EMB ERJ 145 ER/LR
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Conflict Airborne Conflict; Deviation / Discrepancy - Procedural FAR
Flight Crew Took Evasive Action
Human Factors
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.
[Report narrative contained no additional information].
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.