 2000 feet per minute. I immediately disconnected the autopilot; brought thrust to idle; deployed the speed brakes and initiated a descent. I also keyed the ptt to advise 'aircraft X is deviating for a TCAS RA'. The controller advised us that the traffic had just 'popped up' and that additional traffic was below us at 8000 feet. Upon reaching 'clear of conflict'; a second TCAS RA required us to climb due to the oncoming traffic at 8;000. Since I was already leveling off; I continued to pitch up and add thrust. Once clear of the second conflict; I advised that we were climbing through 8;800 and 250 knots and requested new instructions. We were re-cleared direct fytte at 10;000 and 300 knots. The other traffic was VFR; flying level at 10;000 feet (an inappropriate altitude in violation of 91.159); and apparently elected to turn on their transponder only upon sighting other traffic (us). This is a clear violation of 91.215; which requires aircraft to operate transponder on/altitude encoding at and above 10;000 feet. This left us very little warning; as the TCAS RA occurred almost immediately after the traffic appeared on the TCAS display; further; the controller had been left unaware of the conflicting traffic to that point. VFR pilots operating GA aircraft must familiarize themselves with applicable sections of part 91.

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Original NASA ASRS Text

Title: EMB-145 flight crew reported an airborne conflict with an aircraft operating at 10;000' without an encoding transponder. The conflict aircraft reportedly turned on their transponder at some point in the incident.

Narrative: While cruising at 10;000 feet and assigned 300 knots with Rockford Approach; approximately 20 miles northwest of FYTTE; a TCAS traffic advisory (TA) showed an aircraft appearing at our 12 o'clock; same altitude. I took control of the aircraft and began scanning for the traffic. Shortly thereafter; a TCAS resolution advisory (RA) called for a descent of 2000 feet per minute. I immediately disconnected the autopilot; brought thrust to idle; deployed the speed brakes and initiated a descent. I also keyed the PTT to advise 'Aircraft X is deviating for a TCAS RA'. The Controller advised us that the traffic had just 'popped up' and that additional traffic was below us at 8000 feet. Upon reaching 'clear of conflict'; a second TCAS RA required us to climb due to the oncoming traffic at 8;000. Since I was already leveling off; I continued to pitch up and add thrust. Once clear of the second conflict; I advised that we were climbing through 8;800 and 250 knots and requested new instructions. We were re-cleared direct FYTTE at 10;000 and 300 knots. The other traffic was VFR; flying level at 10;000 feet (an inappropriate altitude in violation of 91.159); and apparently elected to turn on their transponder only upon sighting other traffic (us). This is a clear violation of 91.215; which requires aircraft to operate transponder on/altitude encoding at and above 10;000 feet. This left us very little warning; as the TCAS RA occurred almost immediately after the traffic appeared on the TCAS display; further; the Controller had been left unaware of the conflicting traffic to that point. VFR pilots operating GA aircraft must familiarize themselves with applicable sections of Part 91.

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.